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IB HL Biology > 10.1 Meiosis > Flashcards

10.1 Meiosis Flashcards

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1
Q

What does meiosis lead to?

A

An independent assortment of chromosomes and a unique composition of alleles in daughter cells.

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2
Q

What happens in prophase of mitosis?

A

Each replicated chromosome (consisting of two identical sister chromatids connected at the centromere) condenses completely and independently.

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3
Q

What happens in prophase I of meiosis?

A
  • The homologous pairs of replicated chromosomes align closely before condensation is completed.
  • This pairing of homologous chromosomes is called synapsis.
  • The resulting pairs are called tetrads (referring to the four chromatids) or bivalents (referring to the two chromosomes).
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4
Q

Diagram of tetrad formation during prophase I of meiosis

A
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5
Q

What is the difference between sister chromatids and homologous non-sister chromatids?

A
  • Sister chromatids (for example, the two red chromatids) have the same alleles, as well as the same genes, because they are identical copies produced during replication.
  • Homologous non-sister chromatids (for example, a red and a blue chromatid) have the same genes at the same loci, but may have different alleles.
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6
Q

What happens during crossing over?

A

While the bivalent is still in prophase I, exactly equivalent lengths of non-sister homologous chromatids are exchanged in a process called crossing over.

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7
Q

How does crossing over begin?

A

By breaking a covalent bond in the backbone of both strands of the DNA double helix at identical positions on the non-sister chromatids and then causing the helices to switch sides.

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8
Q

What happens in crossing over after the covalent bond is broken

A
  • The position of the chromatids are switched so that, when the backbones are healed (ligated), a chromatid is covalently bonded to the non-sister chromatid.
  • For example, the red chromatid in Figure 2 will bind to the blue non-sister chromatid.
  • This process creates new combinations of alleles by swapping all the genetic information from the point of exchange onward.
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9
Q

Diagarm of crossing over between non-sister chromatids during prophase I

A
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10
Q

How is a chiasma created?

A

The point of exchange between the two non-sister chromatids creates an X-shaped connection called a chiasma.

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11
Q

What happens to chiasmata later on in meiosis?

A
  • Chiasmata remain through metaphase I and help bind the bivalent together as it is pulled to the metaphase plate.
  • Multiple chiasmata may form along the same pair of homologous chromosomes.
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12
Q

Diagram of chiasmata formation during prophase I

A
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13
Q

What is the result of crossing over?

A
  • The exchange of alleles between non-sister chromatids.
  • Each original chromatid is broken up and the sections recombined with those of other chromatids, creating combinations of alleles that were not present in either original chromosome.
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14
Q

Where can crossing over occur and what does this lead to?

A
  • Crossing over can occur at almost any location along the tetrad (though at some locations with greater frequency), leading to a nearly limitless number of possible combinations between non-sister homologous chromatids.
  • Thus, crossing over greatly increases the genetic diversity present in the gametes at the end of meiosis.
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15
Q

Diagram of recombination of non-sister chromatids results in new combinations of alleles

A
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16
Q

Define a chiasma

A

A point of contact between non-sister homologous chromatids where crossing over and exchange of genetic material has occurred.

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17
Q

Tips for drawing crossing over events

A
  • When drawing crossing-over events, use two different colors for the maternal and paternal chromosomes if possible. Label the position of the centromere, which is usually off center, as well as the chiasma.
  • Diagrams of chiasmata should show sister chromatids still closely aligned, except at the point where crossing over occurred and a chiasma was formed.
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18
Q

What event in prophase I does the diagram represent?

A

Crossing over of non-sister chromatids of a homologous pair.

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19
Q

What does a tetrad consist of?

A

Two pairs of sister chromatids that have synapsed.

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20
Q

What is the term for the point of attachment between sister chromatids in a tetrad?

A

Centromere

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21
Q

State the number of chiasmata visible in the figure below.

A

5

Each chiasma appears like a cross in between the two pairs of chromatids. The figure below highlights one chiasma (one cross).

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22
Q

What is independent assortment?

A

Genes found on different chromosomes show independent assortment, meaning that the allele inherited for one gene does not influence which allele is inherited for a different gene.

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23
Q

What did Mendel observe and what did he find?

A
  • Mendel observed the offspring of parents heterozygous for two traits such as pea color and pea shape.
  • He found that the color of the pea, green or yellow, had no effect on whether the pea would have a round or wrinkled shape.
  • These observations led him to formulate the law of independent assortment, which is true of genes on different chromosomes.
24
Q

Why does an independent assortment of genes on the different chromosomes occur?

A

Because during metaphase I of meiosis, tetrads (pairs of homologous chromosomes) line up on the equatorial plate with random orientation.

25
Q

How many possible orientations are there for a tetrad?

A

2

26
Q

Why are there 2 possible orientations for a tetrad?

A
  • Because there are two poles the chromosomes could be pulled toward.
  • A tetrad has an equal chance of having its maternal copy or its paternal copy facing either pole.
27
Q

Diagram showing independent assortment of alleles in meiosis

A
28
Q

What is the result of independent assortment of alleles in meiosis?

