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Unit 11: Genetics and Genetic engineering Learning aim B: Explore how the process of cell division in eukaryotic cells contributes to genetic variation£11.29
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Unit 11: Genetics and Genetic engineering Learning aim B: Explore how the process of cell division in eukaryotic cells contributes to genetic variation
contains all the details needed to get a distinction grade and signposted.
D2 Evaluate how the behaviour of the chromosomes leads to variation.
M2 Demonstrate skilful preparation of microscopic slides to observe and draw the stages of mitosis and meiosis.
B.M3 Discuss the behaviour of the ch...
Unit 11: Genetics and Genetic engineering
11B: Explore how the process of cell division in eukaryotic
cells contributes to genetic variation
Cell division
Chromosomes and cell division
Eukaryotic cells have a nucleus containing a DNA molecule packaged into thread-like
structures. These structures are called chromosomes and they are formed by DNA
tightly curled up around proteins. Chromosomes are very hard to spot in the nucleus
even with the help of a microscope. The only time chromosomes are visible is when
the DNA is tightly packed during cell division. Cell division is vital in the production of
new cells to replace worn-out cells and maintain the growth of an organism, and
proper function. Chromosomes allow the genetic data of an organism to be kept
inside the nuclei, this is helpful because humans have plenty of genetic data. It is so
large that if the DNA molecules in a single cell were uncoiled, they would extend to
be six feet long. The tips of the chromosomes are enclosed by DNA sections called
telomeres, and these telomeres protect chromosomes during DNA replication.
Humans contain 23 pairs of chromosomes of which one pair are sex chromosomes.
The other 22 pairs are known as autosomes. Chromosomes have something called a
centromere.
The centromere is what holds the two chromatids together in a chromosome and
divides the chromosomes into sections named arms. Every chromosome contains
four arms of which two of them are short arms called p arms and two long ones
called q arms. These four arms are held together in the middle by the centromere.
The location of the centromere gives the chromosome its characteristic shape. The
centromere plays a key role in helping the cell divide up its DNA during division. As
the spindle fibres of the cell are attached at the centromere, the 2 sister chromatids
are pulled to opposite sides of the dividing cell during cell division.
,So, then what are the chromatids mentioned above? Before cell division
chromosomes are duplicated and the identical duplicates join at the centromere, and
they are called chromatids. Chromatids are parts of the chromosome that give cells
the ability to store two copies of their information to prepare for cell division. This is
important as this ensures that the daughter cells are healthy, fully functional and
carrying a full complement of the parent cells’ DNA. Attached chromatids are known
as sister chromatids, and these sister chromatids are separated by spindle fibres and
pulled towards opposite ends during mitosis at a stage called anaphase and they are
known as daughter chromosomes. Chromatids are also formed during meiosis.
As mentioned above humans have 23 pairs of chromosomes with 22 of these pairs
being autosomes and 1 pair being sex chromosomes. The 22 pairs (44 individually) of
autosomes are given to an offspring by the parents, 22 single autosomes from the
father and 22 single autosomes from the mother. The one pair of sex chromosomes
is what differs between females and males with males having one X and one Y
chromosome and females having two copies of the X chromosome. Even though the
, sex chromosomes differ in males and females the autosomes are similar. The 22 pairs
of autosomes are numbered by size with the biggest being the first.
Autosomes are responsible for an organism’s characteristics inheritance, and sex
chromosomes are responsible for the sex parts.
But unlike the autosomes where an individual’s characteristics are inherited from
both the father and mother when it comes to determining the biological sex of the
individual it is the father’s genotype that gives an offspring its biological sex. As males
can give out both the X and Y chromosomes but females have two X sex
chromosomes. The absence and presence of the Y chromosome is vital because
humans develop to be females by default and the Y chromosome overrides this and
causes the development of the male reproductive system.
The number of chromosomes in the human body cells or any sexually reproducing
organisms is in pairs and known as diploid. This is twice the number found in sex cells
as they are not in pairs (haploid). The haploid number is produced during meiosis.
Humans or any sexually reproducing organisms get their genetic sequences equally
from both mother and father, this means that they have two copies of each
chromosome, one being from the paternal origin and one from maternal origin.
These paternal and maternal origins are known as homologous chromosomes, they
share similar structural features like the banding patterns size and centromere
positions. They also share the same genes at the same loci positions. But while each
homologous chromosome carries the same genes they can vary in alleles (different
versions of a gene). Homologous chromosomes are vital in the process of mitosis
and meiosis. They are what enables the recombination and random segregation of
genetic material from the parents into new cells. But they don’t react the same in
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