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Meiosis Why Sexual Reproduction?

Lecture 5

Outline

Understanding meiosis: how and why? Phases of meiosis Meiosis and genetic variation Errors in meiosis Comparing asexual and sexual reproduction Advantages of meiosis (sexual reproduction)

A Closer Look at Chromosomes

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Chromosomes and Meiosis

There are 2 chromosomes of each type (23 types) Homologous chromosomes X and Y are sex chromosomes Humans: 2n = 46

MEIOSIS - SEXUAL REPRODUCTION

two haploid gametes—one from each parent—unite in fertilization to form a genetically unique, diploid zygote Explaining meiosis: - Proximate: how ? - Ultimate: why?

Contains two sets of chromosomes

•  Sexual reproduction –  Involves the

alternation of meiosis and fertilization

Contain one set of chromosomes

Meiosis

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Overview of Meiosis

Major Events in Meiosis

Maternal chromosome

Paternal chromosomeNuclear envelope

Replication

Replicated chromosome

Centromere

Sister chromatids

Homologous pair of premeiotic chromosomes

Each chromosome replicates prior to undergoing meiosis.

Major Events in Meiosis

Parent cell contains homologous pair of chromosomes

Homologs separate

Daughter cells contain just one homolog

Sister chromatids separate

ME

IOS

IS II

ME

IOS

IS I

Four daughter cells contain one chromosome each. In animals, these cells become gametes.

During meiosis, chromosome number in each cell is reduced.

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Major Events in Meiosis

Female gamete (egg)

Male gamete (sperm)

Fertilization

Diploid offspring contains homologous pair of chromosomes

A full complement of chromosomes is restored during fertilization.

When Do Meiosis and Fertilization Occur during the Life of an Organism?

Meiosis 1

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Phases of Meiosis I

Meiosis I is a continuous process with five phases

(1) early prophase (2) late prophase (3) metaphase (4) anaphase (5) telophase

The Phases of Meiosis PRIOR TO MEIOSIS MEIOSIS IChromosomes replicate, forming sister chromatids.

Nuclear envelope

Chromatin

1. Interphase: 2. Early Prophase I:

Non-sister chromatids

Tetrad (4 chromatids from homologous chromosomes)

Spindle apparatus

Chiasma

3. Late Prophase I:

Homologous chromosomes separate.

Chromosomes replicate in parent cell, in uncondensed state.

Chromosomes condense, nuclear envelope breaks up, spindle apparatus forms. Synapsis of homologous chromosomes.

Crossing over of non-sister chromatids (often multiple cross- overs between the same chromatids).

Synapsis:

Chromosome Replication, Synapsis, and Crossing Over

A CLOSER LOOK AT THREE KEY EVENTS IN MEIOSIS

Centromere

Sister chromatids

Chromosomes

One homolog

Synaptonemal complex

Second homolog

Non-sister chromatids

Protein complex

1. Replication,

2. Synapsis,

3. Crossing over,

Crossing over usually occurs at least once in each non-sister chromatid, but is only shown on 1 pair here

during interphase. Sister chromatids are held together by proteins along the chromosome “arms” and at the centromere. Shown: early prophase of meiosis I, when chromosomes have condensed.

during prophase I. Complex of proteins forms where crossing over will occur. Chromosome segments are swapped between non-sister chromatids.

during prophase I. Homologous chromosome are held together by proteins in the synaptonemal complex.

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The Phases of Meiosis MEIOSIS I

4. Metaphase I:

Homologous chromosomes separate.

5. Anaphase I:6. Telophase I and Cytokinesis:

Tetrads migrate to metaphase plate.

Homologs separate and begin moving to opposite sides of cell. Chromosomes move to

opposite sides of cell, then cell divides.

Result of meiosis I: one chromosome of each homologous pair is distributed to a different daughter cell. Result:

Phases of Meiosis II

Meiosis II has the folowing phases: (1) prophase II (2) metaphase II (3) anaphase II (4) telophase II

At the start of meiosis II, each daughter cell is haploid; but each replicated chromosome is still composed of two sister chromatids

Meiosis 11

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Meiosis II

7. Prophase II: 8. Metaphase II:

MEIOSIS IISister chromatids separate.

