Chapter 10.1, Meiosis

Chromosomes and Genes

Recall from chapter 9, during mitosis, the DNA (Chromatin) condenses into x-shaped structures called chromosomes.

The chromosomes are arranged into segments called genes. Each gene gives instructions to the cell to create proteins that produce characteristics called traits.

Traits, such as hair color, eye color and height, are inherited through genes on chromosomes.

Genes, Chromosomes, and Traits

Chromosome Numbers

Each organism has a specific chromosome number.

Humans have 46 chromosomes; 23 from your mother, and 23 from your father.

The 46 chromosomes inherited from your parents are homologous, meaning that each chromosome has a matching chromosome that is the same length, has the same centromere position and more importantly, it carries the same genes that code for particular traits.

Homologous Chromosomes

Haploid vs. Diploid Cells

All the cells of our body contain Diploid cells. Diploid cells have all the chromosomes contributed from your mother and father. Diploid cells are represented as 2n.

The sex cells of your body (called gametes) have only half the number of chromosomes as the rest of your body's cells. These cells are called Haploid cells. Haploid cells are represented by the symbol n.

Haploid vs. Diploid Cells

Where do haploid and diploid cells come from?

During sexual reproduction, two gametes (sperm and egg cells) combine by fertilization.

When two haploid cells combine, a diploid cell is formed to make a zygote.

A zygote later develops into an embryo. When an organism becomes sexually mature,

they start to develop sex cells or gametes. The process that forms gametes or sex cells is

called meiosis.

Human Life Cycle and the formation and union of Gametes

What is Meiosis?

Meiosis is a type of cell division that produces gametes or sex cells.

Meiosis is different from mitosis because the number of chromosomes becomes reduced.

Another name for meiosis is reduction division. Meiosis occurs only in organisms that

reproduce sexually. Meiosis involves two cell divisions called

meiosis I and meiosis II.

Mitosis and Meiosis Comparison

Meiosis I

Very similar to mitosis with a few differences, so only the differences will be discussed.

In Prophase I, homologous chromosomes (similar pairs from mother and father) pair up and crossing over occurs.

Crossing over is when segments of homologous chromosomes are exchanged.

This is one way that genetic variety and diversity can occur by sexual reproduction.

Crossing Over in Prophase I

Meiosis I

In Metaphase I, the difference between mitosis and meiosis I is that homologous chromosomes attach to spindle fibers at the equator rather than single chromosomes.

During Anaphase I, the homologous chromosomes separate rather than the sister chromatids, so each chromosome still consists of two sister chromatids after anaphase I.

The sister chromatids might not be identical in Telophase I because of crossing over.

Overview of Meiosis I

Meiosis II

After Meiosis I, the two cells are now reduced from 2n (Diploid) to n (Haploid).

At this point a second division occurs that is very similar to mitosis, so we will only go over the major differences.

During Metaphase II, a haploid number of chromosomes line up at the equator.

During Anaphase II, sister chromatids are pulled apart (rather than homologous pairs).

Overview of Meiosis II

Overview of Meiosis

What is the end result?

At the conclusion of Meiosis I and II, the result is 4 separate haploid (n) daughter cells that are each unique and different from each other.

Remember from mitosis, the end product is two identical diploid (2n) daughter cells.

Variety is produced when homologous chromosomes cross over (during Prophase I), and when homologous chromosomes randomly line up at the equator (during Metaphase I). This is produces Independent Assortment.

Variety from Independent Assortment