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#Post#: 371--------------------------------------------------
Re: Tugasan Chapter 3 4 GE
By: azim-4geo- Date: March 1, 2011, 9:59 pm
---------------------------------------------------------
AZIM [hr]RIZAL
HEREDITY
Heredity is the passing of traits to offspring (from its parent
or ancestors). This is the process by which an offspring cell or
organism acquires or becomes predisposed to the characteristics
of its parent cell or organism. Through heredity, variations
exhibited by individuals can accumulate and cause some species
to evolve. The study of heredity in biology is called genetics,
which includes the field of epigenetics.
CHROMOSOME
A chromosome is an organized structure of DNA and protein that
is found in cells. It is a single piece of coiled DNA containing
many genes, regulatory elements and other nucleotide sequences.
Chromosomes also contain DNA-bound proteins, which serve to
package the DNA and control its functions.
Diagram of a replicated and condensed metaphase eukaryotic
chromosome. (1) Chromatid – one of the two identical parts of
the chromosome after S phase. (2) Centromere – the point where
the two chromatids touch, and where the microtubules attach. (3)
Short arm. (4) Long arm.
Chromosomes vary widely between different organisms. The DNA
molecule may be circular or linear, and can be composed of
10,000 to 1,000,000,000[1] nucleotides in a long chain.
Typically, eukaryotic cells (cells with nuclei) have large
linear chromosomes and prokaryotic cells (cells without defined
nuclei) have smaller circular chromosomes, although there are
many exceptions to this rule. Also, cells may contain more than
one type of chromosome; for example, mitochondria in most
eukaryotes and chloroplasts in plants have their own small
chromosomes.
GENE
A gene is a unit of heredity in a living organism. It normally
resides on some stretches of DNA and RNA that codes for a type
of protein or for an RNA chain that has a function in the
organism. All living things depend on genes, as they specify all
proteins and functional RNA chains. Genes hold the information
to build and maintain an organism's cells and pass genetic
traits to offspring, although some organelles (e.g.
mitochondria) are self-replicating and are not coded for by the
organism's DNA. All organisms have many genes corresponding to
many different biological traits, some of which are immediately
visible, such as eye color or number of limbs, and some of which
are not, such as blood type or increased risk for specific
diseases, or the thousands of basic biochemical processes that
comprise life.
DNA
Deoxyribonucleic acid
(/diˌɒksiˌraɪbɵ.njuːˌkleɪ
;.ɨk
ˈæsɪd/ ( listen)), or DNA, is a nucleic acid that
contains the genetic instructions used in the development and
functioning of all known living organisms (with the exception of
RNA viruses). The main role of DNA molecules is the long-term
storage of information. DNA is often compared to a set of
blueprints, like a recipe or a code, since it contains the
instructions needed to construct other components of cells, such
as proteins and RNA molecules. The DNA segments that carry this
genetic information are called genes, but other DNA sequences
have structural purposes, or are involved in regulating the use
of this genetic information.
DNA consists of two long polymers of simple units called
nucleotides, with backbones made of sugars and phosphate groups
joined by ester bonds. These two strands run in opposite
directions to each other and are therefore anti-parallel.
Attached to each sugar is one of four types of molecules called
bases. It is the sequence of these four bases along the backbone
that encodes information. This information is read using the
genetic code, which specifies the sequence of the amino acids
within proteins. The code is read by copying stretches of DNA
into the related nucleic acid RNA, in a process called
transcription.
Within cells, DNA is organized into long structures called
chromosomes. These chromosomes are duplicated before cells
divide, in a process called DNA replication. Eukaryotic
organisms (animals, plants, fungi, and protists) store most of
their DNA inside the cell nucleus and some of their DNA in
organelles, such as mitochondria or chloroplasts.[1] In
contrast, prokaryotes (bacteria and archaea) store their DNA
only in the cytoplasm. Within the chromosomes, chromatin
proteins such as histones compact and organize DNA. These
compact structures guide the interactions between DNA and other
proteins, helping control which parts of the DNA are
transcribed.
