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Meiosis - PowerPoint PPT Presentation

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Meiosis . Biology 11 Chapter 12. Part I. Meiosis In Males & Females. Meiosis Overview. Meiosis is the process of going from a diploid cell to a haploid cell N = haploid number of any genome For humans n = 23 Your diploid chromosome count is 2n = 46.

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Biology 11

Chapter 12

Part i

Part I

Meiosis In Males & Females

Meiosis overview
Meiosis Overview

  • Meiosis is the process of going from a diploid cell to a haploid cell

    • N = haploid number of any genome

    • For humans n = 23

    • Your diploid chromosome count is

      • 2n = 46

Homologous pair of chromosomes in diploid cell

One diploid cell with 2 chromosomes


Sister Chromatids

Separation of Homologous Chromosomes

Meiosis I

Separation of Sister Chromatids

Meiosis II

Meiosis in males
Meiosis In Males

  • meiosis occurs in the testis

    • Mature sperm begin as spermatogonium diploid cells

  • When a male reaches puberty hormones in the brain signal the testis to make testosterone

  • Testosterone then prompts

    • Maturation of sperm

    • Growth of muscles

    • Increased bone density

    • Deepening of voice

    • Increased hair growth

Testosterone levels over time
Testosterone Levels Over Time

  • In the uterus there is production of testosterone

  • Female development is the default setting

  • When testosterone is present this causes the gonads to drop outside and form the testis

Meiosis in females
Meiosis In Females

  • Meiosis begins in the ovaries

    • Mature oocytes (eggs) develop from diploid oogonium cells

  • When a female reaches puberty hormones in the brain signal the ovaries to develop mature eggs

  • As a follicle develops two major hormones are released

    • Estrogen

    • Progesterone

Female sex hormones
Female Sex Hormones

  • Progesterone

    • Readies the uterus for implantation

  • Estrogen prompts

    • Development of breasts

    • Appearance of pubic hair

    • Increase in fat beneath the skin

    • Widening and lightening of the pelvis

Follicle maturing every month

Middle Stage

Female Ovary

Follicle maturing every month

Beginning Stage

This whole

structure is

a follicle


Immature Eggs

Corpus luteum

After follicle releases egg

Releases estrogen & progesterone

Histology of an ovary
Histology of An Ovary

  • This is a tissue slide from an ovary

  • Can you find the

    • Corpus luteum

    • Oogonia

    • Maturing follicles

    • Eggs arrested in metaphase II

    • Where is estrogen released from

Part ii

Part II

Mechanisms of Meiosis

Color scheme for chromosomes
Color Scheme for Chromosomes

  • We will begin with a hypothetical cell which contains 6 chromosomes

  • Homologous chromosomes are the same size, but not directly attached to one another

  • One homolog will be solid color the other homolog will be the same color with a pattern

  • Sister chromatids are of course attached and the same color

Hypothetical Spermatogonium or Oogonium Cell

How many chromosome does this cell have?

What is n for this cell?

When this cell goes through Meiosis I and II

how many chromosomes will it have?

Meiosis I

Separation of Homologs

Anaphase i
Anaphase I

  • After a cell completes anaphase I the cell is haploid

  • Now you only have one copy of every gene, it is a duplicated copy but still just one copy





Hypothetically – the black dash represents a gene for eye color.

Since the separation of the homologs your cell now has only one

gene for eye color and not two as found in the diploid cell

Anaphase i1
Anaphase I

  • Also during anaphase I in males the Y chromosome pairs with the X chromosome

  • After these separate the cell is haploid and has either one duplicated copy of X or one duplicated copy of Y

Meiosis II

Separation of Sisters

Variation in traits
Variation In Traits

  • Two mechanism by which variation in genetic traits is produced during meiosis

    • Crossing over

    • Random metaphase alignments

Crossing over
Crossing Over

  • Crossing over is an exchange of genetic material between two homologous chromosomes

  • It can only take place in prophase I of meiosis I

  • Produces variation in traits by mixing the maternal and paternal chromosomes

Crossing over1
Crossing Over

  • Homologous chromosomes are 90 to 95% identical

  • Sister chromatids are 100% identical

  • Crossing over allows and exchange of information between non sisters

Crossing over2
Crossing Over

  • The black or white squares is a gene for eye color

When these homologs separate during _____________

this chromosome will end up in a gamete.

