Download Basics of Chromosomes

Basics of Chromosomes
When the cell divides, the chromosomes are thick, compact structures, coiled tightly. At
all other times, the chromosomes are long, thin, uncoiled threads. The granular
appearance at this point means the chromosomes are called chromatin.
Review of DNA
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double helix, paired strands
DNA runs the full length of the
chromosome, supported by protein
(histones)
50% built of protein
some proteins are enzymes involved in
copying and repair
bulk of chromosome protein has a
support and packaging role for DNA
5 features of Chromosomes
1. Number of chromosomes per species
 In one species, the number of
chromosomes is normally constant.
 For example, a mouse has 40
chromosomes per cell, onion has 16 and a human has 46
2. The shape of a chromosome is characteristic
 The chromosome is a long thin structure
 There is a narrow region called the centromere
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Chromosomes occur in pairs in a cell, called homologous pairs.
homologous means similar in structure
one of each pair came from the other parent
For example, humans have 46, 23 from one parent and 23 from the other
homologous chromosomes resemble each other in structure, have the same
sequence of genes, but come from different parents. (Ref to karyotyping)
4. Chromosomes hold the heredity factors – genes
 Gene can be at a specific region of a chromosome and a specific length
 A particular gene always occurs on the same chromosome in the same position,
called a locus (plural – loci)
 Each gene has two or more alternate forms, called alleles
 Total of all genetic information is called a genome
5. Chromosomes copy themselves.
 Mitosis and Meiosis – later
Karyotyping
Karyotyping is a “map” of chromosomes that have been paired up according to their
structures.
Why is this done?
Often used to determine if there are any genetic abnormalities, like missing or extra
chromosomes, or deformity, due to a non-disjuction (homologous pairs do not separate
properly during mitosis or meiosis).
Examples are Down’s Syndrome, Turner (X), and Klinefelter (XXY)
Mutations
Chromosomal Mutations
As mentioned above, the chromosomal homologous pairs do not split evenly during
mitosis or meiosis, creating more than the normal 46 or less.
Down’s Syndrome, is identified by an extra chromosome 21, giving them 47. The
process for identifying the abnormality takes about 3 weeks.
Gene Mutations
A gene mutation is a permanent change in the DNA sequence that makes up a gene.
Therefore, the sequence of base pairs for the production of amino acids, are changed, and
the protein which is coded for may change. Some mutations can be beneficial or nonbeneficial.
Gene mutations occur in two ways.
1. They can be inherited from a parent, called hereditary mutations. These
mutations are present in a person, in virtually every cell in the body. Passed on
through gametes.
2. They can occur in the DNA of a individual’s cells at some time in the person’s
life. These are called acquired or sporadic mutations. These mutations can occur
due to environmental factors or can occur if a mistake is made as DNA copies
itself during cell division. These mutations that occur in cells other than egg or
sperm, cannot be passed on to future generations.
Two types of Gene Mutations
Point Mutations – where only one organic base pair is changed, changing the genetic
code, due to codons changing
This may have no effect because:
 It may involve part of the sense strand of the DNA which is not transcribed
 It involves part of the DNA which that particular cell does not use
 It changes the third (or second) organic base of a codon, and since the genetic
code is degenerate, the same amino acid is still coded for.
Frame Shift Mutation - Insertion / Deletion – where a base is inserted or deleted from a
gene
With insertion and deletion, the codon is definitely altered, as the position of the base is
changed all the way down the strand. An extra base is deleted from a gene or is added.
This causes the AA to change from this point on, which causes major changes in the
protein.
An example of point mutation is Sickle Cell Anemia.
Haemoglobin is made up of 4 polypeptide chains, 2 alpha and 2 beta. Normal red blood
cells are flexible and donut shaped (binoconcave). This allows them to carry four O2
molecules per RBC. They are said to have Hb, which is regular.
Sickle Cell is a base substitution in the gene coding for the 6th amino acid in the beta
chain. This causes valine to be inserted instead of glutamic acid, resulting in abnormally
shaped haemoglobin molecules. They are said to have HbS, or sickle cell haemoglobin.
The shape and rigidity of the sickle shape cause the RBC to bunch together, blocking the
capillaries and the cells cannot carry as much oxygen. This can cause death due to lack
of oxygen to major organs.
Example
But……. the gene has survived for years. Why?
The sickle cell trait is co-dominant with the gene coding for regular haemoglobin
(meaning that it is not found more often in individuals, than regular haemoglobin). This
means that individuals will be heterozygous (meaning the person will have both types of
RBC), with more being normal. They will be mildly anemic.
A person who is homozygous (only contains the sickle cell trait), will die of anemia.
An advantage of having sickle cell anemia, is the sickle shape of the RBC cannot be
infected by the protest, Plasmodium, that causes malaria. In areas where malaria is a
problem, have HbS is an advantage. Natural selection has ensured that the sickle cell trait
is more common among people living in malaria infected areas, such as West Africa.