MITOSIS
Mitosis is the process by which a eukaryotic cell separates the
chromosomes in its cell nucleus into two identical sets in two
nuclei. It is generally followed immediately by cytokinesis,
which divides the nuclei, cytoplasm, organelles and cell
membrane into two cells containing roughly equal shares of these
cellular components. Mitosis and cytokinesis together define the
mitotic (M) phase of the cell cycle—the division of the mother
cell into two daughter cells, genetically identical to each
other and to their parent cell. This accounts for approximately
10% of the cell cycle.
Mitosis occurs only in eukaryotic cells and the process varies
in different species. For example, animals undergo an "open"
mitosis, where the nuclear envelope breaks down before the
chromosomes separate, while fungi such as Aspergillus nidulans
and Saccharomyces cerevisiae (yeast) undergo a "closed" mitosis,
where chromosomes divide within an intact cell nucleus.[1]
Prokaryotic cells, which lack a nucleus, divide by a process
called binary fission
#Post#: 372--------------------------------------------------
Re: Tugasan Chapter 3 4 GE
By: Pian 4 Ge Date: March 1, 2011, 9:59 pm
---------------------------------------------------------
Pian , syafiq
4Ge
HEREDITY
Heredity is the passing of traits to offspring (from its parent
or ancestors). This is the process by which an offspring cell or
organismacquires or becomes predisposed to the characteristics
of its parent cell or organism. Through heredity, variations
exhibited by individuals can accumulate and cause some species
to evolve. The study of heredity in biology is called genetics,
which includes the field of epigenetics.
CHROMOSOME
A chromosome is an organized structure of DNA and protein that
is found in cells. It is a single piece of coiled DNA containing
many genes, regulatory elementsand other nucleotide sequences.
Chromosomes also contain DNA-bound proteins, which serve to
package the DNA and control its functions.
Diagram of a replicated and condensedmetaphase eukaryotic
chromosome. (1)Chromatid – one of the two identical parts of the
chromosome after S phase. (2) Centromere – the point where the
two chromatids touch, and where the microtubules attach. (3)
Short arm. (4) Long arm.
Chromosomes vary widely between different organisms. The DNA
molecule may be circular or linear, and can be composed of
10,000 to 1,000,000,000[1] nucleotides in a long chain.
Typically, eukaryotic cells (cells with nuclei) have large
linear chromosomes and prokaryotic cells (cells without defined
nuclei) have smaller circular chromosomes, although there are
many exceptions to this rule. Also, cells may contain more than
one type of chromosome; for example, mitochondria in most
eukaryotes and chloroplasts in plants have their own small
chromosomes.
In eukaryotes, nuclear chromosomes are packaged by proteins into
a condensed structure called chromatin. This allows the very
long DNA molecules to fit into the cell nucleus. The structure
of chromosomes and chromatin varies through the cell cycle.
Chromosomes are the essential unit for cellular division and
must be replicated, divided, and passed successfully to their
daughter cells so as to ensure the genetic diversity and
survival of their progeny. Chromosomes may exist as either
duplicated or unduplicated. Unduplicated chromosomes are single
linear strands, whereas duplicated chromosomes (copied during
synthesis phase) contain two copies joined by acentromere.
GENE
A gene is a unit of heredity in a living organism. It normally
resides on some stretches of DNA and RNA that codes for a type
of protein or for an RNA chain that has a function in the
organism. All living things depend on genes, as they specify all
proteins and functional RNA chains. Genes hold the information
to build and maintain an organism's cells and pass genetic
traits to offspring, although some organelles (e.g.
mitochondria) are self-replicating and are not coded for by the
organism's DNA. All organisms have many genes corresponding to
many different biological traits, some of which are immediately
visible, such as eye color or number of limbs, and some of which
are not, such as blood type or increased risk for specific
diseases, or the thousands of basic biochemical processes that
comprise life.
