Download Homologous Chromosome www.AssignmentPoint.com A couple of

Homologous Chromosome
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A couple of homologous chromosomes are a set of one maternal chromosome
and one paternal chromosome that pair up with each other inside a cell during
meiosis. These copies have the same genes in the same locations, or loci. These
loci provide points along each chromosome which enable a pair of
chromosomes to align correctly with each other before separating during
meiosis. This is the basis for Gregor Mendel’s laws of genetics that characterize
inheritance patterns of genetic material from an organism to its offspring.
Overview
Chromosomes are linear arrangements of condensed deoxyribonucleic acid
(DNA) and histone proteins, which forms a complex called chromatin.
Homologous chromosomes are made up of chromosome pairs of approximately
the same length, centromere position, and staining pattern, for genes with the
same corresponding loci. One homologous chromosome is inherited from the
organism's mother; the other is inherited from the organism's father. After
mitosis occurs within the daughter cells, they have the correct number of genes
which are a mix of the two parents' genes. In diploid (2n) organisms, the
genome is composed of one set of each homologous chromosome pair, as
compared to tetraploid organisms which may have two sets of each homologous
chromosome pair. The alleles on the homologous chromosomes may be
different, resulting in different phenotypes of the same genes. This mixing of
maternal and paternal traits is enhanced by crossing over during meiosis,
wherein lengths of chromosomal arms and the DNA they contain within a
homologous chromosome pair are exchanged with one another.
History
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Early in the 1900s William Bateson and Reginald Punnett were studying genetic
inheritance and they noted that some combinations of alleles appeared more
frequently than others. That data and information was further explored by
Thomas Morgan. Using test cross experiments, he revealed that, for a single
parent, the alleles of genes near to one another along the length of the
chromosome move together. Using this logic he concluded that the two genes
he was studying were located on homologous chromosomes. Later on during the
1930s Harriet Creighton and Barbara McClintock were studying meiosis in corn
cells and examining gene loci on corn chromosomes. Creighton and McClintock
discovered that the new allele combinations present in the offspring and the
event of crossing over were directly related. This proved intrachromosomal
genetic recombination.
Structure
Homologous chromosomes are chromosomes which contain the same genes in
the same order along their chromosomal arms. There are two main properties of
homologous chromosomes: the length of chromosomal arms and the placement
of the centromere
The actual length of the arm, in accordance with the gene locations, is critically
important for proper alignment. Centromere placement can be characterized by
four main arrangements, consisting of being either metacentric, submetacentric,
telocentric, or acrocentric. Both of these properties are the main factors for
creating structural homology between chromosomes. Therefore when two
chromosomes of the exact structure exist, they are able to pair together to form
homologous chromosomes.
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Since homologous chromosomes are not identical and do not originate from the
same organism, they are different from sister chromatids. Sister chromatids
result after DNA replication has occurred, and thus are identical, side-by-side
duplicates of each other.
In humans
Humans have a total of 46 chromosomes, but there are only 22 pairs of
homologous autosomal chromosomes. The additional 23rd pair is the sex
chromosomes, X and Y. If this pair is made up of an X and Y chromosome,
then the pair is not truly homologous because their size and types of genes differ
slightly. The 22 pairs of homologous chromosomes contain the same genes but
code for different traits in their allelic forms since one was inherited from the
mother and one from the father. So humans have two homologous chromosome
sets in each cell, meaning humans are diploid organisms.
Functions
Homologous chromosomes are important in the processes of meiosis and
mitosis. They allow for the recombination and random segregation of genetic
material from the mother and father into new cells.
In meiosis
Depiction of chromosome 1 after undergoing homologous recombination in
meiosis
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During the process of meiosis, homologous chromosomes can recombine and
produce new combinations of genes in the daughter cells.
Meiosis is a round of two cell divisions that results in four haploid daughter
cells that each contain half the number of chromosomes as the parent cell. It
reduces the chromosome number in a germ cell by half by first separating the
homologous chromosomes in meiosis I and then the sister chromatids in meiosis
II. The process of meiosis I is generally longer than meiosis II because it takes
more time for the chromatin to replicate and for the homologous chromosomes
to be properly oriented and segregated by the processes of pairing and synapsis
in meiosis I. During meiosis, genetic recombination (by random segregation)
and crossing over produces daughter cells that each contain different
combinations of maternally and paternally coded genes. This recombination of
genes allows for the introduction of new allele pairings and genetic variation.
Genetic variation among organisms helps make a population more stable by
providing a wider range of genetic traits for natural selection to act on.
Prophase I
In prophase I of meiosis I, each chromosome is aligned with its homologous
partner and pairs completely. In prophase I, the DNA has already undergone
replication so each chromosome consists of two identical chromatids connected
by a common centromere. During the zygotene stage of prophase I, the
homologous chromosomes pair up with each other. This pairing occurs by a
synapsis process where the synaptonemal complex - a protein scaffold - is
assembled and joins the homologous chromosomes along their lengths. Cohesin
crosslinking occurs between the homologous chromosomes and helps them
resist being pulled apart until anaphase. Genetic crossing over occurs during the
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pachytene stage of prophase I. In this process, genes are exchanged by the
breaking and union of homologous portions of the chromosomes’ lengths.
Structures called chiasmata are the site of the exchange. Chiasmata physically
link the homologous chromosomes once crossing over occurs and throughout
the process of chromosomal segregation during meiosis. At the diplotene stage
of prophase I the synaptonemal complex disassembles before which will allow
the homologous chromosomes to separate, while the sister chromatids stay
associated by their centromeres.
Metaphase I
In metaphase I of meiosis I, the pairs of homologous chromosomes, also known
as bivalents or tetrads, line up in a random order along the metaphase plate. The
random orientation is another way for cells to introduce genetic variation.
Meiotic spindles emanating from opposite spindle poles attach to each of the
homologs (each pair of sister chromatids) at the kinetochore.
Anaphase I
In anaphase I of meiosis I the homologous chromosomes are pulled apart from
each other. The homologs are cleaved by the enzyme separase to release the
cohesin that held the homologous chromosome arms together. This allows the
chiasmata to release and the homologs to move to opposite poles of the cell.
The homologous chromosomes are now randomly segregated into two daughter
cells that will undergo meiosis II to produce four haploid daughter germ cells.
Meiosis II
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After the tetrads of homologous chromosomes are separated in meiosis I, the
sister chromatids from each pair are separated. The two diploid daughter cells
resulting from meiosis I undergo another cell division in meiosis II but without
another round of chromosomal replication. The sister chromatids in the two
daughter cells are pulled apart during anaphase II by nuclear spindle fibers,
resulting in four haploid daughter cells.
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