*

GOAL: To produce cells with HALF the chromosomes of the original cell.

Why would a cell want to do this?

PURPOSE: To make specialized reproductive cells called SEX CELLS!

(OR GAMETES OR SPORES)

*A SEX CELL IN ANIMALS AND PLANTS

(EGGS AND SPERM)

A spore in some types of plants and

eggs and sperm in other plants

*

*Chromosomes in cells come in pairs.

*The pairs originally formed when a egg cell is

fertilized by a sperm cell. During fertilization

the nuclei from each cell fuse together and the

chromosomes from one cell combine with the

chromosomes of the other cell doubling the

number of chromosomes.

*Example in humans 23 + 23 = 46 total

chromosomes. (one set of 23 from mom and

one set of 23 from dad.)

*

Diploid cells Double:

(2N) have two complete

sets of chromosomes.

Most of your cells are

diploid. The body cells

(skin, organ cells, brain

cells) of animals are all

diploid.

Haploid cells: Half

(1N) have one complete

set of chromosomes.

In animals, (sperm and

eggs) are haploid. In

plants ovum, spores,

and sperm are haploid.

*IN NORMAL HUMAN CELLS HOW MANY

CHROMOSOMES ARE PRESENT?

These are DIPLOID CELLS. There are 23 pairs of chromosomes.

Each pair originated from mom donating one representative of

each pair and dad doing the same.

IN SEX CELLS HOW MANY

CHROMOSOMES ARE PRESENT?

These are HAPLOID CELLS. That is why, once they combine with the

other sex cell during fertilization they form a diploid human body cells.

*

EXAMPLES OF GAMETES

IN ANIMALS- sex cells EGG CELLS called Ovules or Ovum

SPERM CELLS called spermatozoon

IN PLANTS- spores EGG CELLS called Ovules or Ovum

The sperm cells are contained in the pollen of plants.

Sperm are in the scrotum

Eggs are in the ovaries

Pollen in the pollen sacs of the

anther and ovules in the ovary

of the plant.

*

*Green students: What are diploid cells?

*How many chromosomes are found in a diploid

human cell?

*Where are they found?

*Gold students: What are haploid cells?

*How many chromosomes in a haploid human

cell?

*Where are they found?

*

*If you don’t have a smart phone you may use a

school computer.

*We want to know the definition for

homologous chromosomes.

*Ready go!

*

*Homologous chromosomes are identical in size and shape

and carry the same genes on them as their partner.

*In cells of eukaryotic organisms chromosomes come in

pairs. Where did these pairs originate again?

*One set from mom, and one set from dad.

*When an egg is fertilized the “pair” of chromosomes is

formed.

*Is the newly fertilized cell haploid or diploid?

*

*What is the purpose of meiosis?

*What is a gamete?

*What is the goal of meiosis?

*Where does meiosis take place in plants?

*What does it mean if chromosomes are

homologous?

*If both homologous chromosomes are present

in a cell is the cell haploid or diploid?

*

If gametes are to form they must organize the chromosomes in

a cells nucleus so that only one representative of each pair of

homologous chromosomes are present in the gamete.

THE PROCESS THAT ACCOMPLISHES THIS IS

CALLED:

*

*

1) REDUCTION DIVISION called

Meiosis I.

2) MITOTIC DIVISION called

Meiosis II

*

MEIOSIS I: There are 5 stages.

Interphase I

Prophase I

Metaphase I

Anaphase I

Telophase I

SOUND FAMILIAR?

*

*

*Same as mitosis

*

*Chromatin condense to

become chromosomes

*Nuclear membrane and

nucleolus disappear

*The centrioles form and

move toward the poles

*CROSSING OVER

(exchange of genes)

takes place. More on

this later!

*

*Chromosomes line up at

the middle IN PAIRS!

with help from spindle

fiber microtublues

*

*Independent Assortment

of chromosomes

*Pairs of chromosomes

move AWAY from one

another. Homologous

chromosomes separate

but chromatids remain

together.

*

*Chromosome number is

cut in half

*Cells start to form new

nuclear membranes

*BUT…Each cell has twice

the amount of DNA

(sister chromatids have

not separated)

*

After meiosis I you are left with the

amount of cells that you started with but they are

NOT identical…they have the amount of

chromosomes that they started with.

You now have 2 HAPLOID cells (but remember they

each still have double the amount of DNA as sister

chromatids have not separated.)

*

*

There is no Interphase II

because DNA has already

replicated.

STEPS OF MEIOSIS II ARE VERY MUCH

LIKE MITOSIS!

*

*Begin with the two new

cells made from meiosis I

*Spindle fibers reform at

centrioles

*

*Chromosomes line up at the

equator NOT in pairs this

time!

*They line up in a single file

line (sister chromatids are

lined up)

*(just like in mitosis)

*

*Centromeres separate so that sister chromatids

can be pulled apart.

*Source of genetic recombination

* Independent Assortment of chromatids

*

*Nuclear membranes

reappear.

*Spindle fibers disappear.

*Chromosomes become

chromatin.

*Then…cytokinesis

*

*From 1 cell we get 4 different daughter

cells.

*Each cell has half the number of

chromosomes but the right amount of

DNA.

*Only one representative of each

homologous chromosome pair remains in

each new cell. One is the loneliest

number

*Gametes (sex cells) have been formed.

*

*

*Why might Life on earth be better because of

sexual reproduction?

*Consider your family history? Are all your

siblings, relatives, mom and dad, exactly like

you? Why?

*There are several ways in which sexual

reproduction and meiosis create genetic

divesity.

*Combination of egg and sperm, crossing over

and independent assortment.

*

*Homologous pairs of chromosomes exchange

sections of DNA, I know, crazy huh?

*Occurs during prophase I

*Why would the homologous chromosomes do this?

*Increases the genetic variation possibilities for the

offspring.

**Homologous

chromosomes will

swap genetic

material in a

process known as

crossing over

* Crossing over

serves to increase

genetic diversity

by creating four

unique chromatids

* This can happen in

many spots

between

homologous

chromosomes.

*

*

*

*

*