Cell Division (Continued) – 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

Parent cellChromosomes

duplicate 2 new daughter cells identical to

parent cell

• Daughter cells are genetically identical to parent cell – same kind and number of chromosomes

• Mitosis occurs in somatic or body cellsEx: 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

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 only• Males (XY) – sex organs are the testes in humans• Females (XX) – sex organs are the ovaries in humans• Meiosis also occurs in the sex organs of other

animals, plants, fungi, etc…

Meiosis

Mitosis

Parent cell

Chromosomes duplicate

Division 1

Division 2

Daughter cells have half as

many chromosomes as parent cell

Meiosis

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

4 sperm • 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.

• Egg and sperm (sex cells) are also called gametes

• 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)

WRONG!!!

23

23

46

Zygote develops into embryo and finally adult

organism by mitosis

Fertilization – process by which an egg and sperm unite

Zygote – fertilized egg

Embryo – organism in early stage of development

Fertilized egg – zygote

• Without meiosis …………

Fertilized egg – zygote

46

46

92

Unique events in Meiosis • 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 2nd 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

No crossing over – daughter cells are identical to

parent cells

Crossing over occurs –causes

genetic variation

(Daughter cells are NOT identical

to parent cell)

Comparing Mitosis and Meiosis

1. Unique features of Meiosis2. Comparison of Meiosis and Mitosis

Mitosis Meiosis

What kind of cells? Somatic cells Male (XY) = SpermFemale (XX) = Egg

When does this occur? Any timeMale (XY) = pubertyFemale (XX) = before birth

# of Divisions(Draw picture) 1 2

# of Daughter cells 2Male (XY) = 4 spermFemale (XX) = 1 viable egg

# of ChromosomesSame as parent cell

diploid or 2N In humans 46

Half as many as parent cellhaploid or NIn humans 23

Type of Reproduction Asexual Sexual

Genetic CompositionDaughter cells

identical / not identical to parent cell

Daughter cells identical / not identical

to parent cellGenetic variation

Pairing of Homologous Chromosomes YES / NO YES / NO

Crossing over of genes

Function/Importance Growth, repair; development of adult from zygote

Production of gametes: eggs and sperm

Sex Cells