BiologyChapter 9-10

Mitosis & Meiosis

Chromosomes

• Rod shaped structures made of DNA and proteins.

• They are the cell’s DNA tightly packaged.DNA wraps around proteins, then keeps coiling until it forms a super coil. The super coil coils further until it becomes a chromosome.

Chromosome StructureChromosomes are only visible in the cell right before cell division.

The rest of the time the cell’s DNA is unraveled and called chromatin.

Right before cell division, the cell’s DNA makes a copy of itself.

X-shaped chromosomes, like this one are really TWO copies of the DNA held together.

Chromatids: The two halves of an x-shaped chromosome.The chromatids are two copies of the EXACT SAME THING

Centromere: The condensed area which holds the two chromatids together.

Chromosome NumbersEach species has a specific number of chromosomes in their cells.

Two Main Types of Chromosomes1. Sex Chromosomes – Those that determine

gender XX = FemaleXY = Male

2. Autosomes – Those chromosomes that do not determine gender.

Chromosome Number continued

Individuals formed through sexual

reproduction have TWO copies of each autosome

One copy from dad

One copy from mom

I’m from the Mom!

I’m from the Dad!

The two copies of each chromosome are called HOMOLOGOUS CHROMOSOMES.

Homologous chromosomes are the same size, the same shape and carry genes for the same traits.

Chromosome Number continued

• Karyotype:– A picture of the chromosomes in a dividing cell.– They pair up the homologous chromosomes.– The only chromosomes which don’t have a

homologous pair are the two sex chromosomes.

What is the gender of

this individual?

Chromosome Number continued

• Diploid and Haploid Cells– Not all cells in your body will have two copies of

each chromosome– Diploid (2n) – (di = two) both chromosomes of each

homologous pair– Haploid (n) – Having only one chromosome of each

homologous pair– Humans have 46 chromosomes in 23 pairs.– Gametes (reproductive cells like egg and sperm)

only have 23 chromosomes.

If egg and sperm weren’t haploid, when the two fused during fertilization, the

resulting baby would have 46 + 46 = 96 chromosomes!

This would be a bad thing. This way, the resulting baby will have 23

+ 23 = 46 chromosomes!

Cell Division

• Prokaryotic (Bacterial) Cell Division– Bacteria just have ONE chromosome.– They copy their chromosome and then split their

cell in half.

Cell Division continued

• Eukaryotic Cell Division– Eukaryotes have LOTS of chromosomes– The cell needs a way of dividing the chromosomes

between the new cells to make sure each new cell has all the chromosomes it needs.

• Two Types of Eukaryotic Nucleus Division– Mitosis – Keeps the number of chromosomes the

same.– Meiosis – Reduces the number of chromosomes by

½

But how do we make sure each new cell gets ONE copy

of each chromosome?

It requires a PROCESS!

The Cell Cycle

• When do cells reproduce?• Cell Cycle – The repeating series of events which

make up the life of the cell.• There are 5 phases to the cell cycle– G1 (First Growth) – The cell grows to mature size– S (Synthesis) – The DNA is copied– G2 (Second Growth) – The cell prepares for cell division– M phase (Mitosis) – Division of the nucleus– Cytokinesis – Division of the cytoplasm (rest of the cell)

The first 3 phases are known as Interphase.

The cell spends most of its time in interphase.

The last 2 phases make up cell division.

Some cells enter G0 phase where they exit the cell cycle.Cells in G0 do not reproduce.

Mitosis

• Mitosis is the division of the nucleus. (How we get each cell a copy of every chromosome.)

• To look at mitosis, we will simplify our cell.

We will use a cell with 4 chromosomes because it’s easier than drawing 46.

These are organelles called centrioles. They are only active during cell division.

Prophase Centrioles move toward opposite sides of the cell and spindle fibers form.

Nuclear membrane breaks down and disappears

Metaphase Centrioles reach opposite sides of the cell and spindle fibers attach to the chromosomes and line them up in the center of the cell.

AnaphaseSpindle fibers pull CHROMATIDS to opposite sides of the cell

TelophaseChromatids reach opposite sides of the cell.Spindle fibers disappear.Nuclear membrane reforms.Cytokinesis begins.

End result of mitosis

Two new cells which are identical to the original cell.Original cell had 4 chromosomes, new cells have 4 chromosomes.

Meiosis

• Type of nuclear division which results in cells which have one half the DNA of the original cell.

• Produces gametes – haploid reproductive cells (egg or sperm)

• Occurs in two parts: Meiosis 1 and Meiosis 2– It’s ONE process split into two halves!

Prophase ICentrioles begin to move to opposite sides of the cell and spindle fibers form.

Nucleus breaks down and disappears.

Synapsis – Homologous chromosomes line up next to each other. Each pair of homologous chromosomes is called a TETRAD.

Crossing Over occurs. Where maternal and paternal homologous chromosomes swap corresponding segments.

Metaphase ICentrioles have reached opposite sides of the cell.Spindle fibers have attached to chromosomes and lined the tetrads up in the center of the cell.

How is this different from metaphase of

mitosis?

Anaphase ISpindle fibers pull homologous chromosomes toward opposite sides of the cell.

How is this different from anaphase of

mitosis?

Telophase IChromosomes reach opposite sides of the cell.Spindle fibers break down.Cytokinesis begins.

After cytokinesis of Meiosis I

We aren’t done yet! We have 2 copies of each chromosome!But we do have 2 HAPLOID cells.

Prophase II

Centrioles duplicate and begin moving to opposite sides of the cell.Spindle fibers form.

Metaphase II

Spindle fibers attach to the chromosomes and line them up in the center of the cell.

Anaphase IISpindle fibers pull chromatids to opposite sides of the cell

Telophase IIChromatids reach opposite sides.Spindle fibers disappear.Nucleus re-forms.Cytokinesis begins.

End Result of Meiosis Meiosis forms 4 haploid cells.

Meiosis in males vs. females

In males the process is called spermatogenesis and the result is 4 spermatids.

In females the process is called oogenesis – and the result is one egg and 3 polar bodies.This is because the cell divisions are not equal, one cell ends up with the bulk of the cytoplasm.

Types of Reproduction

• Asexual Reproduction – Production of offspring from just one parent.– Mitosis is a form of asexual reproduction.

• Sexual Reproduction – Production of offspring through meiosis and the union of sperm and egg.– Requires two parents.

Let’s think about it!

How about a Venn?

Mitosis Meiosis