10.1 Meiosis

Question 1

What is interphase?

Answer 1

Interphase is an active period that precedes meiosis and involves key events needed to prepare the cell for successful division

Question 2

When is DNA replicated during interphase?

Answer 2

DNA is replicated during the S phase of interphase, resulting in chromosomes that contain two identical DNA strands

Question 3

What are the two identical DNA strands held together by and what are they called?

Answer 3

These genetically identical strands are called sister chromatids and are held together by a central region called the centromere

Question 4

When do chromatids separate?

Answer 4

These chromatids separate during meiosis II, becoming independent chromosomes each made of a single DNA strand

Question 5

What would happen if DNA replication did not occur prior to meiosis?

Answer 5

If DNA replication did not occur prior to meiosis there would be no need for a 2nd meiotic division (meiosis I = diploid → haploid)

Question 6

What does the fact that DNA replication does occur before meiosis suggest?

Answer 6

The fact that DNA replication does occur suggests that meiosis evolved from mitosis (where initial DNA replication is necessary)

Question 7

What is an advantage of duplication of chromatids?

Answer 7

One benefit of the duplication of chromatids is that it increases the potential for genetic recombination to occur (more variation)

Question 8

What does meiosis consist of?

Answer 8

Meiosis consists of two divisions, both of which follow the same stages as mitosis (prophase, metaphase, anaphase, telophase)

Question 9

What is meiosis preceded by?

Answer 9

Meiosis is preceded by interphase, in which DNA is replicated to produce chromosomes consisting of two sister chromatids

Question 10

What may occur between the 2 meiosis stages?

Answer 10

A second growth phase called interkinesis may occur between meiosis I and II, however no DNA replication occurs in this stage

Question 11

What is the first meiotic division termed as?

Answer 11

The first meiotic division is a reduction division (diploid → haploid) in which homologous chromosomes are separated

Question 12

What occurs in P-I (prophase one of meiosis)?

Answer 12

P-I: Chromosomes condense, nuclear membrane dissolves, homologous chromosomes form bivalents, crossing over occurs

Question 13

What occurs in M-I?

Answer 13

M-I: Spindle fibres from opposing centrosomes connect to bivalents (at centromeres) and align them along the middle of the cell

Question 14

What occurs in A-I?

Answer 14

A-I: Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell

Question 15

What occurs in T-I?

Answer 15

T-I: Chromosomes decondense, nuclear membrane may reform, cell divides (cytokinesis) to form two haploid daughter cells

Question 16

What is the purpose of the second meiotic division?

Answer 16

The second division separates sister chromatids (these chromatids may not be identical due to crossing over in prophase I)

Question 17

What occurs in P-II?

Answer 17

P-II: Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles (perpendicular to before)

Question 18

What occurs in M-II?

Answer 18

M-II: Spindle fibres from opposing centrosomes attach to chromosomes (at centromere) and align them along the cell equator

Question 19

What occurs in A-II?

Answer 19

A-II: Spindle fibres contract and separate the sister chromatids, chromatids (now called chromosomes) move to opposite poles

Question 20

What occurs in T-II?

Answer 20

T-II: Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis) to form four haploid daughter cells

Question 21

What is the final result of meiosis?

Answer 21

The final outcome of meiosis is the production of four haploid daughter cells

Question 22

How do the daughter cells of meiosis differ?

Answer 22

These cells may all be genetically distinct if crossing over occurs in prophase I (causes recombination of sister chromatids)

Question 23

What does independent assortment describe?

Answer 23

Independent assortment describes how pairs of alleles separate independently from one another during gamete formation

Question 24

What does the law of independent assortment state about inheritance of a gene?

Answer 24

According to independent assortment, the inheritance of one gene/trait is independent to the inheritance of any other gene/trait

Question 25

What causes independent assortment?

Answer 25

Independent assortment is due to the random orientation of pairs of homologous chromosomes in meiosis I

Question 26

What occurs in metaphase I in relation to independent assortment?

Answer 26

During metaphase I, homologous chromosomes line up at the equator as bivalents in one of two arrangements: Maternal copy left / paternal copy right OR paternal copy left / maternal copy right

Question 27

What is the orientation of each homologous pair and what is it therefore not affected by?

Answer 27

The orientation of each homologous pair is random and is not affected by the orientation of any other homologous pair

Question 28

What does it mean that independent assortment is random?

Answer 28

This means an allele on one chromosome has an equal chance of being paired with, or separated from, any allele on another chromosome (their inheritance is independent of one another)

Question 29

When will independent assortment not occur?

Answer 29

Independent assortment will not occur if two genes are located on the same chromosome (linked genes)

Question 30

What is synapsis?

Answer 30

During prophase I of meiosis, homologous chromosomes become connected in a process known as synapsis

Question 31

What are the connected homologues known as?

Answer 31

The connected homologues are known as a bivalent (bi = two chromosomes) or a tetrad (tetra = four chromatids)

Question 32

WHat are the chromosomes connected by?

Answer 32

The chromosomes are connected by a protein-RNA complex called the synaptonemal complex

Question 33

What types of chromosomes undergo synapsis?

Answer 33

While autosomes always undergo synapsis during meiosis, sex chromosomes often remain unpaired

Question 34

What may non-sister chromatids do while in synapsis?

Answer 34

While in synapsis, non-sister chromatids may break and recombine with their homologous partner (crossing over)

Question 35

What are chiasmata?

Answer 35

These non-sister chromatids remain physically connected at these points of exchange – regions called chiasmata

Question 36

What is the role of chiasmata?

Answer 36

Chiasmata (singular = chiasma) hold the homologous chromosomes together as a bivalent until anaphase I

Question 37

What can chiasmata formation lead to?

Answer 37

Chiasmata formation between non-sister chromatids can result in the exchange of alleles

Question 38

What do chiasmata look like?

Answer 38

Chiasmata are X-shaped points of attachment between two non-sister chromatids of a homologous pair

Question 39

What do chiasmata form as a result of?

Answer 39

Chiasmata form as a result of crossing over and hence non-sister chromatids should show an exchange of genetic material

Question 40

When is exchange of genetic material most easily shown?

Answer 40

The exchange of genetic material is most easily shown if homologous chromosomes are differentially colour-coded

Question 41

What does the formation of chiasmata allow for in terms of genetic exchange?

Answer 41

When chiasmata form between bivalents in prophase I, DNA can be exchanged between non-sister homologous chromatids

Question 42

What is the exchange of genetic material called?

Answer 42

This exchange of genetic material is called crossing over and produces new allele combinations on the chromosomes

Question 43

What are recombinant chromosomes?

Answer 43

These chromosomes that consist of genetic material from both homologues are called recombinant chromosomes

Question 44

What does crossing over result in?

Answer 44

Crossing over results in new combinations of alleles in haploid cells and thus increases the genetic diversity of potential offspring