DNA
Deoxyribonucleic acid
(/diˌɒksiˌraɪbɵ.njuːˌkleɪ
;.ɨk
ˈæsɪd/ ( listen)), or DNA, is a nucleic acid that
contains the geneticinstructions used in the development and
functioning of all known living organisms (with the exception of
RNA viruses). The main role of DNA molecules is the long-term
storage of information. DNA is often compared to a set of
blueprints, like a recipe or a code, since it contains the
instructions needed to construct other components of cells, such
as proteins and RNAmolecules. The DNA segments that carry this
genetic information are called genes, but other DNA sequences
have structural purposes, or are involved in regulating the use
of this genetic information.
#Post#: 373--------------------------------------------------
Re: Tugasan Chapter 3 4 GE
By: pakku 4GEO Date: March 1, 2011, 10:00 pm
---------------------------------------------------------
PAKKU & DIK ARD :-X
4GEO
HEREDITY
Heredity is the passing of traits to offspring (from its parent
or ancestors). This is the process by which an offspring cell or
organism acquires or becomes predisposed to the characteristics
of its parent cell or organism. Through heredity, variations
exhibited by individuals can accumulate and cause some species
to evolve. The study of heredity in biology is called genetics,
which includes the field of epigenetics.
CHROMOSOME
A chromosome is an organized structure of DNA and protein that
is found in cells. It is a single piece of coiled DNA containing
many genes,regulatory elements and other nucleotide sequences.
Chromosomes also contain DNA-bound proteins, which serve to
package the DNA and control its functions.
Diagram of a replicated and condensedmetaphase eukaryotic
chromosome. (1)Chromatid – one of the two identical parts of the
chromosome after S phase. (2) Centromere – the point where the
two chromatids touch, and where the microtubules attach. (3)
Short arm. (4) Long arm.
Chromosomes vary widely between different organisms. The DNA
molecule may be circular or linear, and can be composed of
10,000 to 1,000,000,000[1] nucleotides in a long chain.
Typically, eukaryotic cells (cells with nuclei) have large
linear chromosomes and prokaryoticcells (cells without defined
nuclei) have smaller circular chromosomes, although there are
many exceptions to this rule. Also, cells may contain more than
one type of chromosome; for example, mitochondria in most
eukaryotes and chloroplasts in plants have their own small
chromosomes.
GENE
A gene is a unit of heredity in a living organism. It normally
resides on some stretches of DNA and RNA that codes for a type
of protein or for an RNA chain that has a function in the
organism. All living things depend on genes, as they specify all
proteins and functional RNA chains. Genes hold the information
to build and maintain an organism'scells and pass genetic traits
to offspring, although some organelles (e.g. mitochondria) are
self-replicating and are not coded for by the organism's DNA.
All organisms have many genes corresponding to many different
biological traits, some of which are immediately visible, such
as eye color or number of limbs, and some of which are not, such
as blood type or increased risk for specific diseases, or the
thousands of basicbiochemical processes that comprise life.
DNA
DNA, is a nucleic acid that contains the genetic instructions
used in the development and functioning of all known living
organisms(with the exception of RNA viruses). The main role of
DNA molecules is the long-term storage ofinformation. DNA is
often compared to a set of blueprints, like a recipe or a code,
since it contains the instructions needed to construct other
components of cells, such as proteins and RNA molecules. The DNA
segments that carry this genetic information are called genes,
but other DNA sequences have structural purposes, or are
involved in regulating the use of this genetic information.
DNA consists of two long polymers of simple units called
nucleotides, with backbones made of sugarsand phosphate groups
joined by ester bonds. These two strands run in opposite
directions to each other and are therefore anti-parallel.
Attached to each sugar is one of four types of molecules called
bases. It is the sequence of these four bases along the backbone
that encodes information. This information is read using the
genetic code, which specifies the sequence of the amino acids
within proteins. The code is read by copying stretches of DNA
into the related nucleic acid RNA, in a process called
transcription.
