Chapter 13 Meiosis

and Sexual Life

Cycles

Question?

• Reproduction is a characteristic

of Life

• Does Like really beget Like?

• This chapter deals with

reproduction of life.

Heredity

• The transmission of traits from

parents to offspring.

• Comment - Humans have been

aware of heredity for thousands

of years.

Genetics

• The scientific study of heredity.

• Comment - Genetics is only

about 150 years old.

Variation

• Is demonstrated by the

differences in appearance that

offspring show from parents

and siblings.

• Offspring only “resemble” their

parents and siblings.

Genes

• The DNA for a trait.

• Locus - the physical location of

a gene in a chromosome.

Reproduction

• A method of copying genes to

pass them on to offspring.

• Two main types: • Asexual reproduction

• Sexual reproduction

Asexual Reproduction

• Parent passes all of its genes

to its offspring.

• Uses mitosis.

• Also known as cloning.

• Comment - many organisms

reproduce this way.

Asexual Bud

Advantages

• Only need 1 parent.

• Offspring are identical to the

parent.

• Good genetic traits are

conserved and reproduced.

Disadvantages

• No new DNA combinations for

evolution to work on.

• Clones may become extinct if

attacked by a disease or pest.

Sexual Reproduction

• Two parents contribute DNA to

an offspring.

• Comment - most organisms

reproduce this way, but it hasn’t

been proven in some fungi and

a few others.

Advantages

• Offspring has a unique

combination of DNA which may

be an improvement over both

parents.

• New combination of DNA for

evolution to work with.

Disadvantages

• Need two parents.

• Good gene combinations can

be lost.

• Offspring may not be an

improvement over the parents.

Question ?

• Do parents give their whole

DNA copy to each offspring?

• What would happen to

chromosome number if they

did?

Chromosome Number

• Is usually constant for a

species.

• Examples: • Humans - 46

• Corn - 20

• Onions - 16

• Dogs - 72

Life Cycle - if Mitosis

Female 46 Male 46

egg 46 sperm 46

Zygote 92

mitosis mitosis

Mitosis

Result

• Chromosome number would

double each generation.

• Need a method to reduce the

chromosome number.

Life Cycle - if Meiosis

Female 46 Male 46

egg 23 sperm 23

Zygote 46

mitosis mitosis

Meiosis

Result

• Chromosome number will

remain the same with each

sexual reproduction event.

• Meiosis is used to produce the

gametes, sex cells or spores.

Meiosis - Purpose

• To reduce the number of

chromosomes by half.

• Prevents doubling of

chromosome numbers during

sexual reproduction.

Sexual Life Cycle

• Has alternation of meiosis and

fertilization to keep the

chromosome numbers constant

for a species.

Ploidy

• Number of chromosomes in a

"set" for an organism.

• Or, how many different kinds of

chromosomes the species has.

• Usually shown as N = …… • Humans N = 23

Diploid

• 2 sets of chromosomes.

• Most common number in body

or somatic cells. • Humans 2N = 46

• Corn 2N = 20

• Fruit Flies 2N = 8

Human Chromosomes

• Human somatic cells (any cell

other than a gamete) have 23

pairs of chromosomes.

• A karyotype is an ordered display

of the pairs of chromosomes from

a cell.

Human Chromosomes • The two chromosomes in each

pair are called homologous

chromosomes, or homologs.

• Chromosomes in a

homologous pair are the same

length and carry genes

controlling the same inherited

characters.

• Each pair of homologous

chromosomes includes one

chromosome from each parent.

• The 46 chromosomes in a

human somatic cell are two

sets of 23: one from the mother

and one from the father.

• A diploid cell (2n) has two sets

of chromosomes.

• For humans, the diploid number

is 46 (2n = 46).

Haploid

• Single set of chromosomes.

• Number in the gametes or sex

cells. • Humans N = 23

• Corn N = 10

• Fruit Flies N = 4

• A gamete (sperm or egg)

contains a single set of

chromosomes, and is haploid

(N).

• For humans, the haploid number

is 23 (N = 23).

• Each set of 23 consists of 22

autosomes and a single sex

chromosome.

• In an unfertilized egg (ovum),

the sex chromosome is X.

• In a sperm cell, the sex

chromosome may be either X

or Y.

Polyploids

• Multiple sets of chromosomes.

• Examples • 3N = triploid

• 4N = tetraploid

• Common in plants, but often

fatal in animals.

