Warm UpUse the following words in 2-3

sentences to demonstrate what you remember about Mitosis:

division, chromosomes, copying, cells, PMAT

Cell Division – Mitosis (Review) – Division of a somatic (body) 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 cells (ex: liver, heart)

• 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 (body) 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

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 “good” 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

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

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”

• 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)