Lecture 9

Question 1

Nomenclature for genes and cells in a prokaryote is simple (i.e. a+, b-, c+). In diploid eukaryotic cells in G1, there are multiple nomenclatures possible. What does / represent in these expressions for genes and cells? ;? ·?

Answer 1

/ = different homologues. Applies to diploids (i.e. one homolog is allele A, other has allele a)

; = different chromosome

· = don’t know if on same chromosome or different chromosome

Question 2

How are centromeres shown in chromosome diagrams?

Answer 2

circle

Question 3

If one 2n mendy has a/a;b/b, and the other 2n mendy has a/a;b/b, what are the respective n’s derived from the mendys? What about the diploid offspring?

Answer 3

sperm a;b
egg: a/b

offspring: a/a;b/b

Question 4

What is the purpose of alternating between haploid and diploid?

Answer 4

produce offspring with new allele combinations

Question 5

When do meiotic crossovers occur? How do they occur?

Answer 5

prophase I, but easy to observe in metaphase I

1) homologous chromosomes pair at the beginning of prophase I
2) DNA breaks at identical sites on 2 non-sister chromatids
3) DNA is repaired but the wrong ends are joined
4) anaphase I: chromatids break apart
5) 4 products of meiosis (recombinants)

Question 6

What is the major function of meiotic crossovers?

Answer 6

hold homologous chromosomes together during meiosis I

prophase I homologous chromosomes are held together with crossing over and unprotected cohesins (as in, they will be destroyed in anaphase I, allowing homologs to separate)

Question 7

Where can crossovers happen between?

Answer 7

any two non-sister chromatids