SEXUAL REPRODUCTION

Sex cells formed by meiosis

Requires 2 parents

Offspring not identical

The process that results in the formation of gametes

Sex cells, egg or sperm, which have haploid chromosomes

Haploid means HALF, as in the number of chromosomes,written as n

Meiosis

Gametes

Haploid

Fertilization

The union of an egg and a sperm

The cell that is formed when the egg becomes fertilized by thesperm. This results in a full set of chromosomes.

All body cells except for sex cells that have diploid chromosomes

Diploid means they have chromosome found in pairs,written as 2n

n

n 2nZygote

Somatic Cells

Diploid

Homologous Chromosomes

Similar chromosomes that are found in pairs. The paired chromosomes come from the mother and father.

* Human body cells have 46 chromosomes each

* Human body cells have 23 homologous pairs

Meiosis and Fertilization

Important for survival of many species, because these processesresult in genetic variation of offspring.

Meiosis

A kind of cell division that results in gametes (sex cells) withhalf the number of chromosomes.

A. Gametes do not have homologous pairs, but instead they have only half the chromosomes from each pair. Gametes are referred to as haploid (n).

B. When fertilization occurs, the zygote will have a normal number of chromosomes (2n).

C. In meiosis, specialized cells divide 2 times producing 4 daughter cells.

Phases of Meiosis (2 Cell divisions – Meiosis I & Meiosis II)

MEIOSIS I

Prophase I

Homologous chromosomes pair together to form tetrads

During Prophase I, crossing over occurs between homologouschromosomes and genes are exchanged causing genetic variation

Metaphase I

Spindle fibers attach to chromosomes and line them up in themiddle of the cell.

Anaphase I

The fibers pull the homologous chromosomes apart. This is called disjunction. Chromosomes move toward opposite sidesof the cell.

Telophase I

During Telophase I and cytokinesis, the nuclear membrane formsand the cell splits into two new cells.

**** The Chromosome number is reduced to half during Meiosis I forming haploid (n) daughter cells

MEIOSIS II

Prophase II

Spindle fibers form and attach to double stranded chromosomes

Metaphase II

Chromosomes line up in the middle of the cell

Anaphase II

Sister chromatids separate and move towards opposite sides of the cell

Telophase II

Cells divide, resulting in 4 haploid cells – gametes (sex cells)

Males form 4 haploid cells which develop into 4 sperm

K K K K

K K K K

Females form 4 haploid cells which develop into 1 egg and3 small polar bodies

EggPolar Bodies

Small polar bodies are not used and break down.

MeiosisMeiosis

Parent cell – chromosome pair

Chromosomes copied

1st division - pairs split

2nd division – produces 4 gamete cells with ½ the original no. of chromosomes

Meiosis – mouse testes Meiosis – mouse testes Parent cell

4 gametes

1st division

2nd division

Figure 13.7 The stages of meiotic cell division: Meiosis I

Figure 13.7 The stages of meiotic cell division: Meiosis II

One Way Meiosis Makes Lots of One Way Meiosis Makes Lots of Different Sex Cells (Gametes) –Different Sex Cells (Gametes) – Independent AssortmentIndependent Assortment

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

That’s a lot of diversity by this That’s a lot of diversity by this mechanism alone.mechanism alone.

In humans, n = 23In humans, n = 23 and 2 and 22323 = 6,000,0000= 6,000,0000. .

Another Way Meiosis Makes Lots of Different Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-OverSex Cells – Crossing-Over

Crossing-over multiplies the already huge number of different gamete Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.types produced by independent assortment.

The Key Difference Between Mitosis and Meiosis is The Key Difference Between Mitosis and Meiosis is the Way Chromosomes Uniquely Pair and Align in the Way Chromosomes Uniquely Pair and Align in

Meiosis Meiosis

MitosisMitosis The first (and The first (and distinguishing) distinguishing) division of meiosisdivision of meiosis

Boy or Girl? The Y Chromosome “Decides”Boy or Girl? The Y Chromosome “Decides”

X chromosomeX chromosomeY chromosomeY chromosome

Boy or Girl? The Y Chromosome “Decides”Boy or Girl? The Y Chromosome “Decides”

Meiosis – division errorMeiosis – division error

Chromosome pair

Meiosis error - Meiosis error - fertilizationfertilization

Should the gamete with the Should the gamete with the chromosome pair be fertilized chromosome pair be fertilized then the offspring will not be then the offspring will not be ‘normal’.‘normal’.

In humans this often occurs In humans this often occurs with the 21with the 21stst pair – producing pair – producing a child with Downs Syndromea child with Downs Syndrome

21 trisomy – Downs 21 trisomy – Downs SyndromeSyndrome

Can you see the extra 21st chromosome?

Is this person male or female?