22. Cell reproduction. Haploid-Diploid cycle. Aberrations in the normal run of mitosis and meiosis.

Question 1

How do prokaryotes reproduce

Answer 1

Prokaryotes reproduce asexually through binary fission (a single cell is divided into 2 cells of the same size). Eukaryotes divide by mitosis/ meiosis

Question 2

What are the definitions of Mitosis and Meiosis?

Answer 2

Mitosis - cell division where a parent cell forms 2 identical daughter cells

Meiosis - cell division where daughter cell contains half the info from parent cell

Question 3

Give a brief explanation of meiosis

Answer 3

Meiosis

• only found in sex cells - gametes
• somatic cells CANNOT divide by meiosis
• gametes are haploid (n)
• fertilisation = when oocyte and spermatozoan fuse - becomes diploid (2n)
• mitosis of cells to make organs…
• mitosis - diploid progeny cells
• each gamete has either a maternal or paternal version of each chromosome - the choice is random for each pair
• Crossing over: happens only in meiosis for gametes, it’s the exchange of genetic material from maternal + paternal versions of each chromosome. Generates genetic diversity

Question 4

Explain Mitosis interphase

Answer 4

Mitosis:

INTERPHASE
G1:

• growth phase - cell grows
• synthesises mRNA and proteins

S:

• synthesis phase - DNA is replicated
• (DNA packaged in chromosomes are replicated)

G2:

• Rapid cell growth + protein synthesis
• cell gets ready for mitosis
• safety gap - check if DNA has been duplicated properly

Question 5

Explain the (M phase) Of mitosis:

Stages: Prophase, prometaphase, metaphase

Answer 5

M PHASE

Prophase:
- chromosomes condense

• mitotic spindle assembles

Prometaphase:

• nuclear envelope fragments
• chromosomes attach to microtubules

Metaphase:

• chromosomes at the equator (middle)
• chromatids attach to the mitotic spindle

Question 6

Explain the (M phase) Of mitosis:

Stages: Anaphase and telophase

Explain the final stage cytokinesis

Answer 6

Anaphase:

• kinetochore microtubules shorten
• chromosomes separate

Telophase:

• chromosomes at the pole
• nuclear envelope reassembles

Cytokinesis:

• cleavage
• nuclear envelope completed

Question 7

Explain Meiosis

stage: interphase

Answer 7

Meiosis:

INTERPHASE - before meiosis begins, genetic material is duplicated MEIOSIS I

Question 8

Explain Meiosis

stage: Prophase I and its substages

Answer 8

Prophase I:
- duplicated chromatin condenses

• each chromosome consists of 2, closely associated sister chromatids
• (crossing over at the end stage)

Leptotene:

• consists of the condensing of replicated chromosomes
• each sister chromatid is attached to nuclear envelope

Zygotene:

• synapsis between homologous chromosomes begins
• after homologous pairs synapse, they are called tetrads, bivalents

Pachytene:

• synapses snap, allowing crossing-over of genetic info
• 2 sister chromatids separate from each other
• homologous chromosomes are attached

Diplotene:

• 2 homologous chromosomes begin to migrate apart - repel one another
• 2 chromosomes move apart
• chiasma move to end of chromatids

Diakinesis:

• Termination of the condensing of chromosomes
• chromosomes are at there most condensed
• nucleolus disappear

Question 9

Explain Meiosis

stages: Metaphase I, Anaphase I and Telophase I

Answer 9

Metaphase I:

• homologous chromosomes align at the equatorial plate

Anaphase I:

• homologous pairs separate with sister chromatids remaining together

Telophase I:

• 2 daughter cells are formed with each daughter containing only 1 chromosome of the homologous pair

Question 10

Explain MEIOSIS II:

Stages: Prophase II, Metaphase II, Anaphase II and Telophase II.

Answer 10

MEIOSIS II:
Prophase II:
- DNA does NOT replicate

Metaphase II:

• chromosomes align the equatorial plate

Anaphase II:

• centromeres divide + sister chromatids migrate separately to each pole

Telophase II:

• cell division is complete
• 2 haploid daughter cells are obtained

Question 11

Aberrations during Meiosis:

1) What is Aneuploidy

Answer 11

Aneuploidy - addition or loss of one or two chromosomes
- Non-disjunction - chromatids don’t separate properly

• (division failure of sister chromatids)
• multipolar mitosis - 3 or more poles

Question 12

Aberrations during Meiosis:

2) What is Polyploidy

Answer 12

Polyploidy - addition of complete haploid sets of chromosomes

• endomitosis, chromosomal replication not followed by cytokinesis
• endoreduplication, cells grow + enlarge, but don’t go through process of dividing - mitosis
• C mitosis, colchicine block mitotic anaphase (colchicine blocks mitotic spindles contracting