Within cells, DNA is organized into long structures called
chromosomes. These chromosomes are duplicated before cells
divide, in a process called DNA replication. Eukaryotic
organisms (animals,plants, fungi, and protists) store most of
their DNA inside the cell nucleus and some of their DNA
inorganelles, such as mitochondria or chloroplasts.[1] In
contrast, prokaryotes (bacteria and archaea) store their DNA
only in the cytoplasm. Within the chromosomes, chromatin
proteins such as histonescompact and organize DNA. These compact
structures guide the interactions between DNA and other
proteins, helping control which parts of the DNA are
transcribed.[/font]
#Post#: 374--------------------------------------------------
Re: Tugasan Chapter 3 4 GE
By: Aliff4geo Date: March 1, 2011, 10:01 pm
---------------------------------------------------------
SYAFIQ AND ALIFF AMRAN
HEREDITY
Heredity is the passing of traits to offspring (from its parent
or ancestors). This is the process by which an offspring cell or
organism acquires or becomes predisposed to the characteristics
of its parent cell or organism. Through heredity, variations
exhibited by individuals can accumulate and cause some species
to evolve. The study of heredity in biology is called genetics,
which includes the field of epigenetics.
DNA
Deoxyribonucleic acid
(/diˌɒksiˌraɪbɵ.njuːˌkleɪ
;.ɨk
ˈæsɪd/ ( listen)), or DNA, is a nucleic acid that
contains the genetic instructions used in the development and
functioning of all known living organisms (with the exception of
RNA viruses). The main role of DNA molecules is the long-term
storage of information. DNA is often compared to a set of
blueprints, like a recipe or a code, since it contains the
instructions needed to construct other components of cells, such
as proteins and RNA molecules. The DNA segments that carry this
genetic information are called genes, but other DNA sequences
have structural purposes, or are involved in regulating the use
of this genetic information.
CHROMOSOME
A chromosome is an organized structure of DNA and protein that
is found in cells. It is a single piece of coiled DNA containing
many genes, regulatory elements and other nucleotide sequences.
Chromosomes also contain DNA-bound proteins, which serve to
package the DNA and control its functions.
GENE
A gene is a unit of heredity in a living organism. It normally
resides on some stretches of DNA and RNA that codes for a type
of protein or for an RNA chain that has a function in the
organism. All living things depend on genes, as they specify all
proteins and functional RNA chains. Genes hold the information
to build and maintain an organism's cells and pass genetic
traits to offspring, although some organelles (e.g.
mitochondria) are self-replicating and are not coded for by the
organism's DNA. All organisms have many genes corresponding to
many different biological traits, some of which are immediately
visible, such as eye color or number of limbs, and some of which
are not, such as blood type or increased risk for specific
diseases, or the thousands of basic biochemical processes that
comprise life
#Post#: 375--------------------------------------------------
Re: Tugasan Chapter 3 4 GE
By: Akhmal Maee 4GE Date: March 1, 2011, 10:02 pm
---------------------------------------------------------
ali , maee
Heredity is the passing of traits to offspring (from its parent
or ancestors). This is the process by which an offspring cell or
organism acquires or becomes predisposed to the characteristics
of its parent cell or organism. Through heredity, variations
exhibited by individuals can accumulate and cause some species
to evolve. The study of heredity in biology is called genetics,
which includes the field of epigenetics.
A chromosome is an organized structure of DNA and protein that
is found in cells. It is a single piece of coiled DNA containing
many genes, regulatory elements and other nucleotide sequences.
Chromosomes also contain DNA-bound proteins, which serve to
package the DNA and control its functions.
Diagram of a replicated and condensed metaphase eukaryotic
chromosome. (1) Chromatid – one of the two identical parts of
the chromosome after S phase. (2) Centromere – the point where
the two chromatids touch, and where the microtubules attach. (3)
Short arm. (4) Long arm.
Chromosomes vary widely between different organisms. The DNA
molecule may be circular or linear, and can be composed of
10,000 to 1,000,000,000[1] nucleotides in a
long chain. Typically, eukaryotic cells (cells with nuclei) have
large linear chromosomes and prokaryotic cells (cells without
defined nuclei) have smaller circular chromosomes, although
there are many exceptions to this rule. Also, cells may contain
more than one type of chromosome; for example, mitochondria in
most eukaryotes and chloroplasts in plants have their own small
chromosomes.
