Chromosomes and Cell Division

Question 1

What is the structure of chromosomes?

Answer 1

Telomeres at the end of chromosomes
Centromeres
HETEROCHROMATIN (Found at Telomere, Centromere. More condensed structure, silenced genes)
Euchromatin (open structure, open genes)

Question 2

What are telomeres?

Answer 2

• Protective structure found at the end of chromosomes.
• 5’ TTAGGG-3’ repeats
• Hundreds of copies at the ends of the chromosomes
• End-replication problem
• Telomerase, which can add nucleotides at the end of chromosomes.

Question 3

What are centromeres?

Answer 3

• Heterochromatin
• Hold sister chromatids together
• Contains repetitive DNA sequences- satellite DNA.
• Site of kinetochore (protein complex which microtubules attach to)
• Required for chromosome separation during cell division.

Question 4

What is the function of chromatin?

Answer 4

To compress the DNA into a compact unit that will be less voluminous and can fit within the nucleus.
Chromatin consists of complexes of small proteins known as histones and DNA. Histones help to organize DNA into structures called nucleosomes by providing a base on which the DNA can be wrapped around. A nucleosome consists of a DNA sequence of about 150 base pairs that is wrapped around a set of eight histones called an octamer.
The nucleosome is further folded to produce a chromatin fibre.

Question 5

What is the purpose of packaging DNA?

Answer 5

• Negative charged DNA are neutralised by positively charged histone proteins.
• DNA takes up less space.
• Inactive DNA can be folded into inaccessible locations until required.

Question 6

What occurs during the cell cycle?

Answer 6

G1- Gap 1, cell grows
S phase- DNA replication
G2- Gap 2, cell prepares to divide
M- Mitosis and cytokinesis. The cell divides its copied DNA and cytoplasm to make two new daughter cells.

Question 7

What occur during interphase of the cell cycle?

Answer 7

During interphase (G1, S, G2), genes are transcribed and DNA replication occurs in preparation of mitosis.

Question 8

When does mitosis occur?

Answer 8

Used in growth and asexual reproduction.

Question 9

What occurs during mitosis?

Answer 9

Prophase- chromosomes condense. Nuclear membrane disappears. Spindle fibres form from the centriole.

Metaphase- Chromosomes aligned at the equator of the cell. Attached by fibre to each centriole.

Anaphase- sister chromatids (now called chromosomes) are pulled towards opposite poles.

Telophase- chromosome arrives at opposite poles and nuclear membrane surrounds each set of chromsomes.

Question 10

How are chromosomes analysed and what are they useful for?

Answer 10

FISH.

Useful for identifying unique karyotypes of individuals.

Question 11

What are types of FISH probes?

Answer 11

Unique sequence probes

Centromeric probes
- Useful for determining chromosome number

Telomeric probes
- Useful for detecting subtelomeric rearrangements

Whole chromosome probes

Question 12

Meiosis: How chromosome behaviour differs from Mitosis.

Answer 12

Cell division in germ cells.

Diploid cells (in ovaries and testes) divide to form 4 haploid cells + Used to produce gametes.
Chromosomes are passed on as re-arranged (recombined) copies – this creates genetic diversity.

Before division, there is recombination.

• Parent cell divides once and then each cell produced divides again.
• Each daughter cell is un-identical to the parent cell. This is because of DNA replication, followed by crossing over and separation of chromosomes.

Question 13

Fertilisation= what does the embryo contain and where does the mitochondria (and their DNA) come from?

Answer 13

Embryo contains an assortment of genes from each original parent- more genetic diversity.
Mitochondria (and their DNA) come only from mother via the egg- maternal inheritance.

Question 14

Describe X-inactivation in female mammals.

Answer 14

Females have 2 X chromosome. In the zygote, both X chromosome are active. To prevent a double supply of gene products, in the early embryo there is a random inactivation of one X.
Example: tortoiseshell cats. One gene coding for the ginger pigment, if that gets switched off you will get black patches. Non ginger gene switched off- you will still get patches.

Question 15

DNA in our genome.

Answer 15

Only 2% of our DNA are protein-coding.
Extragenic sequences:
-	Tandemly repeated DNA sequences
o	Satellite DNA (at centromeres and telomeres)
o	Minisatellite DNA

-	Highly repeated interspersed DNA sequences
o	SINES (short interspersed nuclear elements)
o	LINES (long interspersed nuclear elements)