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Semester 3 > PP10 > Flashcards

PP10 Flashcards

1
Q

Meiosis produces cell that have:

A
  1. have half the number of chromosomes found in the original parental cell.
  2. are genetically different.
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2
Q

Why does meiosis produce cells that have half the number of chromosomes?

A

If human gametes contained 46 chromosomes like somatic cells then… When a sperm (46) fertilizes an egg (46) the offspring produced would have 92 chromosomes / cell!!! The number of chromosomes / cell would double each generation

Meiosis is the reason that our somatic cells have 2 sets of homologous chromosomes.

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3
Q

What are the steps of meiosis in order?

A

Preceded by Interphase (same as in Mitosis):
G1
S
G2

Involves 2 divisions so it takes longer than Mitosis

Meiosis I:
Prophase I
Metaphase I
Anaphase I
Telophase I
Cytokinesis =
2 daughter cells
Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II
Cytokinesis

4 daughter cells

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4
Q

Explain interphase.

A
  • Chromosomes are full chromosomes
  • The 2 sister identical chromatids are held together by centromeres.
  • Cell has 2 centrosomes with two centrioles each.
  • Cell has two sets of chromosomes, so is in the diploid condition
  • Recall: homologous pair is one chromosome from the mother and the corresponding chromosome from the father.
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5
Q

When does crossing over occur?

A

Crossing over occurs only in meiosis (prophase I)

  • Through the process of crossing over, meiosis allows homologous chromosomes of maternal origin and paternal origin to undergo an exchange of DNA segments.
  • This process is random, resulting in completely unique chromosomes when meiosis is complete. No nucleotides are gained or lost in this process.
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6
Q

Explain prophase 1

A

Nuclear membrane breaks in pieces and disintegrates.
Euchromatin and heterochromatin condense into chromosomes.
Centrosomes start separating and move to opposite poles.
Homologous pairs group and form tetrads

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7
Q

Crossing over leads to the formation of what?

A

Crossing over leads to the formation of recombinant chromatids!

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8
Q

What does crossing over mean?

A

Crossing over: nonsister chromatids exchange genetic material with equivalent part of a chromatid from the other homologous chromosome.

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9
Q

What is the synapsis?

A

Synapsis: homologous chromosomes held together by proteins (allows crossing over to occur.)

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10
Q

What is chiasma?

A

Chiasma (chiasmata, pl): physical manifestation of crossing over: appears as a cross between nonsister chromatids.

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11
Q

Explain metaphase 1

A
  • Spindle fibers move tetrads to cell midline or equator
  • Tetrads line up at the metaphase plate (equator).
  • Tetrads (homologous pairs) orient randomly/independently at the midline. This randomness happens every time the tetrads line up. How one tetrad lines up has nothing to do with how the other lines up.
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12
Q

Explain anaphase 1

A
  • Homologous pairs separate but sister chromatids remain attached at this stage. They separate in anaphase II!
  • Because the homologous pairs separate, each side of the cell only contains one set of chromosomes
  • This cell is still a diploid cell, on its way to become a haploid cell.
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13
Q

Explain telaphase 1.

A
  • Microtubules attached to the kinetochores, pull the chromosomes to opposite end of the cell. Non-kinetochore microtubules provide the tracks for organelles to move to the ends of the cell. Chromatids are still attached by centromeres
    Nuclear membrane surrounds chromosomes and nucleoli reassemble

Cytokinesis I: is followed by interkinesis during which DNA does not replicate

  • Each daughter cell has 23 chromosomes, a mix of parental and maternal. The sister chromatids are not identical to each other because of crossing over.
  • Reduced from 2N to N at this point (daughter cells are haploid & genetically different)
  • Meiosis I is known as the reduction division. Each cell has half the number of chromosomes and half the amount of DNA.
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14
Q

What happened during prophase 2

A
  • Nuclear membrane & nucleoli disappear
  • Spindles (microtubules) attach to kinetochore of each chromosome on both sides
  • Centrosomes separate
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15
Q

What happens during metaphase 2?

A

Chromosomes line up along metaphase plate (equator), with the sister chromatids on either side. How they position themselves is also random. This increases the variation produced by meiosis.

The sister chromatids are not identical because of crossing over

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16
Q

What happens during anaphase 2?

A
  • Centromeres separate, having been pulled away from each other by the microtubule spindles
  • Chromosomes are drawn to opposite ends of the cell
17
Q

explain Telophase II and Cytokinesis.

A

Chromosomes drawn to opposite ends of the cell
Nuclear membrane and nucleoli reform
Through meiosis, each progenitor cell gave rise to 4 daughter cells
Each daughter cell has one chromosome (you could say it’s a little like a daughter chromosome, except it won’t replicate now).

18
Q

Why are we all so different?

A
  • Mutations in DNA are the original source of genetic diversity
  • Mutations can create different versions of genes called alleles
  • Meiosis results in new combinations of the alleles of genes!
  • Reshuffling of alleles during sexual reproduction produces new combinations of genes and increases genetic variation.
19
Q

How is the behaviour of chromosomes during meiosis and fertilization is responsible for most of the genetic variation that arises in each generation. This variation is created by:

A

Crossing over and exchange of parts of chromatids at prophase I
Independent assortment of chromosomes at metaphase I
Independent placement of sister chromatids at metaphase II
Random fertilization (any sperm can fertilize any egg)

20
Q

Explain crossing over.

A
  • Produces recombinant chromosomes that combine genes inherited from each parent.
  • The end result is that chromosomes will have the same number of genes but the sister chromatids are no linger identical!
21
Q

explain Independent Assortment.

A

Homologous pairs of chromosomes orient randomly at metaphase I of meiosis. Each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs. Possible number of combinations is 2n, where n is the ploidy number.

For humans (n=23), there are more than 8.4 million (223) possible combinations of chromosomes, but there’s more!

You have to consider crossing over, which generates more combinations, then there’s arrangement of chromatids at metaphase II, so, the number of possible combinations of alleles in one sex cell is more than infinity!

22
Q

explain random fertilization.

A
  • Contributes to genetic diversity because any sperm can fuse with any ovum
  • The fusion of two gametes produces a zygote with any of about 70 trillion diploid combinations!!!!!!!!!!!!
23
Q 
explain the evolutionary significance of 
mutations
sexual reproduction
natural selection
populations
A

Mutations -> original source of different alleles
Sexual reproduction -> alleles mixed and matched during meiosis

Different combinations of alleles (sex) may work better than others in certain environments

Natural selection results in the accumulation of genetic variations favored by the environment

Populations evolve through differential reproductive success of its variant members.

Semester 3 (9 decks)

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