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.

LE 13-3

5 µm Pair of homologous

chromosomes

Sister

chromatids

Centromere

• This is called a

karyotype. All

23 pairs of

homologous

chromosomes

are lined up.

LE 13-4

Key

Maternal set of

chromosomes (n = 3) 2n = 6

Paternal set of

chromosomes (n = 3)

Two sister chromatids

of one replicated

chromosomes

Two nonsister

chromatids in

a homologous pair

Pair of homologous

chromosomes

(one from each set)

Centromere

• The sex chromosomes are called X

and Y o 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 chromosome

o In an unfertilized egg (ovum), the sex chromosome is always X

o 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

LE 13-9

Propase

Duplicated chromosome

(two sister chromatids)

Chromosome

replication

2n = 6

Parent cell

(before chromosome replication)

Chromosome

replication

MITOSIS MEIOSIS

Chiasma (site of

crossing over) MEIOSIS I

Prophase I

Tetrad formed by

synapsis of homologous

chromosomes

Tetrads

positioned at the

metaphase plate

Metaphase I Chromosomes

positioned at the

metaphase plate

Metaphase

Anaphase

Telophase Homologues

separate

during

anaphase I;

sister

chromatids

remain together

Sister chromatids

separate during

anaphase

Daughter

cells of

meiosis I

Haploid

n = 3

Anaphase I

Telophase I

MEIOSIS II

Daughter cells

of mitosis

2n 2n

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 chromosomes

o Crossing over

o 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 mother

o Another contains 20 from the mother, 3 from the father.

LE 13-10

Key

Maternal set of

chromosomes

Paternal set of

chromosomes

Possibility 1 Possibility 2

Combination 2 Combination 1 Combination 3 Combination 4

Daughter

cells

Metaphase II

Two equally probable arrangements of chromosomes at

metaphase I

Independent Assortment of Chromosomes

• The number of combinations possible when chromosomes assort independently into gametes is calculated by 2n, where n is the haploid number

• For humans (n = 23):

o 223 = 8,388,608 possible combinations!

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 13-11 Prophase I

of meiosis

Tetrad

Nonsister

chromatids

Chiasma,

site of

crossing

over

Recombinant

chromosomes

Metaphase I

Metaphase II

Daughter

cells

Random Fertilization • Random fertilization adds to genetic variation

because any sperm can fuse with any egg.

Genetic Diversity • How many possible combinations of genes are there

from two parents?

• Independent assortment:

223 = 8,388,608 combinations of chromosomes in each

sperm or egg cell.

• Random assortment: 8.4 million possible sperm combinations

+ 8.4 million possible egg combinations

= 16.8 trillion possible embryos

Genetic Diversity • How many possible combinations of genes are there

from two parents?

• Crossing over

o Average of 1,000 genes in each chromosome

o At the most, about half of the chromosome can cross over to

its homologous partner.

o This results in 3.3 novemquardragintillion (1 followed by 150

zeros) gene combinations for each chromosome pair crossing

over.

Genetic Diversity • How many possible combinations of genes are there

from two parents?

• Total

3.3 novemquardragintillion possible chromosome

combinations

x 23 chromosomes

x 16.8 trillion possible sperm-egg combinations

=1.3 quinquinquagintillion (1 followed by 168 zeros)

possible different genetic combinations for two

people.