A gene is a unit of heredity in a living organism. It normally
resides on some stretches of DNA and RNA that codes for a type
of protein or for an RNA chain that has a function in the
organism. All living things depend on genes, as they specify all
proteins and functional RNA chains. Genes hold the information
to build and maintain an organism's cells and pass genetic
traits to offspring, although some organelles (e.g.
mitochondria) are self-replicating and are not coded for by the
organism's DNA. All organisms have many genes corresponding to
many different biological traits, some of which are immediately
visible, such as eye color or number of limbs, and some of which
are not, such as blood type or increased risk for specific
diseases, or the thousands of basic biochemical processes that
comprise life.
DNA consists of two long polymers of simple units called
nucleotides, with backbones made of sugars and phosphate groups
joined by ester bonds. These two strands run in opposite
directions to each other and are therefore anti-parallel.
Attached to each sugar is one of four types of molecules called
bases. It is the sequence of these four bases along the backbone
that encodes information. This information is read using the
genetic code, which specifies the sequence of the amino acids
within proteins. The code is read by copying stretches of DNA
into the related nucleic acid RNA, in a process called
transcription.
Within cells, DNA is organized into long structures called
chromosomes. These chromosomes are duplicated before cells
divide, in a process called DNA replication. Eukaryotic
organisms (animals, plants, fungi, and protists) store most of
their DNA inside the cell nucleus and some of their DNA in
organelles, such as mitochondria or chloroplasts.[1] In
contrast, prokaryotes (bacteria and archaea) store their DNA
only in the cytoplasm. Within the chromosomes, chromatin
proteins such as histones compact and organize DNA. These
compact structures guide the interactions between DNA and other
proteins, helping control which parts of the DNA are transcribed
Mitosis is the process by which a eukaryotic cell separates the
chromosomes in its cell nucleus into two identical sets in two
nuclei. It is generally followed immediately by cytokinesis,
which divides the nuclei, cytoplasm, organelles and cell
membrane into two cells containing roughly equal shares of these
cellular components. Mitosis and cytokinesis together define the
mitotic (M) phase of the cell cycle—the division of the mother
cell into two daughter cells, genetically identical to each
other and to their parent cell. This accounts for approximately
10% of the cell cycle.
Mitosis occurs only in eukaryotic cells and the process varies
in different species. For example, animals undergo an "open"
mitosis, where the nuclear envelope breaks down before the
chromosomes separate, while fungi such as Aspergillus nidulans
and Saccharomyces cerevisiae (yeast) undergo a "closed" mitosis,
where chromosomes divide within an intact cell nucleus.[1]
Prokaryotic cells, which lack a nucleus, divide by a process
called binary fission.
#Post#: 389--------------------------------------------------
Re: Tugasan Chapter 3 4 GE
By: adiaiman 4geo Date: March 2, 2011, 6:59 am
---------------------------------------------------------
AdiAiman
Adib
4Geo
Heredity
is the passing of traits to offspring (from its parent or
ancestors). This is the process by which an offspring cell or
organism acquires or becomes predisposed to the characteristics
of its parent cell or organism. Through heredity, variations
exhibited by individuals can accumulate and cause some species
to evolve. The study of heredity in biology is called genetics,
which includes the field of epigenetics.
