Meiosis

Question 1

How are genetically different daughter cells produced

Answer 1

Through independent segregation of homologous chromosomes

Question 2

Describe meiosis

Answer 2

• two nuclear divisions

Question 3

Describe the genetic variation caused by meiosis

Answer 3

Every new cell produced in likely to be different from the parent cell (unlike mitosis, barring mutations in interphase)

Question 4

What is independent segregation

Answer 4

• orde of chromosomes in metaphase 1 in random, therefore the combination of maternal and paternal chromosomes which each daughter cell receives is random
• can happen because chromatids are unidentical; can attach either way to the centromere
• doubles genetic variation- hence why siblings never have identical DNA (except monozygotic twins)

Question 5

Describe crossing over

Answer 5

• occurs in first nuclear division, second nuclear division does not involve DNA replication- results in haploid daughter cells
• chromatids twist around each other in the homologous pair
• portions of chromatids break off
• can rejoin the chromatid is its homologous partner
• normally, equivalent portions of homologous chromosomes are exchanged- if not, a chromatids would have 2 of the same gene and absence of one, which could mean that a protein cannot be produced

Question 6

What is recombination?

Answer 6

• when chromatids join

Question 7

How do you calculate number of possible gametes in independent segregation?

Answer 7

2 to the power of n where n= haploid number

Question 8

Why does DNA coil and condense

Answer 8

• less risk of breakage and damage (more stable)
• less likely to tangle
• easier to separate

Question 9

Define chromosome

Answer 9

Linear structures made of DNA and protein containing specific genes

Question 10

Define chromatid

Answer 10

One of the two strands of a chromosome (post synthesis) that are joined together by a single centromere

Question 11

Describe homologous chromosomes

Answer 11

• chromosomes which consist of the same genes at the same loci
• same genes, different alleles

Question 12

Describe a bivalent

Answer 12

• pair of homologous chromosomes, coiled around each other and joined at multiple points
• makes sure homologous parts are next to each other at the equator- increases genetic variation

Question 13

Describe chiasmata

Answer 13

• point where where non-sister chromatids of homologous chromosomes are joined and become a single unit
• makes sure 1 chromosome from each homologous pair ends up in each daughter cell so that when you fuse gametes, you get the homologous chromosomes back and there is a complete genome

Question 14

What is an aneuploidy?

Answer 14

• an incorrect number of chromosomes
• monoploidy
• triploid/trisme -> causes Down’s syndrome
• other ones would cause the foetus to die very early
• caused by microtubules not separately the homologous chromosomes or chromatids correctly, meaning gametes have extra/lack of chromosomes

Question 15

Describe prophase 1

Answer 15

• chromosomes coil and condense
• nucleolus and envelope breaks down
• bivalents form

Question 16

Metaphase 1

Answer 16

• homologous pairs line up at the equator

* independent segregation

Question 17

Anaphase 1

Answer 17

Homologous pairs separate; drawn to opposite poles of the cell

Question 18

Prophase 2

Answer 18

Only happens sometimes

Question 19

Describe gamete meiosis

Answer 19

• undergo M1 during fœtal development
• freezes at M2 during puberty, only continues after fertilisation
• sperm cells are constant after puberty

Question 20

Describe metaphase 2

Answer 20

• chromosomes line up at the equator

* independent segregation of chromatids

Question 21

Describe anaphase 2

Answer 21

Chromatids separate; drawn to opposite poles of cell