Chromosomes and DNA packaging

Question 1

Why do we need DNA packaging?

Answer 1

The human genome is 2.2 metres (2,200,000 micrometer). The diameter of a nucleus is 11 micrometers. This creates a size issue as we need to fit such a large DNA molecule inside a nucleus of such a size. Therefore, the packaging and access to genetic info must be regulated.

Question 2

What is the solution to the size issue?

Answer 2

Proteins called Histones (via DNA packaging)

Question 3

what are histones?

Answer 3

Proteins that DNA wrap around.

Question 4

What are the 4 main core histones?

Answer 4

H2A, H2B, H3, H4

H1 = linker

Question 5

why do basic amino acids need to assist in these histone proteins?

Answer 5

Due to the amino acid charge - it is acidic so we need basic amino acids to bind together (25% of amino acids in these proteins are basic - arginine and lysine)

Question 6

what do histones form?

Answer 6

octamers

Question 7

what is the solenoid?

Answer 7

Packing of DNA as a 30nm finer of chromatin and results in the helical winding of nucleosome strands.

Question 8

what are the levels of DNA compaction?

Answer 8

1. Naked DNA - sequence of base pairs - double helix, 2 chromatin strands - 2nm
2. Nucleosomes - octamers of histones, linker DNA = 10nm fiber
3. Solenoid - 30nm fiber - several nucleosomes
4. Chromatin fiber - super helically arranged solenoids (300nm)
5. Chromosome - viewed during metaphase in mitosis

Question 9

What is euchromatin?

Answer 9

form of chromatin that is uncoiled and active (high transcriptional activity loosely packed)

Question 10

What is heterochromatin?

Answer 10

form of chromatin that is condensed and inactive (low transcriptional activity densely packed)

Question 11

what are the 2 types of DNA modification (chromatin modification)?

Answer 11

Acetylation and methylation

Question 12

What is acetylation?

Answer 12

Histones in euchromatin acetylated. Keeps structure open and prevents further condensation. (doesn’t occur in heterochromatin)

Question 13

What is methylation?

Answer 13

Opposite to acetylation during condensing DNA. Histones in euchromatin are not methylated. Histones in heterochromatin have high methylation (compact)

Question 14

what is a chromosome?

Answer 14

single long strand of DNA containing a linear array of genes.

Question 15

What are karyotypes?

Answer 15

The arrangement of a full chromosome set

Question 16

what is the method to Karyotype?

Answer 16

1. add tissue sample and add a chemical that stimulates mitosis (culture in growth medium)
2. incubate for 2-3 days
3. Add chemical to stop mitosis in metaphase.
4. Transfer cells to tube and centrifuge to concentrate in layers.
5. transfer cells to new tube containing a fixative
6. Put cells onto a microscope and add stain to enhance the chromosomes
7. identify and photograph
8. Cut out pictures and arrange into a karyotype

Question 17

What are the methods to generate karyotypes?

Answer 17

1. G-banding / giemsa stain

2. FISH - fluorescent in situ hybridisation

Question 18

What is G - banding?

Answer 18

• heterochromatic regions e.g adrenine/thymine are gene poor and stain darker.
• Each chromosome has a specific banding pattern as we have different sets of genetic information on each chromosome. Thus, specific banding patterns can be seen (only occur on 2 sister chromatids)

Question 19

What is FISH?

Answer 19

• Can visualise where a gene/DNA sequence is within a genome.
• uses specific probes for defined DNA sequences. Then hybridise the defined DNA sequences - hybridisation probe is a short fragment of DNA that has a fluorescent dye attached.
• Can then arrange chromosomes based on size and specific fluorescence

Question 20

What is a centromere?

Answer 20

Each pair of chromatids is attached at a centromere - DNA hidden beneath kinetochore proteins. They divide chromosomes into 2 arms - the short arm (p arm) and long arm (q arm).

Question 21

What is another name for centromeres?

Answer 21

Primary constrictions (based on microscopic appearance)

Question 22

What are telomeres?

Answer 22

Caps at the end of chromosomes. They contain unique repetitive sequences called telomere tandem repeats.

Question 23

Why do telomeres shorten with each cell division?

Answer 23

Because DNA polymerases only work in 1 direction. We have to deal with discontinuous replication at the lagging strand of the DNA. This leaves an unprotected end of DNA which is prone to being digested by enzymes.

Question 24

What types of cell avoid cellular ageing?

Answer 24

Stem cells - regenerate cells & tissues

Tumour cells - generate new telomeric sequences - immortal

Question 25

What enzyme do the cells use to avoid cellular ageing? and what is this?

Answer 25

Telomerase - Has DNA polymerase activity and contains RNA sequence that provides a template for synthesis of telomeric repeat DNA

Question 26

If chromosomes fail to divide properly - what does this lead to?

Answer 26

1. Numerical aberrations - caused by the failure of chromosome division - cells with extra chromosome or a deficiency.
2. Structural aberrations - due to a loss of genetic material, or rearrangement in the location of genetic material. Includes- duplications, deletions, inversions, translocations & ring formations (5)

Question 27

what is trisomy 21?

Answer 27

Down syndrome. Have reduced muscle tone (hypotonia), slanted eyes upwards and outwards. Flat back of head. Small mouth with protruding tongue. Have some degree of learning disability

Question 28

What is XXY trisomy?

Answer 28

Klinefelter syndrome. Sex chromosome abnormality - occurs in males with 2 or more X chromosomes instead of Male norm of XY. The more X chromosomes = the more severe version of the syndrome.
Symptoms include: Taller, less muscular, larger breasts, weaker bonds, delay in puberty and less hair.

Question 29

What is Turner syndrome?

Answer 29

One normal X instead of the female norm of XX.

Smaller than average, underdeveloped ovaries, lack of monthly periods and infertility.