In humans, eye colour is an inherited characteristic and an
individual might inherit the "brown-eye trait" from one of the
parents.[1] Inherited traits are controlled by genes and the
complete set of genes within an organism's genome is called its
genotype.[2]
The complete set of observable traits that make up the structure
and behaviour of an organism is called its phenotype. These
traits come from the interaction of its genotype with the
environment.[3] As a result, many aspects of an organism's
phenotype are not inherited. For example, suntanned skin comes
from the interaction between a person's genotype and sunlight;
thus, suntans are not passed on to people's children. However,
some people tan more easily than others, due to differences in
their genotype; a striking example are people with the inherited
trait of albinism, who do not tan at all and are very sensitive
to sunburn.[4]
Heritable traits are known to be passed from one generation to
the next via DNA, a molecule that encodes genetic
information.[2] DNA is a long polymer composed of four types of
bases. The sequence of bases along a particular DNA molecule
specify the genetic information, in a manner similar to a
sequence of letters spelling out a sentence. Before a cell
divides, the DNA is copied, so that each of the resulting two
cells will inherit the DNA sequence. Portions of a DNA molecule
that specify a single functional unit are called genes;
different genes have different sequences of bases. Within cells,
the long strands of DNA form condensed structures called
chromosomes. The specific location of a DNA sequence within a
chromosome is known as a locus. If the DNA sequence at a locus
varies between individuals, the different forms of this sequence
are called alleles. DNA sequences can change through mutations,
producing new alleles. If a mutation occurs within a gene, the
new allele may affect the trait that the gene controls, altering
the phenotype of the organism.[5]
However, while this simple correspondence between an allele and
a trait works in some cases, most traits are more complex and
are controlled by multiple interacting genes within and among
organisms.[6][7] Developmental biologists suggest that complex
interactions in genetic networks and communication among cells
can lead to heritable variations that may underlay some of the
mechanics in developmental plasticity and canalization.[8]
Recent findings have confirmed important examples of heritable
changes that cannot be explained by direct agency of the DNA
molecule. These phenomena are classed as epigenetic inheritance
systems that are causally or independently evolving over genes.
Research into modes and mechanisms of epigenetic inheritance is
still in its scientific infancy, however, this area of research
has attracted much recent activity as it broadens the scope of
heritability and evolutionary biology in general.[9] DNA
methylation marking chromatin, self-sustaining metabolic loops,
gene silencing by RNA interference, and the three dimensional
conformation of proteins (such as prions) are areas where
epigenetic inheritance systems have been discovered at the
organismic level.[10][11] Heritability may also occur at even
larger scales. For example, ecological inheritance through the
process of niche construction is defined by the regular and
repeated activities of organisms in their environment. This
generates a legacy of effect that modifies and feeds back into
the selection regime of subsequent generations. Descendants
inherit genes plus environmental characteristics generated by
the ecological actions of ancestors.[12] Other examples of
heritability in evolution that are not under the direct control
of genes include the inheritance of cultural traits, group
heritability, and symbiogenesis.[13][14][15] These examples of
heritability that operate above the gene are covered broadly
under the title of multilevel or hierarchical selection, which
has been a subject of intense debate in the history of
evolutionary science.
Chromosome
is an organized structure of DNA and protein that is found in
cells. It is a single piece of coiled DNA containing many genes,
regulatory elements and other nucleotide sequences. Chromosomes
also contain DNA-bound proteins, which serve to package the DNA
and control its functions.
Chromosomes vary widely between different organisms. The DNA
molecule may be circular or linear, and can be composed of
10,000 to 1,000,000,000[1] nucleotides in a long chain.
Typically, eukaryotic cells (cells with nuclei) have large
linear chromosomes and prokaryotic cells (cells without defined
nuclei) have smaller circular chromosomes, although there are
many exceptions to this rule. Also, cells may contain more than
one type of chromosome; for example, mitochondria in most
eukaryotes and chloroplasts in plants have their own small
chromosomes.
In eukaryotes, nuclear chromosomes are packaged by proteins into
a condensed structure called chromatin. This allows the very
long DNA molecules to fit into the cell nucleus. The structure
of chromosomes and chromatin varies through the cell cycle.
Chromosomes are the essential unit for cellular division and
must be replicated, divided, and passed successfully to their
daughter cells so as to ensure the genetic diversity and
survival of their progeny. Chromosomes may exist as either
duplicated or unduplicated. Unduplicated chromosomes are single
linear strands, whereas duplicated chromosomes (copied during
synthesis phase) contain two copies joined by a centromere.