Spindle apparatus forms.

Chromosomes line up at middle of cell (metaphase plate).

Note: each daughter cell is haploid

Meiosis II

9. Anaphase II: 10. Telophase II and Cytokinesis:Sister chromatids

separate, begin moving to opposite sides of cell.

Chromosomes move to opposite sides of cell, then cell divides.

MEIOSIS IISister chromatids separate.

Figure 9.17 – Part 2

Figure 9.17 – Part 2

MEIOSIS: SUMMARY

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Genetic Variation: Independent Assortment of Chromosomes

Genetic Variation: Crossing Over

•  DNA exchanges between maternal and paternal chromatid pairs

•  new combination of alleles that did not exist in each parent

•  a form of genetic recombination

Crossing over

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Meiotic Errors: Nondisjunction Leads to Aneuploidy

Nondisjunction: failure of homologous chromosomes to separate -  aneuploid zygotes: those with too few or too many chromosomes

monosomy

trisomy

Down Syndrome (trisomy)

1 in 1,500 if mother is under 30

1 in 16 if mother is over 45

Nondisjunction of the X chromosome

1/1000

1/1000 1/5000

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Why Do Errors Occur? •  Most instances of aneuploidy in humans involve

the sex chromosomes or chromosome 21. 2 hypotheses proposed: 1.  either embryos with other aneuploidies are

spontaneously aborted before birth so we do not see them

2.  and/or the sex chromosomes and chromosome 21 may be more prone to aneuploidy than others

Comparing Mitosis and Meiosis

SEXUAL REPRODUCTION

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ASEXUAL REPRODUCTION

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Asexual Reproduction Has an Advantage

Asexual reproduction

Generation 1

Generation 2

Generation 3

Sexual reproduction

The mathematical model of John Maynard Smith predicts that asexually reproducing organisms should reproduce faster and thus outcompete similar organisms that invest in sexual reproduction

only ½ of the offspring in each generation can produce their own offspring

Every individual in each generation can produce 4 offspring

WHY DOES MEIOSIS EXIST? WHY SEXUAL REPRODUCTION ?

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MEIOSIS LEADS TO GENETIC VARIATION

Genetic Variation

–  Humans have 23 pairs of chromosomes

•  223 possible chromosome combinations •  8,388,608

Three chromosome pairs 23 combinations

•  The zygote is formed by the union of two independently-produced male and female gametes

•  Therefore, the possible combinations in a zygote is 8,388,608 X 8,388,608

70,368,744,177,664 > than 70 trillion diploid combinations!

Random Fertilization

Genetic variation:

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•  Genetic diversity is the raw material that fuels evolution

Answering the Question Why: Importance of Generating Diversity

Purifying Selection Hypothesis Asexually reproducing organisms: - a damaged gene that hinders function will be inherited by all of the individual’s offspring (all deleterious alleles are passed to offspring) -  deleterious alleles lower fitness Sexually reproducing organisms: - if one mate has one damaged allele but the other mate has the normal allele, about half the offspring will have the deleterious allele Natural selection against deleterious alleles is referred to as purifying selection

Changing-Environment Hypothesis

Offspring that are genetically different from their parents (sexual reproduction) may be more likely to survive and produce offspring if the environment changes compared to offspring that are genetically identical to their parents (asexual reproduction)

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Genetic Diversity in a Asexually Reproducing Bdelloid Rotifer

found in streams, lakes, puddles, lichens, tree bark, decaying vegetation -  reproduce only asexually, so how

do they increase their genetic diversity?

-  obtain DNA from other rotifers while in a state of suspended animation (increase genetic diversity by horizontal transfer)

Advantages of Meiosis (Sex) Summary

•  adaptation that creates genetic diversity among offspring and within a species

•  sexual populations may have

higher fitness than asexual populations (purifying selection hyp)

•  adaptations to changing environmental conditions (changing environment hypothesis)