MEIOSIS

Mendel: Forgotten?

Gregor Mendel published his findings on genetics in 1865 but his work didn’t get much attention

In the early 1900’s scientists did experiments similar to what Mendel had done

They searched for papers detailing results similar to theirs and realized that their discoveries were not new.

Genes were still a mystery though. Scientists didn’t know where they were located or how they passed information from once cell to another.

One Makes Two

Asexual reproduction: only one parent cell is needed for reproduction

Parent cell goes through mitosis and divides into two daughter cells

Most single-celled organisms reproduce in this way as do most of the cells in your body

Two Make One

Sexual reproduction: two parent cells join together to form a new individual

Parent cells are known as sex cells, different from ordinary body cells

Human body cells have 46 chromosomes or 23 pairs of homologous chromosomes

Human sex cells have only 23 chromosomes

Male sex cells are called sperm

Female sex cells are called eggs, or ova

Each sperm and each egg has only one of the chromosomes from each homologous pair

Less is More

When an egg and sperm join to form a new individual, each parent donates ½ of a homologous pair of chromosomes

This ensures the offspring will have a normal number of chromosomes in each body cell

Meiosis to the Rescue

Sex cells are made during meiosis

Meiosis produces new cells with half the usual number of chromosomes

When sex cells are made, the chromosomes are copied once, and then the nucleus divides twice.

Resulting sperm and eggs have ½ the number of chromosomes found in a normal body cell

Back at the Lab

Walter Sutton read Mendel’s studies which showed that the egg and sperm must contribute the same amount of genetic information to the offspring

Using some of his own observations, he came up with the idea that:

Genes are located on chromosomes

What was that again?

Mitosis: P M A T/C

Meiosis: First Time Through

Prophase I Metaphase I Anaphase I: Chromosomes separate

from their homologous partners and move to opposite sides of cell

Telophase/Cytokinesis I: Two cells are formed, each with the same amount of chromosomes as the parent cell

Chromatids vs. Homologous Chromosomes

Meiosis: Going Again

Prophase II: Nuclear membranes dissolve again, meiotic spindle fibers form

Metaphase II: Spindle fibers attach and line paired chromatids up at equator

Anaphase II: Chromatids pull apart and move to opposite poles

Telophase/Cytokinesis II: Four new cells have formed from original cell, each with half the number of chromosomes present

Differences Between Mitosis and Meiosis

Mitosis Meiosis

One cell divisision Two cell divisionsTwo daughter cells Four daughter cells

Daughter cells have same number of chromosomes as parent cells

Daughter cells have half the number of chromosomes as parent cells

Male or Female?

Sex chromosomes carry genes that determine whether the offspring is male or female.

In humans, females have two X chromosomes while males have one X chromosome and one Y chromosome.

Each egg cell produced by a female contains one X chromosome. A sperm cell produced by a male can contain either an X or Y chromosome.

Male or Female?

Male or Female?

Think/Pair/Share

Explain the difference between sex cells and sex chromosomes.

If there are 14 chromosomes in pea plant cells, how many chromosomes are present in a sex cell of a pea?

How many times do the chromosomes make copies of themselves during meiosis? How many times do cells divide during meiosis?