Compaction of the duplicated chromosomes during mitosis and
meiosis results in the classic four-arm structure (pictured to
the right). Chromosomal recombination plays a vital role in
genetic diversity. If these structures are manipulated
incorrectly, through processes known as chromosomal instability
and translocation, the cell may undergo mitotic catastrophe and
die, or it may unexpectedly evade apoptosis leading to the
progression of cancer.
In practice "chromosome" is a rather loosely defined term. In
prokaryotes and viruses, the term genophore is more appropriate
when no chromatin is present. However, a large body of work uses
the term chromosome regardless of chromatin content. In
prokaryotes, DNA is usually arranged as a circle, which is
tightly coiled in on itself, sometimes accompanied by one or
more smaller, circular DNA molecules called plasmids. These
small circular genomes are also found in mitochondria and
chloroplasts, reflecting their bacterial origins. The simplest
genophores are found in viruses: these DNA or RNA molecules are
short linear or circular genophores that often lack structural
proteins.
Gene
is a unit of heredity in a living organism. It normally resides
on some stretches of DNA and RNA that codes for a type of
protein or for an RNA chain that has a function in the organism.
All living things depend on genes, as they specify all proteins
and functional RNA chains. Genes hold the information to build
and maintain an organism's cells and pass genetic traits to
offspring, although some organelles (e.g. mitochondria) are
self-replicating and are not coded for by the organism's DNA.
All organisms have many genes corresponding to many different
biological traits, some of which are immediately visible, such
as eye color or number of limbs, and some of which are not, such
as blood type or increased risk for specific diseases, or the
thousands of basic biochemical processes that comprise life.
A modern working definition of a gene is "a locatable region of
genomic sequence, corresponding to a unit of inheritance, which
is associated with regulatory regions, transcribed regions, and
or other functional sequence regions ".[1][2] Colloquial usage
of the term gene (e.g. "good genes, "hair color gene") may
actually refer to an allele: a gene is the basic instruction, a
sequence of nucleic acid (DNA or, in the case of certain viruses
RNA), while an allele is one variant of that gene. Thus, when
the mainstream press refers to "having" a "gene" for a specific
trait, this is generally inaccurate. In most cases, all people
would have a gene for the trait in question, but certain people
will have a specific allele of that gene, which results in the
trait variant. In the simplest case, the phenotypic variation
observed may be caused by a single letter of the genetic code -
a single nucleotide polymorphism.
DNA
Deoxyribonucleic acid or DNA is a nucleic acid that contains the
genetic instructions used in the development and functioning of
all known living organisms (with the exception of RNA viruses).
The main role of DNA molecules is the long-term storage of
information. DNA is often compared to a set of blueprints, like
a recipe or a code, since it contains the instructions needed to
construct other components of cells, such as proteins and RNA
molecules. The DNA segments that carry this genetic information
are called genes, but other DNA sequences have structural
purposes, or are involved in regulating the use of this genetic
information.
DNA consists of two long polymers of simple units called
nucleotides, with backbones made of sugars and phosphate groups
joined by ester bonds. These two strands run in opposite
directions to each other and are therefore anti-parallel.
Attached to each sugar is one of four types of molecules called
bases. It is the sequence of these four bases along the backbone
that encodes information. This information is read using the
genetic code, which specifies the sequence of the amino acids
within proteins. The code is read by copying stretches of DNA
into the related nucleic acid RNA, in a process called
transcription.
Within cells, DNA is organized into long structures called
chromosomes. These chromosomes are duplicated before cells
divide, in a process called DNA replication. Eukaryotic
organisms (animals, plants, fungi, and protists) store most of
their DNA inside the cell nucleus and some of their DNA in
organelles, such as mitochondria or chloroplasts.[1] In
contrast, prokaryotes (bacteria and archaea) store their DNA
only in the cytoplasm. Within the chromosomes, chromatin
proteins such as histones compact and organize DNA. These
compact structures guide the interactions between DNA and other
proteins, helping control which parts of the DNA are
transcribed.
