Chromosomes, Meiosis, Cell division and cellular abnormalities

Question 1

Describe the basic structure of chromosomes

Answer 1

Chromosomes are linear (in eukaryotes) forms of DNA. They consist of 2 sister chromatids that are joined by a Centromere, this is also the site of the kinetochore of the chromatids. Kinetochores are where the spindle fibres attach.
At the ends of the chromosomes there are telomeres to protect them and prevent fraying or tangling.
The extended form of chromatin is euchromatin, and the condensed form is Heterochromatin - both are present in chromosomes.

Question 2

Describe genetic disease in terms of abnormalities in chromosome number.

Answer 2

Abnormalities in chromosome numbers are called e.g. monosomy (only 1 chromosome), trisomy (3 chromosomes).
It has been understood that chromosomal abnormalities in number have to do with ‘non-disjunction’, this occurs within meiosis.

Question 3

Describe the types of structural chromosomal abnormalities

Answer 3

Balanced or unbalanced rearrangements
Translocations
Deletions
Insertions
Inversions

Question 4

Describe the basic packing of chromosomes.

Answer 4

1) Nucleosomes - These are DNA packaged with histone proteins to form chromatin. The histones have a positive charge - 146 base pairs wrapped around core of 8 histone proteins.
2) Solenoid - This is DNA compacted by a factor of 40 - occurs by the further wrapping of nucleosomes into a 30nm-wide fibre (approx. 6 nucleosomes per turn)
3) Fibre-scaffold complex - These are sections of chromosomes in an extended form.
4) Chromosome - Entire visible metaphase chromosome = very very condensed.

Question 5

Name the two types of translocations that lead to structural abnormalities.

Answer 5

Reciprocal.
Robertsonian

Question 6

Describe the effects of a balanced rearrangement carrier.

Answer 6

If a balanced rearrangement occurs, there is no immediate problem for that cell - this only poses a problem to the cells produced from this specific cell.
- chromosomes with balanced translocation + normal chromosomes = unbalanced translocation.

Question 7

Describe Robertsonian translocation.

Answer 7

This is the fusion of two acrocentric chromosomes - this results in the loss of p (short) arms, so two q arms.
With a Robertsonian Translocation carrier involved in reproduction, it could ultimately lead to Down syndrome.

Question 8

Describe deletions in chromosomes.

Answer 8

The deletion of genetic material is ultimately a normal part of mutations, however it could potentially cause a nonsense codon (for example) and lead to cell death.

Question 9

Describe inversions in chromosomes.

Answer 9

Paracentric inversion, happens on one arm - no contact with centromere.
Pericentric inversion - happens with the centromere.

Question 10

Name some genetic mutations.

Answer 10

Germline or somatic
Gene disruption/ disease-associated
Polymorphism ( SNP- single nucleotide polymorphism) - no phenotypic effect.

Question 11

Describe the two types of mutations.

Answer 11

Non-coding
Coding; silent; missense; nonsense; frameshift

Question 12

Describe how mutations are detected in the lab.

Answer 12

PCR (polymerase chain reaction)
Gel electrophoresis (following PCR to view results)
Restriction fragment length polymorphism (RFLP) analysis
Amplification refractory mutation system (ARMS)
DNA sequencing.

Question 13

Describe the requirements for PCR.

Answer 13

For PCR:
- Sequence information
- Oligonucleotide primers (5’ to 3’ direction) , these primers tell polymerase enzyme where to bind.
- DNA (template)
- Nucleotides labelled with fluorescence
- DNA polymerase

Question 14

Describe the function of restriction endonucleases.

Answer 14

They identify the location of the mutation.
= enzymes from bacterial cells
They are a protective mechanism
They degrade the DNA of invading viruses
They recognise specific DNA sequences
Always cut DNA at the same time