Mohammed Salam Unit 11 LA: B
Explore how the process of cell division in eukaryotic cells contributes to genetic variation
Structure and function of human chromosomes
Throughout the nucleus of cells, there are thread-like structures called chromosomes. They are
composed of DNA coiled around proteins called histones, which act as structural support. The
chromosomes ensure DNA replication and distribution correctly during cell division. They have
constriction points called centromeres which divide each chromosome into two parts or arms.
Chromosomes have their distinctive shapes due to the position of the centromeres. They can be
used to identify individual genes by their location on chromosomes. Each living species distinguishes
itself by its distinct set of instructions contained in DNA, which are transmitted from parents to
offspring through chromosomes. Chromosomes ensure that every cell in an organism contains the
same genetic information. Chromosomes also control gene expression, which determines whether
genes are activated or deactivated in a cell. Chromosomes can't be seen in a cell nucleus when it's
not dividing, but they become visible when it divides under a microscope. A human contains 23 pairs
of chromosomes, a total of 46 chromosomes.
Centromere:
During mitosis and meiosis, the centromere helps the cell divide up its DNA by constricting the
chromosomes. The centromere is the region of the cell where the spindle fibres attach, after which
the sister chromatids forming the replicated chromosome are separated by their spindle fibres. DNA
is identical in both daughter cells as a result of this process. Cell division occurs when one "parent"
cell divides into two, and the daughter cells are useful to the cell's growth and reproduction. To
survive, each daughter cell must contain a copy of each parent cell's chromosome. The cell can die or
develop genetic mutation if it receives inaccurate information in the form of incomplete copies of a
chromosome or if too many copies of the same chromosome are present. Firstly, the cell must make
a copy of its DNA. The two copies adhere to each other, eventually condensing to form sister
chromatids before being separated during cell division. This process is done so that there is full copy
of DNA given to each daughter cell.
Centromeres plays a role in cell replication like it act as a binding site for the spindle fibre that pulls
sister chromatids to opposite ends of the parent cell to become their cytoplasm during mitosis.
There is consequence if the centromeres don’t function properly, like cells cannot divide
successfully. If there are any attempts been made to divide the daughter cells with the lack of
genetic instructions, it may cause genetic disorders. When centromere lacks function, it can create
huge chromosome imbalance and it is believed that it is responsible for causing cancer cells forming.
There is a theory that inherited centromere disorders can result in early embryonic death during
miscarriages due to chromosome sorting problems caused by centromere dysfunction.
Chromatids:
A sister chromatid contains two identical DNA strands. They are held together at the centromere
during the S-phase, and during mitosis, they separate and enter separate daughter cells carrying the
same DNA. In order to ensure daughter cells are healthy and fully functional, chromatids store two
copies of the DNA from the parent cell in preparation for cell division. Each daughter cell can pass
one copy of the cell's DNA down to each daughter cell by keeping two copies of its DNA together.
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,Mohammed Salam Unit 11 LA: B
The consequence can be major if the sister chromatids fail to separate during the process of cell
division.
Autosomes:
Autosomes comes in two forms in human cell which are called X and Y. 22 sets of autosomes are
currently present in humans and they are referred to numerically like chromosome 1 and 2.
Normally, each chromosome follows a "map" that is shared by all members of a species, which
contains thousands of genes performing unique functions within the cells. Gene-expression factors
may use this "map" to understand when a cell needs to express a particular gene. Cells also use this
"map" when they need to express specific genes. It is possible for cells to perform a wide range of
functions when their autosomes are healthy. Our bodies are composed of hundreds of subtly
different cell types that express different combinations of genes at the right time. This allows us to
carry out a wide range of cellular functions.
Sex chromosomes:
There are two types of sex chromosomes which are identified in human body such as chromosome x
and y. The sex chromosome determines the gender of the offspring. Males have one x and y
chromosomes and females have two x chromosomes in their cells. The two types of sex
chromosomes are structured differently which gives them different functions, the X chromosome
has a long arm and a short arm. The Y chromosome features one long arm, but the second arm is
much more shorter than X chromosome.
Chromosome number:
Chromosome number, the number of chromosomes in a species. Every species reproducing
asexually has exactly the same number of chromosomes. Asexually reproducing organisms typically
contain a diploid number of chromosomes (2n), twice as many as haploid (1n) gametes. The cell
generates haploid gametes through meiosis.
Homologous:
One homologous chromosome is inherited from the father, while the other is inherited from the
mother. Homologous chromosomes have the same gene sequence, chromosomal length, and
centromere location. It is during meiosis that homologous chromosomes are paired, but not during
mitosis. Meiosis results in genetic diversity because homologous chromosomes recombine and
exchange genetic information. Sister chromatids, which segregate to opposite poles and divide
among daughter cells, are formed by homologous chromosomes, as they contain identical genes
that encode the same protein.
Non – homologous chromosomes:
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, Mohammed Salam Unit 11 LA: B
Chromosomes that are not identical to each other, or which have different genes, are termed non-
homologous chromosomes. Non-homologous chromosomes include the X and Y chromosomes,
which contain different genes. Non-homologous chromosomes do not pair during meiosis, with the
exception of small regions of similarity required for sex cells. There is a single molecule of DNA in
each chromosome, including non-homologous chromosomes, which contains specific instructions
that determines the species.
Mitosis
Microscopic slides:
Prophase Magnification = 1000x
This image may not be
clear, but it is sufficient
to determine where
prophase has occurred
as this can be seen in
this cell where it has an
region that is dark
which suggests that
the chromatin has
been condensed this
results the cells to have
darker area. The
chromatin has been
coiled up and has been
increasingly
compacted.
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