Mitosis
is the process by which a eukaryotic cell separates the
chromosomes in its cell nucleus into two identical sets in two
nuclei. It is generally followed immediately by cytokinesis,
which divides the nuclei, cytoplasm, organelles and cell
membrane into two cells containing roughly equal shares of these
cellular components. Mitosis and cytokinesis together define the
mitotic (M) phase of the cell cycle—the division of the mother
cell into two daughter cells, genetically identical to each
other and to their parent cell. This accounts for approximately
10% of the cell cycle.
Mitosis occurs only in eukaryotic cells and the process varies
in different species. For example, animals undergo an "open"
mitosis, where the nuclear envelope breaks down before the
chromosomes separate, while fungi such as Aspergillus nidulans
and Saccharomyces cerevisiae (yeast) undergo a "closed" mitosis,
where chromosomes divide within an intact cell nucleus.[1]
Prokaryotic cells, which lack a nucleus, divide by a process
called binary fission.
The process of mitosis is fast and highly complex. The sequence
of events is divided into stages corresponding to the completion
of one set of activities and the start of the next. These stages
are interphase, prophase, prometaphase, metaphase, anaphase and
telophase. During mitosis the pairs of chromatids condense and
attach to fibers that pull the sister chromatids to opposite
sides of the cell. The cell then divides in cytokinesis, to
produce two identical daughter cells.[2]
Because cytokinesis usually occurs in conjunction with mitosis,
"mitosis" is often used interchangeably with "mitotic phase".
However, there are many cells where mitosis and cytokinesis
occur separately, forming single cells with multiple nuclei.
This occurs most notably among the fungi and slime moulds, but
is found in various different groups. Even in animals,
cytokinesis and mitosis may occur independently, for instance
during certain stages of fruit fly embryonic development.[3]
Errors in mitosis can either kill a cell through apoptosis or
cause mutations that may lead to cancer.
Meiosis
is a special type of cell division necessary for sexual
reproduction. In animals, meiosis produces gametes like sperm
and egg cells, while in other organisms like fungi it generates
spores. Meiosis begins with one diploid cell containing two
copies of each chromosome—one from the organism's mother and one
from its father—and produces four haploid cells containing one
copy of each chromosome. Each of the resulting chromosomes in
the gamete cells is a unique mixture of maternal and paternal
DNA, ensuring that offspring are genetically distinct from
either parent. This gives rise to genetic diversity in sexually
reproducing populations, which enables them to adapt during the
course of evolution.
Before meiosis, the cell's chromosomes are duplicated by a round
of DNA replication. This leaves the maternal and paternal
versions of each chromosome, called homologs, composed of two
exact copies called sister chromatids and attached at the
centromere region. In the beginning of meiosis, the maternal and
paternal homologs pair to each other. Then they typically
exchange parts by homologous recombination, leading to
crossovers of DNA from the maternal version of the chromosome to
the paternal version and vice versa. Spindle fibers bind to the
centromeres of each pair of homologs and arrange the pairs at
the spindle equator. Then the fibers pull the recombined
homologs to opposite poles of the cell. As the chromosomes move
away from the center, the cell divides into two daughter cells,
each containing a haploid number of chromosomes composed of two
chromatids. After the recombined maternal and paternal homologs
have separated into the two daughter cells, a second round of
cell division occurs. There, meiosis ends as the two sister
chromatids making up each homolog are separated and move into
one of the four resulting gamete cells. Upon fertilization, for
example when a sperm enters an egg cell, two gamete cells
produced by meiosis fuse. The gamete from the mother and the
gamete from the father each contribute half to the set of
chromosomes that make up the new offsping's genome.
Meiosis uses many of the same mechanisms as mitosis, a type of
cell division used by eukaryotes like plants and animals to
split one cell into two identical daughter cells. In all plants,
and in many protists, meiosis results in the formation of
spores, haploid cells that can divide vegetatively without
undergoing fertilization. Some eukaryotes, like Bdelloid
rotifers, have lost the ability to carry out meiosis and have
acquired the ability to reproduce by parthenogenesis. Meiosis
does not occur in archaea or bacteria, which reproduce via
asexual processes such as binary fission.
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