Cellular Replication and Variation: Unit 3, Topic 1

Question 1

What is meiosis?

Answer 1

• A form of eukaryotic cell division that is involved in sexual reproduction
• nuclear division results in the formation of four daughter cell, each containing half the number of chromosomes of the original nucleus.

Question 2

What happens in Meiosis I?

Answer 2

1. Prophase I (before):
   - chromosomes condense and thicken
   - line up with homologous pairs (sane size, gene, etc.)
   - crossing over occurs (chromosomes transfer genetic information between each other in the homologous pair).
   - creates recombinant chromosomes
2. Metaphase I (middle of cell):
   - Chromosomes are going to be in pairs in the middle of the cell.
   - NOT SINGLE FILE LIKE
3. Anaphase I (away):
   - chromosomes pairs are pulled away from each other by spindle fibers
4. Telophase I (two):
   - two newly formed nuclei
   - 2 new cells

After meiosis I:
- there are 23 chromosomes in each cell
- with 46 chromatids

Question 3

What happens in Meiosis II

Answer 3

Prophase II:
- cells condense (preparing for division)
- spindle fibers form again (used to pull apart the chromosomes)
- DOES NOT INCLUDE CROSSING OVER

Metaphase II:
- Chromosomes are lined in the middle (single file line, no pairs)

Anaphase II:
- Chromatids are pulled away from each other by the spindle fibers

Telophase II:
- nuclei begin to reform
- the two cells divide into 4 daughter cells.
- cytokinesis splits the cytoplasm

After Meiosis II
- 23 chromosomes
- 23 Chromatids

All four cells will be different from each other due to variety

Question 4

What is the role of Homologous chromosomes?

Answer 4

• pass on genetic material
• allow for recombination
• allow for random segregation of genetic material from the parents into new cells.

Gene code for proteins -> determines characteristics/traits Eukaryotic cells have 2 copies of each gene.

Each copy residing on one of a pair of homologous chromosomes (copies are not identical - difference in DNA sequences between copies create alleles)

Question 5

What is crossing over & recombination?

Answer 5

• In prophase I, homologous pairs coil around each other (bivalent)
• non-sister chromatids attach at points called chiasmata (where the exchange of genetic material aka crossing over occurs)
• scrambles maternal and paternal genes and rearranges the combinations of alleles available on each homologous chromosome.
• recombinant genes produce new combinations of genes = increasing genetic diversity in offspring.

Question 6

What is spermatogenesis?

Answer 6

• begins at puberty, occurs in the testes
• Germ cells divide by mitosis, forming diploid (2 or more) spermatogonia.
• Spermatogonia divide many times and become primary spermatocytes.
• primary spermatocytes undergo meiosis I and produce 2 haploid secondary spermatocytes.
• secondary spermatocytes undergo meiosis II = forming 4 spermatids
• 4 spermatids undergo differentiation and mature into a sperm cell.

PRODUCES HAPLOID GAMETES

Question 7

What is oogenesis?

Answer 7

• process begins during embryonic development, with interphase (while still in the mother’s womb)
• occurs in the ovaries, and stops until the individual hits puberty
• Begins again once individual hits puberty and gets their period.
• Every menstrual cycle, chromosomes undergo meiosis I

1. The process begins during embryonic development, with interphase (while still in the mother’s womb)
   - Germ cells divide by mitosis, forming oogonia -> oogonia grow to form primary oocytes (diploid cells as well)
2. primary oocytes start to divide by meiosis I and stop at Prophase I (which starts again during puberty).
3. Primary oocyte completes meiosis I forming a haploid secondary oocyte and a polar body. unequal division of cytoplasm (half of the chromosomes go to one cell, the other half to another, large is usually the secondary ovum, and smaller is the polar body)
4. The secondary oocyte grows in the ovary until it reaches maturation (begins meiosis II but stops at Metaphase II, until fertilisation occurs.
5. The secondary ovum breaks loose and is carried to the fallopian tubes.
6. once in the fallopian tubes, the secondary egg is now suitable for fertilisation by the male sperm cells.

(if fertilisation does not occur, the secondary ovum is expelled during menstruation)

Produces HAPLOID gametes.

Question 8

What is independent assortment?

Answer 8

• occurs during meiosis anaphase I
• homologous pairs are divided in half to form haploid cells
• this separation/assortment is random
• ensures the haploid cell contains a mixture of mother and father genes.
  increases the number of different combinations carried by the gametes.

Question 9

What is random fertilisation?

Answer 9

• increase possible combinations of alleles inherited by offspring
• any egg (contain more than 8 million possible chromosome combinations), can be fertilised by any sperm (containing more than 8 million possible chromosomes).
• the random fertilisation (independent assortment) shuffle existing alleles into different combinations in each individual
• greatly increasing the potential for variation in the genotypes of the offspring.