2. Organisation of Eukaryotic Genome

Question 1

Genome definition

Answer 1

The genetic material of an organism or virus; the complete complement of an organism’s genes along with non-coding nucleic acid sequences

Question 2

Gene definition

Answer 2

A section of DNA that contains a specific sequence of nucleotides to direct synthesis of a polypeptide chain or RNA. It is also unit of inheritance in a fixed position (locus) on a chromosome which specifies a particular character of an organism.

Question 3

Role of DNA condensation

Answer 3

1. To organise and pack DNA molecules into structures that will facilitate their separation into daughter cells
2. Ensure DNA molecule do not entangle and break during separation at anaphase

Question 4

What happens during 1st level of DNA condensation?

Hint: form nucleosome fibre

Answer 4

Histone proteins have high concentration of positively charged residues (lysine and arginine) which form ionic bonds with negatively charged DNA sugar phosphate backbone
DNA coils around (histone proteins which form an octamer)
form nucleosome core, beads on a string look
Many nucleosomes pack together to form 10nm Nucleosome fibre

Question 5

What happens during 2nd level of DNA condensation?

Hint: form solenoid

Answer 5

further coiled to form 30nm chromatin fibre called solenoid

Histone H1 and linker DNA involved

Question 6

What happens during 3rd level of DNA condensation?

Answer 6

Non-histone proteins, scaffold proteins condense solenoid to form looped domains
In mitotic/meiotic chromosomes, looped domains form chromosome

Question 7

4 Functions of telomere

Answer 7

1. Protective function (telomeric DNA form t-loops that protect 5’ end and 3’ single-stranded overhang of linear chromosomes from degradation by cellular exonuclease, prevent it from being recognised as damaged DNA)
2. Regulates replicative cell senescence (when telomeres are shortened to critical length then reach Hayflick limit they enter replicate cell senescence cycle, eventually die)
3. Prevent loss of genes (telomeres shorten with each cell replication cycle due to end replication problem, ensure DNA replication can occur without loss of important coding sequences)
4. Maintain stability (t-loops prevent ends of chromosomes from fusing with ends of other chromosomes)

Question 8

Replicative cell senescence definition

Answer 8

period in which cell withdraws permanently from cell cycle and stops dividing after reaching Hayflick limit (20-30 cell divisions)

Question 9

How does telomerase maintain telomere length? 4 points

Answer 9

1. RNA template binds complementarily to 3’ overhang of parental DNA strand
2. Telomerase extends the 3’ overhang in 5’-3’ direction by adding sequence repeats via complementary base pairing
3. Hence the synthesis of shorter daughter strand can be extended during the next round of DNA replication resulting in longer telomere
4. Telomerase maintains the number of repeats, delaying cell senescence and enabling cells to proliferate indefinitely

Question 10

Function of centromere

Answer 10

1. Sister chromatid adhesion (where two sister chromatids join)
2. Kinetochore formation (site of assembly of kinetochore which attaches to microtubules of spindle)
3. Proper chromosome segregation (allow for correct segregation of daughter chromosomes after DNA replication, ensure only one centromere on each chromosome. Without centromere, daughter chromosomes segregate randomly, leading to loss or duplication of chromosomes in daughter cells)

Question 11

Difference between centromere and telomere

Answer 11

Centromere is region where two sister chromatids are joined in a replicated chromosome during cell division
Telomere is at the tips of a linear eukaryotic chromosome
Centromere consist of regular satellite DNA
Telomere consist of minisatellite DNA

Question 12

Similarities

Answer 12

Both consist of tandem repetitive, non-coding satellite DNA

Both are embedded in heterochromatin