JM Lecture 40: Mitosis and Meiosis

Question 1

List the human somatic cell cycle in order including their DNA content.

Answer 1

G1: 2N
S (DNA synthesis): between 2N and 4N
G2: 4N
M (Mitosis/cell division): 4N

Question 2

What are the steps of mitosis?

Answer 2

G2 (one cell)
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
G1 (two cells)

Question 3

Describe what happens in prophase.

Answer 3

centrosomes (microtubule organizing centers) move to the poles
nucelar membrane breaks down and chromosomes condense (each pair of sister chromatids are held together by the centromere)

Question 4

Describe what happens in metaphase.

Answer 4

alignment of chromosomes in the center of the mitotic spindle
stage at which karyotyping can be done

Question 5

Describe what happens in anaphase.

Answer 5

sister chromatids separate and move to opposite poles

cell elongates

Question 6

Describe what happens in telophase.

Answer 6

nuclear membrane re-forms
chromosomes decondense
spindle disappears

Question 7

Describe what happens in cytokinesis.

Answer 7

separation of daughter cells driven by actin filaments

Question 8

Describe the general division of DNA content in germ cells.

Answer 8

germ cells have 1N DNA content (haploid)
must get there from somatic cells which are 2N
done by Meiosis I (separation of homologous chromosomes) and Meiosis II (separation of sister chromatids)

Question 9

What happens in prometaphase?

Answer 9

spindle fibers form

Question 10

What are the types of spindle fibers?

Answer 10

astral microtubules
kinetochore
microtubules
polar microtubules

Question 11

Describe the events of meiosis I.

Answer 11

pairing and segregation of homologues –> daughter cells containing one maternal copy or one paternal copy (but not both)

Question 12

Describe the events of meiosis II.

Answer 12

pairing and segregation of sister chromatids (as in mitosis)

Question 13

Why does meiosis involve DNA synthesis followed by two distinct divisions?

Answer 13

to allow for crossing-over of chromosomes at areas of homology to exchange genetic material (recombination)
allows for limitless genetic diversity

Question 14

What is the synaptonemal complex?

Answer 14

highly ordered interlocking of two homologous chromosomes that facilitates recombination between paternal and maternal homologues
consists of 2 lateral elements and a central element
found during Meiosis I

Question 15

What are chiasma?

Answer 15

sites of crossing-over that persist into metaphase of Meiosis I
remnants of the synaptonemal complex that hold homologues together until segragation occurs (like centromeres do for sister chromatids)

Question 16

Describe the pairing of X and Y chromosomes during Meiosis I.

Answer 16

small regions of homology are found between X and Y chromosomes allowing for pairing until segregation is ready to occur but the genes that could exchange are non-coding

Question 17

What is non-dysjunction?

Answer 17

when homologues fail to separate in Meiosis I resulting in germ cells with an abnormal number of chromosomes (which further result in abnormal embryos, most of which die)
can also happen in Meiosis II (but less likely because sister chromatid separation is much more tightly controlled)