MEIOSISReproduction that occurs from

recombining genetic material from two parents.

ReproductionIs a characteristic that all living things demonstrate … in 2 different forms:

Asexual and Sexual

Asexual ReproductionOne parent cell goes through MITOSIS in

order to produce 2 daughter cells that are practically identical to the parent.

Mitosis looks like:

Sexual ReproductionThrough MEIOSIS, 2 parent cells (sex cells) combine

genetic material to produce offspring that have similar (but not identical) characteristics of both

parents.

Meiosis looks like:

But hold on … there is a lot more to Meiosis:

In order for a new organism to be created, it must have the right amount of DNA (found in your chromosomes). If human sex cells had the same amount of chromosomes

as all of your other cells, then when they combined together, you would make a cell with 98 chromosomes

instead of 46 … and that just wouldn’t work!

So MEIOSIS is a process that makes sure sex cells are formed

with exactly HALF the genetic material an organism needs.

That way, 1 sex cell from each parent (1 sperm and 1 egg) will

combine to provide the right amount of chromosomes needed!

This process is similar to MITOSIS, but also very different.

We’ll now go through the phases of MEIOSIS:

Interphase G1:

- The cell starts to increase in size- The cell contains 23 PAIRS of chromosomes- The chromosomes pair up to the one that

carries the same types of genes … these are called HOMOLOGOUS CHROMOSOMES- Each member of a homologous

chromosome pair is called a homologue, one comes from the father, and the other

from the mother!

Interphase S:

- The DNA replicates to create 2 copies of everything.

- Identical copies attach together at a midpoint called a centromere.

- The centriole pairs also duplicate (for animals – plants do not have centrioles).

Prophase I :

- This is the beginning of Meiosis I- Homologous chromosomes move together and

pair up, making a tetrad- Once a tetrad is formed, DNA is exchanged

between the homologues in a process called Crossing Over

- Crossing over is RANDOM!!! This gives us the benefit of variation!!!

- Also, the chromosomes thicken and condense, the nuclear membrane fades away and the centrioles

start to go to opposite sides.

Crossing Over

Metaphase I :

- The tetrads are pulled to the “equator” by spindle fibers

- More genetic variation occurs here as the chromosomes line up randomly, which is

called INDEPENDENT ASSORTMENT- The centrioles are now in place, the nuclear

membrane is gone and the spindles can be fully formed.

Anaphase I :

- The spindle fibers attach to each homologous pair and start pulling them to opposite sides (they don’t pull apart, as in

Mitosis).- The cell elongates to prepare for division.

Telophase I :

- This is the end of the FIRST part of Meiosis- 2 distinct poles are defined with one set of NON-

IDENTICAL DNA on each side- The cell starts to pinch in the middle, this is the

beginning of cytokinesis – which will result in 2 NON-IDENTICAL daughter cells

- The DNA stretches back out again and a nuclear membrane forms, spindle fibers go away.

- The centrioles will duplicate again before the next phase.

Prophase II:

- This is the beginning of Meiosis II- This is a LOT like Mitosis, only the cells that

result are not identical to the parent- Nuclear membranes dissolve

- Chromosomes condense and thicken again- Centriole pairs (now there are 2 pairs in the

new cells) migrate away from each other

Metaphase II:

- Chromosomes move to the middle- This is another event (Independent Assortment) that allows for variation (the

chromosomes line up randomly)- Centrioles are on opposite sides

- Nuclear membrane is completely dissolved- Spindle fibers are fully formed

Anaphase II:

- Spindle fibers attach to the chromatids and pull them apart, taking half to each pole

- The cell elongates

Telophase II:

- This is the final phase, the DNA gets long and stringy again

- Nuclear membranes start to form- Spindle fibers dissolve

- Cell membrane begins to pinch cell in half- In males, 4 sperm are created, in females, 1 egg

and 3 polar bodies are created

Variation:

The process of meiosis results in gametes that are all different from one another, as well as the

parent cells.

Variation occurs in 3 SPECIFIC events:1. Crossing over in Prophase I

2. Independent Assortment in Metaphase I3. Independent Assortment in Metaphase II

Meiosis Model:

Use 2 different colors of yarn for the chromosomes.

Draw everything else.

Show ALL phases of Meiosis using the yarn, blank paper and pens or markers.

Label each phase clearly!