Life Cycle Variations

Life cycle variation

• Plants and some algae exhibit

an alternation of generations.

• This life cycle includes both a

diploid and haploid multicellular

stage.

• The diploid organism, called

the sporophyte, makes

haploid spores by meiosis.

Plants

• Each spore grows by mitosis

into a haploid organism called a

gametophyte.

• A gametophyte makes haploid

gametes by mitosis.

• Fertilization of gametes results

in a diploid sporophyte.

Another variation

• In most fungi and some

protists, the only diploid stage

is the single-celled zygote;

there is no multicellular diploid

stage.

• The zygote produces haploid

cells by meiosis.

Fungi

• Each haploid cell grows by

mitosis into a haploid

multicellular organism.

• The haploid adult produces

gametes by mitosis.

Meiosis/Mitosis Preview of

differences

• Two cell divisions, not one.

• Four cells produced, not two.

• Synapsis and Chiasmata will

be observed in Meiosis

Meiosis - Uniqueness

• Three events are unique to meiosis, and all three occur in meiosis l: 1. Synapsis and crossing over in

prophase I: Homologous chromosomes physically connect and exchange genetic information.

Continued…

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

3. At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate.

Meiosis/Mitosis Preview of

differences

• 1st division separates PAIRS of

chromosomes, not duplicate

chromosomes.

• Interkinesis is present.

Meiosis

• Has two cell divisions.

Steps follow the names for

mitosis, but a “I” or “II” will be

added to label the phase.

Prophase I

• Basic steps same as in

prophase of Mitosis.

• Synapsis occurs as the

chromosomes condense.

• Synapsis - homologous

chromosomes form bivalents or

tetrads.

Prophase I

• Chiasmata observed.

• Longest phase of division.

Metaphase I

• Tetrads or bivalents align on

the metaphase plate.

• Centromeres of homologous

pairs point toward opposite

poles.

Anaphase I

• Homologous PAIRS separate.

• Duplicate chromosomes are

still attached at the

centromeres.

Anaphase I possibilities

Anaphase I

• Maternal and Paternal

chromosomes are now

separated randomly.

Telophase I

• Similar to Mitosis.

• Chromosomes may or may not

unwind to chromatin.

• Cytokinesis separates

cytoplasm and 2 cells are

formed.

Interkinesis

• No DNA synthesis occurs.

• May last for years, or the cell

may go immediately into

Meiosis II.

• May appear similar to

Interphase of Mitosis.

Meiosis II

• Steps are the same as in

Mitosis. • Prophase II

• Metaphase II

• Anaphase II

• Telophase II

Meiosis - Results

• 4 cells produced.

• Chromosome number halved.

• Gametes or sex cells made.

• Genetic variation increased.

Sexual Sources of Genetic

Variation

1. Independent Assortment of

Chromosomes during Meiosis.

2. Random Fertilization.

3. Crossing Over.

Independent Assortment

• There are 23 pairs of

chromosomes in humans.

• The chance to inherit a single

chromosome (maternal or

paternal) of each pair is 1/2.

Gamete Possibilities

• With 23 pairs of chromosomes,

the number of combinations of

chromosome types

(paternal and maternal) are:

223 or 8,388,608

Random Fertilization

• The choice of which sperm fuses

with which egg is random.

Random Fertilization

• Therefore, with 8,388,608 kinds

of sperms and 8,388,608 kinds

of eggs, the number of possible

combinations of offspring is

over 64 million kinds.

Result

• Is it any wonder that two

offspring from the same human

parents only resemble each

other and are not identical

twins?

Crossing-Over

• The exchange of sister

chromatid material during

synapsis.

• Occurs ONLY in Prophase I.

Chiasmata

• The point of contact where two

chromosomes are crossing-

over.

Importance

• Breaks old linkage groups.

• Creates new linkage groups

increases genetic variation.

Importance

• Very common during meiosis.

• Frequency can be used to map

the position of genes on

chromosomes.

Comments

• With crossing over, offspring can never be 100% like a parent if sexual reproduction is used.

• Multiple cross-overs are common, especially on large chromosomes.

Comments

• Genes near the centromere do

not cross-over very often.

Summary

• Know how the chromosomes

separate during Meiosis.

• Know how Meiosis differs from

Mitosis.

• Know how sexual reproduction

increases genetic variation.