Meiosis Notes

Meiosis- cell division that producing reproductive cells called gametes (or egg and sperm)

Meiosis involves replicating DNA once and dividing the nucleus two times in steps called

Meiosis I and Meiosis II.

By dividing twice, the chromosome number is halved in gametes.

Slide 1

- 2 parents

- offspring genetically similar to parents

(combination of parent chromosomes) (offspring not identical)

Gamete: Sex cellFemale: egg

Male: sperm

Zygote: Union of sperm and egg 46

23

23

2n

n

n

(Diploid)

(Haploid)

Gonads: Male – Testes Female – Ovaries

Slide 2

Meiosis: Process of cell division used to producegametes (sex cells)

46SC

46DC

23 DC

23 DC

Interphase

PMAT

23SC

23SC

23SC

23SC

PMAT

IIII

IIIIIIII

Primary Sex Cell

Secondary Sex Cells

Sex Cells

2n

2n

n n

n n n n

Slide 3

What happens in PMAT is the same in meiosis as in mitosis

EXCEPT in…….

Prophase I

Anaphase I

Slide 4

Prophase I of Meiosis I

Sister chromatids pair with their homologous pair, forming a tetrad

*Double chromosomes made during interphase*

Slide 5

Homologous ChromosomesHumans have 23 chromosome pairs, each chromosome in the pair contains

the same kind of traits, like eye color; they are called homologous chromosomes. (Humans have 23 homologous chromosomes for a total of 46 chromosomes

Having two copies of every gene can be an advantage. For example, if the one gene from Dad was damaged, Mom’s good gene could take over.

Slide 6

Sister Chromatid

Sister Chromatid

Slide 7

TetradMaternal chromosome replicated

Paternal chromosome replicated

Slide 8

Prophase I

tetrad

Joining of homologous chromosomeat the centromere forming a tetrad

Slide 9

A Way Meiosis Makes Lots of Different Sex Cells A Way Meiosis Makes Lots of Different Sex Cells Crossing-OverCrossing-Over

Slide 10

Crossing Over: - increases variation

-changes chromosome package -(not same as either parent)

Pieces of homologous chromosomes switch locations

P

Bld

Blu

Fair

M

Brn

Brn

Drk

P

Bld

Blu

Drk

M

Brn

Brn

Fair

Slide 11

Metaphase I of Meiosis I

Homologous pairs (tetrads) line up in the center of the cell.

Slide 12

Anaphase I of Meiosis I

Spindle fibers pull the homologues (tetrad)

away from each other during disjunction.

Slide 13

Telophase I of Meiosis I

The chromosome number in these cells divides in half; starting with an initial four (2n) and ending with two (n) in each new cell.

Slide 14

Meiosis II: Similar to Mitosis

In Meiosis II each chromosome lines up and sister chromatids separate from each other.

Prophase II Metaphase II Anaphase II Telophase II

Slide 15

http://www.biologyinmotion.com/cell_division/

Meiosis in males Meiosis in femaleOccurs in testes Occurs in ovaries

2n 2n

2n 2n

n n n n

n n n n n n n4 sperm 1 egg polar bodies

1:4 1:1

n3 polar bodies

Slide 16

Meiosis does two things -Meiosis does two things -

1) Meiosis takes a cell with 1) Meiosis takes a cell with two copiestwo copies of every of every chromosome (diploid) and makes cells with a chromosome (diploid) and makes cells with a single copysingle copy of every chromosome (haploid). of every chromosome (haploid).

This is a good idea if you’re going to combine This is a good idea if you’re going to combine two cells to make a new organism. This trick two cells to make a new organism. This trick is accomplished by is accomplished by halvinghalving chromosome chromosome number. number.

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

Slide 17

2) Meiosis 2) Meiosis scramblesscrambles the specific forms of the specific forms of each gene that each sex cell (egg or sperm) each gene that each sex cell (egg or sperm) receives. receives.

This makes for a lot of This makes for a lot of genetic diversitygenetic diversity. This . This trick is accomplished through trick is accomplished through independent independent assortmentassortment and and crossing-overcrossing-over..

Genetic diversity is important for the Genetic diversity is important for the evolution evolution of populations and speciesof populations and species..

Slide 18

MeiosisMeiosis

Parent cell – chromosome pair

Chromosomes copied

1st division - pairs split

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

What if Meiosis doesn’t happen as planned?!?!

Slide 19

Meiosis – division errorMeiosis – division error

Chromosome pair

Slide 20

Meiosis error - fertilizationMeiosis error - fertilization

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

Slide 21

Trisomy 21– Downs SyndromeTrisomy 21– Downs Syndrome

Can you see the extra 21st chromosome?

Is this person male or female?

Slide 22

Anaphase I

Disjunction: Separation of tetrad at the centromere

Nondisjunction: Uneven separation of a tetrad during disjunction

Ex. Down’s Syndrome (nondisjunction of 21st chromosome tetrad)

Polyploidy: Nondisjunction of all tetrads-gamete has complete extra set of chromosomes

Animal cells – fatalPlant cells – larger more vigorous variety

Slide 23

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

X chromosomeX chromosomeY chromosomeY chromosome

Slide 24

Boy or Girl? The Y Chromosome “Decides”Boy or Girl? The Y Chromosome “Decides” Slide 25

How does the image in the first picture become the person in the last picture?

Slide 26

http://www.pbs.org/wgbh/nova/baby/divide.html#

Type of cells that perform:

Uses:

Number of cell divisions:

Chromosome counts:

Genetic Variation?:

Number of daughter cells:

Mitosis: Meiosis:

Somatic

1

2n2n

None

Gametes

Sexual Reproduction

2

2nn

Yes (crossing over)

Growth, Repair,Asexual reproduction

2 4 sperm/1 egg

Slide 27

- Organism that normally contains both sexes (ovaries and testes) - Usually does not self fertilize

Example: earthworm

-Development of an unfertilized egg to a complete organismExample: male drones bees

- Fertilized eggs give rise to queen and female workers

Slide 28

Union of the sperm and the egg forming a zygote

Occurs outside body of female

Occurs inside bodyof female

Produces more eggs (1000+) Produces less eggs (1-12)

Occurs in water Secretes fluid to providewatery environment

FishAmphibians

BirdsReptilesMammals

Slide 29