Mrs. Griffin. The birds and the bees… Humans and most other animals reproduce sexually. This...

Mrs. Griffin

The birds and the bees…

Humans and most other animals reproduce sexually.

This means that two cells fuse to make a new “baby” organism.

These specialized cells are called gametes or sex cells

Sperm and egg cells are human gametes

Meiosis What is it?

Who does it?

Where does it happen?

Why does it happen?

Why is a different process needed?

Making sex cells (sperm and egg)

All sexually reproducing organismsIn males- testes

In females- ovaries

To all organisms to reproduce

1) Chromosome # needs to be cut in half2) Creates genetic diversity

Meiosis

MITOSISDiplo

idzygot

e

Diploid

adult

MEIOSIS

Haploidgamete

s

Remember, humans have 23 pairs of chromosomes

Pairs 1-22 are said to beHomologous chromosomes(they go together becausethey have same traits but arenot identical)

Gene for eye color

Gene for hair color

MeiosisMeiosis is a special kind of cell

divisionthat halves the number of

chromosomesin gametes. This insures that the

correctnumber of chromosomes are

passed on.

The goal is to makehaploid sex cells

First division

Second division

MotherFather

Polar Bodies (not

functional)

What would happen if twocells with 46 chromosomes

fused?

46 chromosomes+

46 chromosomes

92 chromosomes….

Nondisjunction =chromosomes fail to “dis-join” or separate:

Trisomy=gamete with 3 of one type

Monosomy=gamete with 1 of one type

Tripoidy= zygote with three of one type

Down Syndrome= 3 of #21

Karyotype

Klinefelter’s = XXY

Some symptoms… Delayed puberty Loss of libido - reduced interest in sexual activity Impotence Reduced facial hair - reduction in need to shave Tiredness Reduced muscle power and stamina Change in body shape - increased fat and breast

development Infertility Osteoporosis - brittle bones resulting in fractures Depression and sometimes dissocial behavior Early heart disease

Turner’s Syndrome

XO sex chromosome

Instead of XX female

or XY for male

Meiosis has two stages:Meiosis 1and Meiosis 2

Meiosis 1: This is the stage where the chromosome number is halved

Includes 4 stages:Prophase 1Metaphase 1Anaphase 1Telophase 1

Meiosis 1 Prophase 1 is

similar in some ways to prophase in mitosis.Chromosomes

condense

Spindle fibers appear

Nucleus and nucleolus disappear

UNLIKE in mitosis, homologouschromosomes line up next to each other during prophase

•This process iscalled synapsing

•Lined uphomologues arecalled tetrads

When homologous chromosomeshang out so close to each

other,they sometimes swap parts.This is called “crossing over”

Synapsis and Crossing Over in Action

Crossing over increases

genetic diversity

Why is this important

for a species’long-term survival?

During metaphase 1, tetradsline up on a metaphase plate

Then spindle fibers pull thehomologous chromosomes apart inanaphase 1

In telophase 1, two daughter cells are formed. They are NOT identical! (Why?)

After Meiosis 1…

Homologous chromosomes have been separated.

Two non-identical daughter cells have been formed.

The chromosome number has been cut in half.

The big picture

Chromosomes are NOT

duplicated again between

Meiosis 1 and Meiosis 2

Why not?

Meiosis 2 is similar to mitosis Two identical daughter cells are

formed from each of the cells created in Meiosis 1

Includes 4 Stages: Prophase 2 Metaphase 2Anaphase 2Telophase 2

Meiosis II

Meiosis 2 Prophase 2: spindle reforms and

chromosomes move toward the metaphase plate

Metaphase 2: sister chromatids lined up on the metaphase plate

Anaphase 2: sister chromatids are separated and pulled toward opposite poles of the cell

Telophase 2 and Cytokinesis: nuclei form at either pole and each cell is finally divided into two identical daughter cells

All together now…

Let’s compare mitosiswith meiosis…