Meiosis and Sexual ReproductionChapter 10

Outline• Reduction in Chromosome Number

–Meiosis Overview

–Homologous Pairs

• Phases of Meiosis

–Meiosis I

–Meiosis II

• Meiosis Compared to Mitosis

• Genetic Variation

–Crossing-Over

–Independent Assortment

–Fertilization

• Human Life Cycle

Discovery of Meiosis

• Meiosis was first observed by the Belgian cytologist Pierre-Joseph van Beneden in 1887

• Gametes (eggs and sperm) contain half the number of chromosomes found in other cells (haploid)

• The fusion of gametes is called fertilization

• It creates the zygote, which contains two copies of each chromosome (diploid)

Contains two sets of chromosomes

• Sexual reproduction– Involves the

alternation of meiosis and fertilization

• Asexual reproduction– Does not involve

fertilization

Contain one set of chromosomes

The Sexual Life Cycle in Animals

Meiosis:Halves the Chromosome Number

• Special type of nuclear division

• Used only for sexual reproduction

• Halves the chromosome number prior to fertilization

–Parents diploid

–Meiosis produces haploid gametes

–Gametes fuse in fertilization to form diploid zygote

–Becomes the next diploid generation

Homologous Pairs of Chromosomes

• In diploid body cells chromosomes occur in pairs

• Humans have 23 different types of chromosomes

• Diploid cells have two of each type

• Chromosomes of the same type are said to be homologous

–They have the same length

–Their centromeres are positioned in the same place

–One came from the father (the paternal homolog) the other from the mother (the maternal homolog)

–When stained, they show similar banding patterns

–Because they have genes controlling the same traits at the same positions

Homologous Chromosomes

The Stages of Meiosis

• Meiosis consists of two successive divisions, but only one DNA replication

– Meiosis I• Separates the two versions of each

chromosome (homologous chromosomes)

– Meiosis II• Separates the two sister chromatids of each

chromosome

• Meiosis halves the number of chromosomes

Overview of Meiosis

• Meiosis I (Reductional Division)– Prophase I

• Nuclear membrane breaks down• Homologous chromosomes pair up and exchange segments

(crossing over)

– Metaphase I• Homologous chromosome pairs align at random in the

equatorial plane such that maternal or paternal member may be oriented toward either pole (independent assortment)

– Anaphase I• Homologous chromosomes (each still consisting of 2

chromatids) separate and move to opposite poles

– Telophase I• Individual chromosomes gather together at each of the two

poles• Cytokinesis produces 2 daughter cells which are haploid

Interkinesis

Meiosis I

Meiosis I

Prophase I• The longest and

most complex stage of meiosis

• Homologous chromosomes undergo synapsis

– Pair up along their lengths

• Crossing over occurs

Crossing Over

Independent Assortment

Meiosis IMetaphase I

• In humans, a gamete receives one homologue of each of the 23 chromosomes– Humans have 23 pairs of chromosomes

• 223 combinations in an egg or sperm• 8,388,608 possible kinds of gametes

Independent Assortment

Three chromosome pairs23 combinations

• Meiosis II

– After meiosis I there is a brief interphase (interkinesis)• No DNA synthesis occurs

– Meiosis II is similar to mitosis, but with two main differences

• 1. Haploid set of chromosomes

• 2. Sister chromatids are not identical

• Meiosis II

– Prophase II• Brief and simple, unlike prophase I• Cells have 1 member of each homologous pair

– Metaphase II• Chromosomes line up at the equator

– Anaphase II• Spindle fibers contract, splitting the centromeres• Sister chromatids move to opposite poles

– Telophase II• Nuclear envelope reforms around four sets of daughter

chromosomes• Cytokinesis occurs

Interkinesis

Meiosis I

No two cells are alike

Meiosis II

• Overview of meiosis– 2 divisions, 4 daughter cells (not identical)– Cells are diploid at beginning of meiosis– Pairs of chromosomes are called homologues (homologous chromosomes)

– Meiosis I• Homologues line up side by side at equator-synapsis• When pairs separate, each daughter cell receives one member of

the pair• Cells are now haploid

– Meiosis II• No replication of DNA occurs in this division• Centromeres divide and sister chromatids migrate to opposite poles

to become individual chromosomes• Each of the four daughter cells produced has the haploid

chromosome number and each chromosome is composed of one chromatid

Genetic Variation: Crossing Over and Independent Assortment

• Meiosis I brings about genetic variation in two key ways:• Crossing over-exchange of segments of DNA between

homologues (Prophase I)

• Independent assortment of chromosome pairs (Metaphase I)– When homologues align at the metaphase plate– They separate in a random manner– The maternal or paternal homologue may be oriented

toward either pole of mother cell

– Promotes genetic variability– Both assure that gametes will contain different combinations of

chromosomes– When fertilization occurs, the resulting offspring will genetically

unique

• In comparison of meiosis to mitosis note that:– DNA replication occurs only once prior to both– Meiosis requires 2 divisions, mitosis only 1– Meiosis produces 4 daughter cells, mitosis produces

2– Daughter cells from meiosis are haploid, those from

mitosis are diploid– Daughter cells from meiosis are genetically unique,

while those from mitosis are genetically identical

Comparing Meiosis and Mitosis

• Meiosis and mitosis have much in common• However, meiosis has two unique features

– 1. Crossing over• Homologous chromosomes pair all along their

lengths in meiosis I and exchange pieces of DNA

– 2. Reduction division• There is no chromosome duplication between the

two meiotic divisions• This produces haploid gametes

Meiosis Compared to Mitosis

Meiosis versus Mitosis

• Meiosis–Requires two nuclear

divisions

–Chromosomes synapse and cross over

–Halves chromosome number

–Produces four daughter nuclei

–Produces daughter cells genetically different from parent and each other

–Used only for sexual reproduction

• Mitosis–Requires one nuclear division

–Chromosomes do not synapse nor cross over

–Preserves chromosome number

–Produces two daughter nuclei

–Produces daughter cells genetically identical to parent and to each other

–Used for asexual reproduction, growth, development, and repair

Comparison of Mitosis and Meiosis

Evolutionary Consequences of Sex

• Sexual reproduction increases genetic variation through three key mechanisms

– 1. Crossing over

– 2. Independent assortment

– 3. Random fertilization

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

• Therefore, the possible combinations in an offspring

– 8,388,608 X 8,388,608 =

– 70,368,744,177,664– More than 70 trillion!

