Mitosis and Meiosis

• Chromosomes are made up of DNA which carries the cells genetic information

• Human body cells have 46 chromosomes (in 23 pairs)

• Sex cells (gametes aka egg and sperm) only have 23 chromosomes

• Before cell division occurs, each chromosome replicates (or copies itself)

ChromosomesSister Chromatids

Centromere

Cell Growth and Division

Cell Cycle is a series of events that cells go through as they grow, prepare for division and divide into two identical daughter cells

Two major stages in a cells life are:1.Interphase2.M Phase or cell division

Interphase

Interphase is the phase where the cell grows and prepares to divide.

Interphase is divided into three phases:• G1 Phase - growth• S Phase – replication (copying) chromosomes• G2 Phase – further growth* A lot is happening during G1, S and G2 phases!

M Phase

M Phase is cell division which includes two steps:

1.Mitosis- this is division of the nucleus

2.Cytokinesis – this is division of the cytoplasm

MitosisMitosis is divided into four phases:1.Prophase – chromosomes condense,

becoming visible; centrioles move to opposite sides of nucleus; nuclear envelope dissolves

2.Metaphase – chromosomes line up across middle of cell

3.Anaphase – sister chromatids split4.Telophase - sister chromatids move to

opposite sides of cell and new nuclear envelopes form

Cytokinesis

• Cytokinesis is occurring at the same time as telophase

• The cell membrane pinches the cytoplasm into two equal parts.

• Two new daughter cells are now formed and are genetically identical to the parent cell.

• This is the end of the cell cycle.

MEIOSISMeiosis is process where the number of

chromosomes per cell is divided in half by separating homologous chromosomes in a diploid cell.

Gametes are produced by meiosis.

What does all that mean???

• Chromosomes are made up of DNA which carries the cells genetic information

• Genes are found on the chromosomes.• Genes are chemical factors that determine

traits. (Ex. Eye color, hair color, height, etc.)• Different forms of a gene are called alleles (Ex. Blue or brown eyes, brunette or auburn)

* The symbol N represents the number of chromosomes in a gamete (sex cell)

Homologous chromosome means that each chromosome that comes from the male has a corresponding chromosome from the female

Homologous chromosome have DNA segments, or genes, for the same trait

• Diploid cells have both sets of homologous chromosomes (one set from momma and one from the baby’s daddy)– All cells in your body (except egg and sperm) are

diploid– These cells are called somatic cells– 2N

• Haploid cells have half the number of chromosomes as their parent cells

• Gametes (egg and sperm) are haploid

• Gametes are genetically different than the parent cell and one another

• N

2N

N

N

N

N

N

N

Human has 46 chromosomes. How many chromosomes in human gametes?

23Pineapple has 25 chromosomes in its gametes.

What is it’s haploid number?

25A giraffe has a diploid number of 62. What is it’s

haploid number?

31

Draw this!

46

First Step

Replication• During meiosis, chromosomes are replicated

once in S phase (just like mitosis) into sister chromatids.

• This occurs so that each of the 46 chromosomes exists as a duplicated sister chromatid.

Meiosis I

• HOMOLOGUES (1-23) are segregated into different cells.

• Results in 2 cells that each have 1 copy of each of the 23 chromosomes (still duplicated from DNA replication).

Prophase ICrossing over• Homologous chromosomes in

the cell pair up alongside each other lengthwise forming a tetrad

• Swap bits and pieces of their chromosomes, shuffling the genome

• Results in NEW Chromosomes that did not exist before in the parent

Meiosis II

• SISTER CHROMATIDS of each of the 23 chromosomes are pulled apart just as they are in mitosis

• Results in 4 cells that have 1 copy of each of the 23 chromosomes (now unduplicated again)

• Now officially haploid

Meiosis occurs in two stagesMeiosis I • Two cells form with

sets of chromosomes and alleles that are different from each other

Meiosis II• Cells divide again

without DNA replication forming four cells with half the number of chromosomes of the parent cell

Summary of Mitosis/Meiosis

Mitosis• Results in 2 identical cells that are diploid (2N)

and are exactly like the parent cell

Meiosis• Results in 4 cells that have half the number of

chromosomes, haploid (N), as the parent cell and are genetically different from one another

• Genetic Recombination

Meiosis Square Dance

Linked Genes - Genes located closely together are linked

Crossing over is more likely to occur between genes that are far apart rather than linked which are close together.

Polyploidy – The occurrence of one or more extra chromosomes in an organism. This is not good in animals but is beneficial in plants (crop plants are stronger and bigger)