Meiosis and Chromosome Assortment

Introduction to Biology

Chromosomes in Human Cells

• Somatic cells include all cells in the human body except sperm and eggs.• Gametes are human sperm and egg cells.

• Each human somatic cell has 23 pairs of chromosomes, 46 total.• Each pair of chromosomes are called homologous

chromosomes.

• Each homologous chromosome carries a copy of the same genes, either from the father or mother.

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5 µmPair of homologouschromosomes

Sisterchromatids

Centromere

• This is called a karyotype. All 23 pairs of homologous chromosomes are lined up.

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Key

Maternal set ofchromosomes (n = 3)

2n = 6

Paternal set ofchromosomes (n = 3)

Two sister chromatidsof one replicatedchromosomes

Two nonsister chromatids in a homologous pair

Pair of homologouschromosomes(one from each set)

Centromere

• The sex chromosomes are called X and Yo Human females have two X chromosomes.o Human males have one X and one Y

chromosome

• The 22 pairs of chromosomes that do not determine sex are called autosomes.

Inheritance of Genes• A gene is a unit of heredity that carries the

information for a specific trait or body function.o A gene is made of a segment of DNA.o Each gene is located on a specific chromosome.o Everyone has two copies of each gene (one on each

homologous chromosome).

• A cell with a full pair of each chromosome is called diploid.o Diploid is written shorthand as 2n.o All somatic cells are diploid (46 chromosomes).

• A cell with only one of each homologous chromosome is called haploid. o Haploid is written shorthand as n.o All gametes are haploid and have 23 total chromosomes.

• Gametes are haploid cells, containing only one set of chromosomes

• For humans, this means 23 total chromosomes (no pairs)o This includes 22 autosomes and a single sex chromosomeo In an unfertilized egg (ovum), the sex chromosome is

always Xo In a sperm cell, the sex chromosome may be either X or Y

Chromosomes and the Human Sex Cycle

• At sexual maturity, the ovaries and testes begin producing sperm and eggs through meiosis.o Gametes are the only types of human cells produced by

meiosis, rather than mitosis

• Meiosis is a form of cell division that results in one set of chromosomes in each gamete instead of two.o The resulting daughter cells are haploid.

• When fertilization occurs, the haploid sperm and haploid egg fuse together to form a diploid embryo.

Interphase• At the end of interphase, each cell has

grown into its full size, produced a full set of organelles, and duplicated its DNA.o The cell is diploid at this point.

• The nucleus contains 23 homologous chromosome pairs.

• Each chromosome is made of two sister chromatids (copies).

• The cells begin to divide, and the chromosomes pair up, forming a structure called a tetrad, which contains four chromatids.

Prophase I

Prophase I

• As homologous chromosomes pair up and form tetrads, they undergo a process called crossing-over.• First, the chromatids of the homologous

chromosomes overlap each other.• Then, the crossed sections of the chromatids are

exchanged.• Crossing-over is important because it produces new

combinations of genes in the cell.

Metaphase I

• As prophase I ends, a spindle forms and attaches to each tetrad.

• During metaphase I of meiosis, paired homologous chromosomes line up across the center of the cell.

 

Anaphase I

• During anaphase I, spindle fibers pull each homologous chromosome pair toward opposite ends of the cell.

• When anaphase I is complete, the separated chromosomes cluster at opposite ends of the cell.

Telophase I and Cytokinesis

• During telophase I, a nuclear membrane forms around each cluster of chromosomes.

• Cytokinesis follows telophase I, forming two new cells.

Summary of Meiosis I• Two new haploid cells have been produced.• Each haploid cell contains one chromosome out of

the original pair.• Each chromosome still contains two sister

chromatids.

Prophase II

• As the cells enter prophase II, their chromosomes—each consisting of two chromatids—become visible.

• The chromosomes do not pair to form tetrads, because the homologous pairs were already separated during meiosis I.

Metaphase II

• During metaphase of meiosis II, chromosomes line up in the center of each cell.

Anaphase II• As the cell enters anaphase,

the paired chromatids separate.

Telophase II and Cytokinesis

• The two daughter cells from Meiosis I divide, resulting in four daughter cells, each with two chromatids.

• These four daughter cells now contain the haploid number (N)—just two chromosomes each.

Summary of Meiosis II

• A total of four cells have been produced.• Each cell is haploid and only contains one out of the

original pairs of homologous chromosomes.• Each chromosome only contains a single chromatid.

A Comparison of Mitosis and Meiosis

• Mitosis produces cells that are genetically identical to the parent cell.

• Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid).

• Meiosis allows crossing over of chromosomes.o This produces cells that are genetically different from the parents and

each other.

• Three events are unique to meiosis, and all three occur in meiosis l:o Synapsis and crossing over in prophase I:

Homologous chromosomes physically connect and exchange genetic information

o At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes

o At anaphase I, it is homologous chromosomes, instead of sister chromatids that separate and are carried to opposite poles of the cell

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Propase

Duplicated chromosome(two sister chromatids)

Chromosomereplication

2n = 6

Parent cell(before chromosome replication)

Chromosomereplication

MITOSIS MEIOSIS

Chiasma (site ofcrossing over) MEIOSIS I

Prophase I

Tetrad formed bysynapsis of homologouschromosomes

Tetradspositioned at themetaphase plate

Metaphase IChromosomes positioned at themetaphase plate

Metaphase

AnaphaseTelophase

Homologuesseparateduringanaphase I;sisterchromatidsremain together

Sister chromatidsseparate duringanaphase

Daughtercells of

meiosis I

Haploidn = 3

Anaphase ITelophase I

MEIOSIS II

Daughter cellsof mitosis

2n2n

n

Sister chromatids separate during anaphase II

n n n

Daughter cells of meiosis II

Mitosis Meiosis

DNA replication

During interphase

During interphase

Divisions One Two

Synapsis and crossing over

Do not occur Form tetrads in prophase I

Daughter cells, genetic composition

Two diploid, identical to parent cell

Four haploid, different from parent cell and each other

Role in animal body

Produces cells for growth and tissue repair

Produces gametes

Genetic Variation Among Offspring

• The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation

• Three mechanisms contribute to genetic variation:o Independent assortment of chromosomeso Crossing overo Random fertilization

Independent Assortment of Chromosomes

• In independent assortment, each pair of chromosomes sorts maternal and paternal homologous chromosomes into daughter cells independently of the other pairs.

• Example: o One human sperm cell could contain 15 chromosomes

from his father, and 8 from his mothero Another contains 20 from the mother, 3 from the father.

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Key

Maternal set ofchromosomes

Paternal set ofchromosomes

Possibility 1 Possibility 2

Combination 2Combination 1 Combination 3 Combination 4

Daughtercells

Metaphase II

Two equally probablearrangements ofchromosomes at

metaphase I

Crossing Over• Crossing over produces new chromosomes with a

mixture of genes from each parent.• Instead of a chromosome that is 100% from the

person’s father or mother, it might now be 95% from the father, 5% from the mother.

LE 13-11Prophase Iof meiosis

Tetrad

Nonsisterchromatids

Chiasma,site of crossingover

Recombinantchromosomes

Metaphase I

Metaphase II

Daughtercells

Random Fertilization• Random fertilization adds to genetic variation

because any sperm can fuse with any egg.