Now it is a mixture of maternal and paternal DNA

Blue = Paternal

Purple = Maternal

Random alignments
Random Alignments

  • There is no particular pattern to the metaphase I positions of maternal and paternal chromosomes

  • Possible Combinations

    • The rule is 2n possible combinations of chromosomes

    • For our hypothetical cell with 6 chromosomes how many combinations of chromosomes are possible

Mistakes in meiosis
Mistakes In Meiosis

  • Nondisjunction during Meisosis I

  • When one set of homologs does not separate during anaphase I

  • Certain gametes will have too many or too few chromosomes

    • Aneuploid cells

      • 2n + 1 = trisomy

      • 2n – 1 = monosomy

Mistakes in meiosis1
Mistakes In Meiosis

  • Nondisjunction during Meisosis II

  • Sister chromatids do not separate during anaphase II

  • Relatively rare event leads to the production of two normal gametes and two abnormal gametes

Mistakes in meiosis2
Mistakes In Meiosis

  • Mistakes in meiosis are a major cause of spontaneous abortion in humans

  • These seem to be random errors

  • The sex chromosomes and chromosome 21 are more prevalent in the population because they are not embryonic lethal

    • Down syndrome

    • Turners syndrome XO

    • Klinefelter syndrome XXY

    • Triple X syndrome XXX

Meiosis in males1
Meiosis In Males

Many Complex and physiological

and morphological changes

happen during sperm development

  • Sperm formation occurs in specialized cells in the testis

  • It takes approximately 30 days for a mature sperm cell to form

  • Sperm production begins in puberty and continues until death

Meiosis in males2
Meiosis In Males

  • One spermatogonium should result in the production of 4 functional haploid cells

Meiosis in females1
Meiosis in Females

  • Females produce eggs from puberty until menopause

  • Female oogonia undergo meiosis but do not complete meiosis II unless the egg becomes fertilized

    • Arrested in metaphase II

Meiosis in females2
Meiosis In Females

  • In Females the oogonium undergo two cellular division to yield four haploid cells

  • However, the division of the cytoplasm is not equal

  • Why?

Meiosis in females3
Meiosis In Females

  • Polar bodies are not functional

  • They only contain genetic material and are reabsorbed by the body

Unequal division of cytoplasm
Unequal Division of Cytoplasm

Proteins – instruct neural tube development

  • The female egg must have

    • Proteins ready to direct development

    • Mitochondria for energy

    • Many rRNA molecules ready to make proteins

  • Biologically sperm is motile and small

    • Its purpose is to carry half to the genetic information to the egg




Proteins – instruct formation of heart

Proteins – instruct the formation of gut

Comparing meiosis to mitosis
Comparing Meiosis to Mitosis

  • Mitosis

    • PMAT

    • 1 2n cell to 2 2n cells

  • Meiosis

    • 2 divisions

    • PMAT I & II

    • 1 2n cell to 4 n cells

      • Not all haploid cells functional

Why sex
Why Sex

  • All things considered equal asexual reproduction confers a large numerical advantage

Asexual reproduction is more efficient because everyone is

capable of reproduction

Purifying selection hypothesis
Purifying Selection Hypothesis

  • What if a critical gene is damaged in an asexually producing organism

  • This gene would be transferred to offspring and may hinder “fitness”

  • In sexual reproduction an individual will probably have only one damaged allele

  • If they mate with an unaffected individual then the probability of inheriting the damaged allele decreases

Purifying selection
Purifying Selection

  • Natural selection against deleterious alleles is called purifying selection

  • Sexual reproduction reduces the impact of purifying selection

Changing environment hypothesis
Changing Environment Hypothesis

  • If environmental conditions change parents may be poorly adapted

    • Temperature

    • Moisture

    • Predation/parasites

    • Competitors/food resource

  • But the genetically different offspring may have acquired alleles in different combinations that give them an advantage

Changing environment hypothesis1
Changing Environment Hypothesis

  • Study using a snail species that reproduces sexually and asexually

  • This snail is susceptible to parasitic worm infections

  • The variation in offspring via sexual reproduction should have the chance of a higher “fitness” in the parasitic worm environment

  • Sexual reproduction should increase as parasitism rates increase

Sexual reproduction
Sexual Reproduction

  • Is an adaptation that reduces the impact of purifying selection

  • Is an adaptation that increases the fitness of individuals in an environment where disease causing organisms are common