Cell Division – Meiosis

Chromosomes are DNA! Chromosomes contain genetic information

Cell Division – Mitosis (Review) – Division of a somatic cell that results in 2 genetically

identical daughter cells

Cells must divide for growth, repair of tissues, and asexual reproduction

Cell division begins in Interphase when the chromosomes duplicate

Daughter cells genetically identical to parent cell – same kind and number of chromosomes Mitosis occurs in somatic or body cells

Ex: liver, heart, skin, stomach Every organism has its own unique number of

chromosomes. Humans have 46. This is called its diploid number or the total number of chromosomes in a somatic cell.Diploid means “2 sets” and is written as “2N”.

Body cells of adult organisms have 2 sets of homologous (matching) chromosomes – 1 set from female parent and 1 set from male parent

2 new daughter cells

identical to parent cell

Chromosomes duplicateParent cell

Cell Division –Meiosis – the process in which the number of chromosomes in the original cell is reduced by HALF through the

separation of homologous chromosomes

Meiosis occurs in sex organs only1. Males (XY) – sex organs are the testes in humans2. Females (XX) – sex organs are the ovaries in humans3. Meiosis also occurs in the sex organs of other animals, plants, fungi, etc…

Meiosis produces sex cells – cells with ½ the number of chromosomes as the original cell

1. Males – meiosis produces 4 sperm

2. Females – meiosis produces 1 (viable) egg The other 3 cells are called polar bodies – they give up their cytoplasm to nourish the 1 good egg.

3. Egg and sperm (sex cells) are also calledgametes

Daughter cells (gametes)have half as many

chromosomes as parent cell

Division 2

Division 1Chromosomes

duplicateParent cell

4. Gametes have ½ the number of chromosomes as somatic (body) cells. We call this the haploid number. Haploid means “1 set” and is written as “N”.

If human diploid number is 46, what is its haploid number? 23

Diploid # of a dog – 78 Haploid # of a dog – 39Diploid # of a fly – 8 Haploid # of a fly – 4

When does meiosis occur in humans?

1. Males beginning at puberty2. Females before birth – all eggs are produced before birth and at puberty eggs mature

Chromosome Number Remember, chromosome number is unique to each kind of organism and all cells (except sex cells) in an

organism have the same kind and number of chromosomes.Ex: All humans have 46 chromosomes and all cells in the human body (except sperm and egg) have 46 chromosomes.

This is why the chromosome number in sex cells must be reduced in half by meiosisEx: Humans have 46 chromosomes in their somatic cells, but 23 chromosomes in their sex cells (egg and sperm)

Fertilization – process by which an egg and sperm uniteZygote – fertilized eggEmbryo – organism in early stage of development

Without meiosis …………

Unique events in Meiosis

Fertilized egg – zygote

Zygote develops into embryo and finally adult

organism by mitosis46

23

23

Fertilized egg – zygote

92

46

46

Homologous (matching) chromosomes pair up before 1st cell divisionHomologous chromosomes:

-look alike-code for same traits-receive one from each parent

During 1st division, homologous chromosomes exchange genes during process called “crossing over”

These homologous chromosomes separate during 2 nd division of meiosis – so chromosomes in gametes are different from each other due to crossing over

Crossing over increases genetic variation and is the reason why siblings look different

Comparing Mitosis and Meiosis

No crossing over – daughter cells are

identical to parent cells

Crossing over occurs –causes genetic variation(Daughter cells are NOT identical to parent cell)

Meiosis Mitosis

What kind of cells?Male (XY) = Testes

Female (XX) = OvariesSomatic cells

When does this occur?

Male (XY) = puberty

Female (XX) = before birthAny time

# of Divisions(Draw picture)

2 1

# of Daughter cellsMale (XY) = 4 sperm

Female (XX) = 1 viable egg2

# of ChromosomesHalf as many as parent cell

haploid or NIn humans 23

Same as parent celldiploid or 2NIn humans 46

Type of Reproduction Sexual Asexual

Genetic Composition

Daughter cellsidentical / not identical

to parent cellGenetic variation

Daughter cellsidentical / not identical

to parent cell

Pairing of Homologous Chromosomes

YES / NOCrossing over of genes

YES / NO

Function/ImportanceProduction of gametes:

eggs and spermGrowth, repair; development of

adult from zygote

Sex Cells