Why do you share some but not all characters of

each parent?

Heredity• Passing of

genetic traits from parent to offspring

Chromosome theory of heredity• Chromosomes

carry genes• Gene = unit of

heredity

Meiosis allows the creation of unique individuals through sexual reproduction.

Somatic Cells are “body” cells and contain the normal number of chromosomes . Examples would be … skin cells, brain cells, etc. Diploid

Gametes are the “sex” cells and contain only ½ the normal number of chromosomes. Haploid Sperm cells and ova are gametes.

Pair of chromosomes Pair of chromosomes (maternal and paternal) similar in shape and size.

Each locuslocus (position of a gene) is in the same position on homologues.

Humans have Humans have 23 pairs 23 pairs of of homologous chromosomeshomologous chromosomes..• 22 pairs of autosomespairs of autosomes• 1 pair of sex chromosomespair of sex chromosomes

Homologous pairs Homologous pairs (tetrads are duplicated) carry genes controlling the same inherited traits.

PaternalPaternal MaternalMaternal

eye coloreye color locuslocus

eye coloreye color locuslocus

hair colorhair color locuslocus

hair colorhair color locuslocus

• In Humans the In Humans the “Autosomes” “Autosomes” are sets 1 – 22are sets 1 – 22

• (The Autosomes (The Autosomes code for most of code for most of the offspring’s the offspring’s traits)traits)

XX chromosome - femaleXX chromosome - female XY chromosome - maleXY chromosome - male

In Humans the “Sex In Humans the “Sex Chromosomes” are Chromosomes” are

the 23the 23rdrd set set

Sex Chromosomes Sex Chromosomes the 23the 23rdrd set set

23

X chromosome

Y chromosome

Reproduction where the genetic material combined is called sexual reproduction

Two cells, a sperm and an egg, unite to form a zygote, the single cell from which the organism develops

Meiosis is the process of producing sperm and eggs (gametes) – the number of chromosomes are halved

1) Meiosis takes a cell with 1) Meiosis takes a cell with two copies of every two copies of every chromosome (chromosome (diploiddiploid) and ) and makes cells with a single makes cells with a single copy of every chromosome copy of every chromosome ((haploidhaploid). ).

In meiosis, one diploid cells In meiosis, one diploid cells produces produces four haploid cellsfour haploid cells..

2) Meiosis scrambles the genes that each sex cell (egg or sperm) receives.

This makes for a lot of genetic diversity. This happens through independent assortment and

crossing-over. Genetic diversity is important for the evolution of

populations and species.

MeiosisMeiosisParent cell – chromosome pair

Chromosomes copied

1st division - pairs split

2nd division –4 gamete cells with ½ the original number of chromosomes

Gametes have exactly one set of chromosomes, this state is called haploid (1n)

Regular cells have two sets of chromosomes, this state is called diploid (2n)

Why?

Meiosis – mouse testes

Parent cellParent cell

4 gametes4 gametes

11stst division division

22ndnd division division

Similar to mitosis interphase.

Chromosomes replicate (S phase).

Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres.

Centriole pairs also replicate.

The Key Difference Between Mitosis and Meiosis is the Way Chromosomes

Uniquely Pair and Align in Meiosis

Mitosis

Meiosis 1st division

Unlike mitosis, homologous chromosomes line up next to each other during prophase

This process is called synapsing

Lined up homologues are called tetrads

Longest and most complex phase.

90% of the meiotic process is spent in Prophase I

Chromosomes condense. Synapsis occurs:

homologous chromosomes come together to form a tetrad.

Tetrad is two chromosomes or four chromatids (sister and non-sister chromatids).

centriolesspindle fiber

homologues

Homologous chromosomes

sister chromatids sister chromatidsTetrad

Prophase 1– Crossing Over

Homologous chromosomes connect at areas called chiasmata

Segments break and reform at similar locations.

This results in new genetic combinations

Non-sister chromatids

chiasmata: site of crossing over

variation

Tetrad

metaphase plate

OR

metaphase plate

Shortest phaseTetrads align on the metaphase plate.

INDEPENDENT ASSORTMENT

Independent AssortmentIndependent Assortment

Independent assortment produces 2n distinct gametes, where n = the number of unique chromosomes.

In humans, n = 23 223 = 6,000,0000.

That’s a lot of diversity by this mechanism alone

Homologous chromosomes separate and move towards the poles.

Sister chromatids remain attached at their centromeres.

Each pole now has haploid set of chromosomes.

Cytokinesis occurs and two haploid daughter cells are formed.

No interphase II (or very short - no more DNA replication)

same as prophase in mitosissame as prophase in mitosis spindle reforms and chromosomes spindle reforms and chromosomes

move toward the metaphase platemove toward the metaphase plate

same as metaphase in mitosissame as metaphase in mitosis sister chromatids lined up on the metaphase platesister chromatids lined up on the metaphase plate

metaphase platemetaphase plate

same as anaphase in mitosissame as anaphase in mitosis sister chromatids are separated and

pulled toward opposite poles of the cell

Same as telophase in mitosisSame as telophase in mitosisNuclei form, Cytokinesis occurs.Nuclei form, Cytokinesis occurs.Four haploid daughter cells produced.Four haploid daughter cells produced.

Example: 23 chromosomes in haploidn = 23

2n = 223 = ~ 6 million possible combinations!

At least 6 million combinations from Mum and another 6 million from Dad …

>36 trillion combinations for a diploid zygote!!!

In males is called spermatogenesisspermatogenesis and produces sperm.

In females is called oogenesisoogenesis and produces ova.

2n=46

n=23

n=23

meiosis I

n=23

sperm

meiosis II

4 sperm cells are produced from each primary spermatocyte

Primary Spermatocyt

e

Secondary Spermatocyte

n=23

n=23

n=23

The male gamete is called a spermatozoa Four spermatocytes are formed during meiosis Men are busy - meiosis produces roughly

250,000 sperm a day.

The female gamete is called an ovum

At birth each female carries a lifetime supply of developing oocytes, each of which is in Prophase I.

A developing egg (secondary oocyte) is released each month from puberty until menopause, a total of 400-500 eggs.

Only one ovum is produced during meiosisOogenesis places most of the cytoplasm into the large egg. The other cells, the polar bodies, do not develop.

Any Questions?