lecture 14 - meiosis

Question 1

What is the process in which single called organisms asexually reproduce?

Answer 1

Binary fission

Question 2

How many chromosomes does a diploid cell have?

Answer 2

46 single chromosomes in 23 homologous pairs

Question 3

How many different chromosomes do haploid cells have?

Answer 3

23 unique chromosomes. (though may have sister chromatid replicas in certain circumstances)

Question 4

What is the overall product of meiosis?

Answer 4

4 haploid cells (gametes) each with n chromsomes (23).

Question 5

Why do gametes have only 23 chromsomes?

Answer 5

So that the zygote, which is formed from two gametes, has 46 chromosomes, with one chromosome from each gamete in each of the 23 homologous pairs.

Question 6

What are the two parts of meiosis?

Answer 6

Meiosis I, Meiosis II

Question 7

What are the overall phases of both Meiosis I and Meiosis II?

Answer 7

Prophase, metaphase, anaphase, telophase (and cytokinesis)

Question 8

What occurs in the interphase so that Meiosis can begin?

Answer 8

Chromosomes duplicate forming sister chromatids for all 46 chromosomes.

Question 9

What is the overall goal of Meiosis I?

Answer 9

To form 2 haploid cells from 1 parent cell, by separating homologous chromosomes.

Question 10

What occurs at prophase I in meiosis?

Answer 10

Nuclear envelope disintegrates. Homologous chromosomes align with each other (they synapse) and align at corresponding positions along their length, joining at chiasmata. Crossing over occurs and the spindle microtubules begin to form.

Question 11

What is the purpose and process of crossing over in prophase I of meiosis I?

Answer 11

DNA is broken at the same spot on each homologous and equivalent sections are exchanged and reconnect in a cross-cross pattern. Produces homologues that are different to their sister chromatid

Question 12

What occurs at metaphase I of meiosis I?

Answer 12

Paired homologous chromosomes move to the metaphase plate and line up. Their orientation is random (i.e. either maternal or paternal sister chromatid pairs/homologues may be on either side of the line).

Question 13

How does the orientation of homologous chromosomes at the metaphase plate in metaphase I create variation?

Answer 13

Each sister chromatids of the homologous pair is oriented randomly, on either side of the metaphase plate, so that when the cell divides a random combination of paternal and maternal chromosomes end up on each side

Question 14

What occurs at anaphase I of meiosis I?

Answer 14

Kinetochore microtubules shorten, pulling the homologous chromosomes apart

Question 15

What are the products of telophase I/cytokinesis in meiosis I?

Answer 15

Haploid cells with 23 duplicated chromosomes (sister chromatids) are formed, containing half of the parent cell’s genetic information (with alterations due to crossing over)

Question 16

What is the overall process occurring in meiosis II?

Answer 16

The two haploid daughter cells formed in Meiosis II each split to form 2 haploid daughter cells (gametes) with 23 unpaired chromosomes.

Question 17

What occurs in prophase II of meiosis II?

Answer 17

Chromosomes condense and the nuclear envelope breaks down, centrosomes move apart and the spindle forms

Question 18

What occurs in metaphase II of meiosis II?

Answer 18

Kinetochore microtubules attach to the kinetochore of the the sister chromatids from opposite spindle poles, causing them to line up at the metaphase plate.

Question 19

What occurs in anaphase II of meiosis II?

Answer 19

Kinetochore microtubules shorten, pulling the two chromatids of each pair of sister chromatids apart to opposite ends of the cell.

Question 20

What occurs in telophase II & cytokinesis of Meiosis II?

Answer 20

Nuclear membranes form around each set of 23 chromosomes and they decondense, cytokinesis splits the cell into two new cells at the cleavage furrrow, forming two haploid cells

Question 21

What are the 2 sources of variation produced by meiosis?

Answer 21

Crossing over and random orientation of homologue pairs in Metaphase I. (independent assortment)

Question 22

Why does the random orientation of homologue pairs in Metaphase I lead to variation?

Answer 22

The daughter haploids will get a mixture of chromosomes from either the paternal or maternal chromosome from each pair. Each carries different genetic information, so the different combinations of chromosomes leads to variation in the different haploids.

Question 23

How can the number of possible homologue combinations in the 1st daughter cells of meiosis be calculated?

Answer 23

no. of possibilities = 2^n

Question 24

How many different possible homologue combinations are possible in humans during the first phase of meiosis?

Answer 24

2^23 = approx. 8.3 million

Question 25

How does lining up of the chromosomes differ between mitosis and meiosis I?

Answer 25

Mitosis = homologues align independent to their pair. Meiosis = homologues align together and ‘synapse’

Question 26

How does the position of the chromosomes along the metaphase plate differ in mitosis and meiosis I?

Answer 26

Mitosis = Kinetochore microtubules position the centromeres of sister chromatids along the plate Meiosis = Pair of homologous chromosomes line up a at chiasmata

Question 27

What is the overall ‘equation’ for chromosome number in mitosis in terms of n?

Answer 27

2n -> 2n

Question 28

What is the overall ‘equation’ for chromosome number in meiosis in terms of n?

Answer 28

2n -> n

Question 29

Which phenomenon, which leads to genetic variation between gametes, occurs at Metaphase 1 / Anaphase 1?

Answer 29

Segregation