Chromosomes and Protein Synthesis

Question 1

Homologous chromosomes

Answer 1

One paternal and one maternal chromosome (same genes in the same loci but may have different alleles)

Question 2

Sister chromatids

Answer 2

the copy of either the paternal or maternal chromosome joined at the centromere

Question 3

What are telomeres?

Answer 3

Protective caps on chromosomes which prevent them from fusing and deteriorating

Question 4

What its the role of telomerase?

Answer 4

Maintains telomere length in germ line cells (most somatic cells have decreased levels of telomerase and therefore these shorten with each cell division)

Question 5

Describe X-inactivation

Answer 5

Female mammals have 2 X chromosomes however, only one is required for normal gene expression so one is inactivated. Inactivation occurs at the blastocyst stage and is random therefore different tissues can have different X chromosomes inactivated.

Question 6

Briefly describe the cell cycle

Answer 6

G0 phase - resting phase, cell has stopped dividing
Interphase - lasts for 90% of cell cycle (consists of G1, S and G2)

G1 - cell increases its supply of proteins and organelles and grows in size
S - DNA replication, DNA. in cell effectively doubles
G2 - period of protein synthesis

Mitotic phase

Question 7

Prophase (P m a t)

Answer 7

• chromosomes condense and become visible
• centrosomes that duplicate during interphase migrate to opposite poles
• centrosomes lay out spindle fibres

Question 8

Metaphase (p M a t)

Answer 8

• nuclear membrane breaks down

- sister chromatids become attached by their centromere to the equator of the spindle fibres / microtubules

Question 9

Anaphase (p m A t)

Answer 9

• centromere of each sister chromatid splits and chromosomes move to opposite poles of the cell
• separation caused by shortening of spindle microtubules

Question 10

Telophase (p m a T)

Answer 10

• nuclear membrane re-dorms
• chromosomes de-condense into chromatin
• daughter cells return into interphase

Question 11

Meiosis - Phrophase 1 breakdown

Answer 11

• Zygotene: homologous chromosomes become closely associated along their length via synapsis forming bivalents
• Pachytene: bivalent held via synaptonemal complex, crossover occurs forming recombinant chromosomes
• Diplotene: homologous chromosomes appear to repel each other, they are held only at chiasmata

Question 12

Meiosis - Metaphase 1

Answer 12

• Bivalents attach to spindle microtubule and are lined up at the equator
• Independent assortment

Question 13

Meiosis - Anaphase 1 and Telophase 1

Answer 13

Anaphase 1:
- a pair of sister chromatids are pulled to opposite poles by spindle microtubule

Telophase 1:

• each daughter cell consists of a pair of sister chromatids (23 chromosomes)
• nuclear membrane reforms
• chromosomes de-condense and cell returns to interphase ready for the next meiotic division.

Question 14

What is Mendel’s first law?

Answer 14

• law of segregation of characteristics
• gamete only contains either maternal or paternal gene
• concept of genetic dominance

Question 15

What is Mendel’s second law?

Answer 15

• law of independent assortment: all combinations of gametes are equally probable
• exception when 2 genes are linked

Question 16

Give examples of X-linked recessive disorders

Answer 16

• Red-green colourblindness
• Heamophilia A and B
• Duchenne muscular dystrophy

Question 17

What is Hardy Weinberg’s principle?

Answer 17

• allele and genotype frequencies in a population will remain constant between generations in the absence of other evolutionary influences.
• p + q = 1 and p^2 + 2pq + q^2 = 1