A
  • As a result, for two heterozygous genes, all four possible combinations of alleles in the haploid cells are equally likely.
  • In Figure 1, the genes for pea color and pea shape produce equal proportions of gametes with the allele combinations RY, Ry, rY, and ry, as seen at the bottom of the image.
29
Q

Probability calculations

A
  • A gamete produced by a parent heterozygous for pea color (Yy) has a 50% chance of inheriting the dominant allele (Y).
  • Similarly, the gamete has a 50% chance of inheriting the dominant allele for pea shape (R) if the parent is heterozygous.
  • The chance of both events occurring together can be calculated by multiplying the probability of each event: 0.5 × 0.5 = 0.25, or 25%.
  • Each of the four possible combinations of alleles is equally likely, so 25% of gametes are expected to have each combination.
30
Q

What causes independent assortment in genes?

A

The random orientation of pairs of homologous chromosomes in meiosis I.

31
Q

What was Mendel not aware of and why?

A
  • He was not aware that some genes are found along the same piece of DNA, even though some of the traits he studied were on the same chromosome.
  • This is because even when genes are on the same chromosome, new allele combinations can be formed by crossing over.
32
Q

Diagram of crossing over resulting in new combinations of alleles

A
33
Q

Description and explanation of this diagram:

A
  • The chromosome carries two different genes of interest, and the individual is heterozygous for both genes.
  • The blue homologue carries the dominant alleles (A and B) for these genes, while the red homologue has the recessive alleles (a and b).
  • As a result of crossing over, there are two recombinant chromatids with two new combinations of alleles, Ab and aB.
  • The chromatids that did not cross over still have the original, parental combinations of alleles, AB and ab.
34
Q

What are recombinant gametes?

A

Gametes containing a new combination of alleles, different from the two parents.

35
Q

When does independent assortment occur and why?

A
  • Genes that are far enough apart on the same chromosome show independent assortment.
  • This is because crossing over occurs so frequently that they are inherited together and apart with equal frequency, just as if they were on different chromosomes.
36
Q

Mendel’s peas

A

In Mendel’s peas, pea color and flower color are both located on chromosome 1 but are far enough apart that due to crossing over, the alleles are inherited independently.

37
Q

When we see chiasmata under a microscope, which has occurred?

A

Prophase I

38
Q

The diagram below shows a tetrad (bivalent) during prophase I. How many chromatids and chiasmata are visible?

A

Chromatids: 4

Chiasmata: 2

There are two chiasmata visible. The connection between the chromatids in the center shows the centromeres. There are four chromatids present, two for each of the homologous chromosomes.

39
Q

The diagram below shows three gene loci on the same chromosomes, with the individual being heterozygous for each gene.

Given the diagram below, which is not a combination of alleles that would be expected in the gametes?

A

Ab

No crossing over is shown between alleles A and B, so inheriting A and b together is not expected.

40
Q

What are mitosis and meiosis both forms of?

A

Nuclear division

41
Q

Describe the cells produced by mitosis

A
  • They are genetically identical to each other and to the parent cell.
  • These cells become part of the body or can be used in asexual reproduction to create cloned offspring.
42
Q

Describe the cells produced by meiosis

A
  • They are genetically unique and contain only half the genetic information of the parent cell.
  • These cells eventually form gametes (sperm and egg) for use in sexual reproduction.
43
Q

Gametes increase ___

A

Genetic diversity in offspring

44
Q

How do gametes increase genetic diversity in offspring?

A

By each having a unique combination of alleles and by combining their DNA with DNA from a second parent.

45
Q

Diagram showing the hared start and different products of mitosis and meiosis

A
46
Q

How are mitosis and meiosis similar?

A
  • The processes of mitosis and meiosis are both preceded by the same cell cycle.
  • They both replicate their DNA during interphase and, when preparing to divide, begin to condense their DNA into chromosomes using nucleosome structure for supercoiling.
47
Q

How are mitosis and meiosis different?

A

The differences between mitosis and meiosis begin during the process of condensation.

48
Q

Diagram summarizing the phases of meiosis

A
49
Q

There are three meiotic events that contribute to genetic diversity, and they all occur during ___

A

Meiosis I

50
Q

Prophase I of meiosis

A

Crossing over leads to the exchange of DNA between homologous chromosomes, resulting in allele combinations not found in either original chromosome.

51
Q

Metaphase I of meiosis

A
  • Random orientation of tetrads (pairs of homologous chromosomes) on the equatorial plate results in an independent assortment of maternal and paternal homologues.
  • In other words, each gametic nucleus receives one copy of each type of chromosome, but the version it receives is equally likely to be from the mother or the father.
52
Q

Anaphase I of meiosis

A
  • Reduction division from the diploid to the haploid number of chromosomes in the nucleus, as a result of homologous chromosomes moving to opposite poles.
  • When the gametes fuse and combine the DNA from two genetically different parents, the resulting offspring possesses the standard diploid number of chromosomes.
53
Q

Diagram showing the first three phases of meiosis I in an animal cell

A
54
Q

What is produced in meiosis?

A
  • In meiosis, a diploid cell (one cell, 2n) produces two haploid cells in meiosis I (two cells, n) and four haploid cells total in meiosis II (four cells, n).
  • These haploid cells may develop into gametes (sperm or egg) for use in sexual reproduction.
55
Q

What separates in meiosis I and II?

A
  • Homologous chromosomes separate in meiosis I.
  • Sister chromatids separate in meiosis II.
56
Q

What is synapsis?

A

This pairing of homologous chromosomes.

IB HL Biology (68 decks)

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