• And this number does not count crossing-over

Random Fertilization

Genetic Variation: Significance

• Asexual reproduction produces genetically identical clones

• Asexual reproduction is advantageous when environment is stable

• Sexual reproduction produces genetically unique combinations

• However, if environment changes, genetic variability introduced by sexual reproduction may be advantageous

• Genetic diversity is the raw material that fuels evolution

• The Human Life Cycle– Requires both mitosis and meiosis– The formation of gametes (eggs and sperm) is called

gametogenesis.– In females meiosis is part of the process of oogenesis– In males meiosis is part of spermatogenesis– At fertilization, the resulting zygote divides by mitosis for

the processes of growth and development– Mitosis is used for repair throughout life

Life Cycle of Humans

• Spermatogenesis– Begins at puberty and continues throughout life– Occurs in seminiferous tubules of testes– Primary spermatocytes (2n) divide in meiosis I to form 2

secondary spermatocytes (1n)– Secondary spermatocytes divide in meiosis II to

produce 4 sperm

• Oogenesis– Occurs in the ovaries– Primary oocyte (2n) divides in meiosis I to produce 1 secondary

oocyte (1n) and 1 polar body• Division is unequal as secondary oocyte receives most of the

cell contents (nearly all cytoplasm and organelles) and half the chromosomes

– Allows ovum to have all the cellular “machinery” it needs for embryonic development

• Polar body functions only to receive half of the chromosomes– Secondary oocyte begins meiosis II but stops at metaphase II;

polar body may also divide– At puberty, after ovulation secondary oocyte is activated if fertilized

to complete division– Meiosis II produces 1 ovum and up to 3 polar bodies

Spermatogenesis and Oogenesis

Summary

– Spermatogenesis and oogenesis both utilize meiosis– Spermatogenesis begins at puberty and continues

throughout life– Spermatogenesis produces 4 sperm per primary

spermatocyte• Results in production of many sperm

– Oogenesis results in 1 oocyte and up to 3 polar bodies per primary oocyte

• Divisions are unequal, ovum receives most cell contents

– Oogenesis begins prior to birth, stops until puberty, then resumes in a cyclic pattern

– Cyclic release of oocytes continues until menopause when the process stops

Human Chromosomes

• Human somatic cells have 23 pairs of chromosomes– 22 pairs of autosomes

• Autosome-any chromosome other than a sex chromosome

– 1 pair of sex chromosomes• XX in females• XY in males

Human Chromosomes

• Failure of chromosomes to separate correctly in meiosis I or II is termed nondisjunction.– This leads to an abnormal number of chromosomes, or

aneuploidy.

• Humans with one less autosome are called monosomics. – These do not survive development.

• Humans with one extra autosome are called trisomics.– The vast majority do not survive– Trisomy for only a few chromosomes is compatible with

survival• However, there are severe developmental defects• The only one compatible with a reasonable chance of

survival is trisomy 21 (Down Syndrome).

Down Syndrome

1 in 1,500 if mother is under 30

1 in 16 if mother is over 45

Most common trisomy in humans.

Short stature, eyelid fold, flat face, stubby fingers,, round head, mental retardation

3 copies of chromosome 21

75% of cases- egg has 2 copies, sperm has 1

Can be detected by a karyotype

When nondisjunction occurs during meiosis I both members of a homologous pair migrate into the same daughter cell.

When nondisjunction occurs in meiosis II, the centromere fails to divide and both daughter chromatids enter the same gamete.

– Egg with 24 chromosomes fertilized by sperm with 23- trisomy

» 47 chromosomes in zygote– Egg with 22 chromosomes fertilized by sperm

with 23 chromosomes- monosomy» 45 chromosomes in zygote

– Normal development depends on the presence of exactly 2 of each kind of chromosome

Nondisjunction of Chromosomes During Oogenesis Followed by Fertilization with

Normal Sperm

Nondisjunction Involving Sex Chromosomes

• Aneuploidies of sex chromosomes have less serious consequences than those of autosomes– Chances of survival are greatest if monosomy or

trisomy involves the sex chromosomes– However, they can lead to sterility

Nondisjunction of the X Chromosome

Syndrome-disorders characterized by groups of symptoms.

Turner’s syndrome– Monosomy X (XO), zygote has one X chromosome and no other X

or Y

– Capable of survival, phenotypically female, infertile– Ovarian failure

Klinefelter syndrome– XXY

– Underdeveloped testes and prostate gland, no facial hair

– Phenotypically male, infertile

Poly-X females– More than 2 X chromosomes

– XXX females may be unusually tall

– XXXX females are usually severely retarded

Turner and Klinefelter Syndromes

• Nondisjunction of the Y chromosome

Jacob’s syndrome – Due to nondisjunction in meiosis II– Yields YY gametes and ultimately XYY zygotes– XYY genotype can only result from nondisjunction in

spermatogenesis– Frequency of XYY is 1 in 800 males (live births)

• In general, these individuals are phenotypically normal• Taller than average, persistent acne

Syndromes from Abnormal Chromosome Numbers