Vocabulary:Homologous

Diploid

Haploid

Meiosis I and II

Tetrad

Crossing-over

Gametes

Key Concepts:

What happens during the process of

meiosis?

How is meiosis different than

mitosis?

Genes, Chromosomes, and Numbers

● Organisms have tens of thousands of genes

that determine individual traits;

● Where are genes??...

lined up on chromosomes!

● a typical chromosome can contain a

thousand or more genes

Chromosomes and Genes

● in the “body cells” of animals and most

plants, chromosomes occur in pairs;

● one chromosome in each pair came from

the female parent, and the other came from

the male parent

VOCABULARY:

● Homologous chromosomes = paired

chromosomes; 1 came from mom, and

the corresponding chromosome came

from dad

● Diploid = has 2 sets of chromosomes

(2n); all “body cells” are diploid

● Haploid = one set of chromosomes (n)

-in the gametes (sex cells)

-when egg joins with sperm, the diploid # of

chromosomes is restored!

● MEIOSIS is a specialized type of cell division

that occurs in the formation of gametes such

as egg and sperm.

● Meiosis appears much more complicated than

mitosis…

-it is really just two divisions in sequence

-each one of which has strong similarities to

mitosis.

INTERPHASE:● Interphase prior to meiosis is

identical to interphase prior to

mitosis

● This includes the copying of

DNA in the S phase!

● Meiotic division will only occur in cells associated with male or female sex organs.

MEIOSIS I

● Meiosis I, the first of the two divisions, is often called reduction division

it is here that the chromosome number is reduced from 2n (diploid) to n (haploid).

Ex: In humans, the

diploid number = 46

(haploid = 23)

PROPHASE I● Prophase I of meiosis

is similar to prophase

in mitosis

chromosomes

condense and become

visible

spindle fibers

develop

nuclear envelope

breaks down

Prophase I - Important Events

● Homologous chromosomes pair up to form a TETRAD

● The sister chromatids press together at points along their length

● It is during this alignment that chromatid arms may overlap and temporarily fuse and exchange segments resulting in crossing over

Result: chomosomes (consisting of 2 sister chromatids) are no longer just from “mom” or “dad”—the chromosomes are a mixture!!!

More VOCABULARY:

• Tetrad—each chromosome pairs with homologous chromosome

– 4 sister chromatids!

• Crossing over– when chromosomes are in tetrads, they may exchange parts

– This happens in Prophase I

– Result – increased variety in offspring (a good thing)

Result: INCREASED genetic variety!!!

METAPHASE I

• Here is where the critical

difference occurs between

Metaphase I in meiosis and

metaphase in mitosis.

METAPHASE I

• In MITOSIS, all the chromosomes line up in single file on the metaphase plate in no particular order.

• In Metaphase I (of Meiosis), the chromosome PAIRS are aligned on either side of the metaphase plate.

ANAPHASE I

● During Anaphase I the

spindle fibers shorten

-the homologous pairs

are pulled away from

each other toward

each pole of the cell.

TELOPHASE I● spindle fibers break down;

● the nuclear membrane (envelope) may or may not

reform, and the chromosomes do not

disappear.

At the end of Telophase I:

● each daughter cell has a single set of

chromosomes

● half the total number in the original

cell where the chromosomes were

present in pairs.

● chromosomes are still in doubled

state (1 chromosome = 2 sister

chromatids)

● While the original cell was diploid (2n), the daughter cells are now haploid (1n).

● This is why Meiosis I is often called reduction division.

Example: if the

-Diploid # = 4, then the

-Haploid # = 2

It was 4, now it will be

2 in each cell!!

MEIOSIS II

● Meiosis II is quite simple in that it is simply

a mitotic division of each of the haploid cells

produced in Meiosis I.

● There is no interphase between Meiosis I

and Meiosis II…

● Meiosis II begins with…

…PROPHASE II

● A new set of spindle

fibers forms

● the chromosomes

begin to move toward

the center of the cell

● no longer with their

homologous partner!

METAPHASE II

● The chromosomes

in each haploid cell

align in the center of

the cell… this time

in single file!!

ANAPHASE II

● The centromeres split

● The spindle fibers shorten

-pulls the single

chromosomes toward each

pole of the cell.

TELOPHASE II

● the nuclear envelope reforms

● the chromosomes begin to uncoil, converting back to chromatin

● total of four daughter cells, each with half the total number of chromosomes as the original cell.

● followed by CYTOKINESIS

NOVA video

http://www.pbs.org/wgbh/nova/baby/divi_flash.html

● MALE GAMETES:

-all four haploid

cells will eventually

develop into mature,

functional sperm

cells.

● FEMALE GAMETES:

(as seen in life cycles in

“higher” organisms)

-the cytoplasm and cellular

organelles are divided

unequally

-three of the cells will

typically abort

-leaves a single cell to

develop into a mature egg cell

**usually much larger than a typical sperm cell.

Gametes