Creating Genetic Diversity Meiosis Creating Genetic Diversity

Homologous Chromosomes Chromosomes that have the genes for the same traits. These genes must be in the same order on the chromosome. They do not have to be identical alleles. EXAMPLE: Can be S and S, S and s, or s and s. ARRANGE THE CHROMOSOMES AT YOUR DESK INTO HOMOLOGOUS PAIRS

Homologous Chromosomes Before Replication After Replication Before Replication After Replication

Cell Types A cell with two of each kind of chromosome is called a DIPLOID cell. All Body cells Chromosomes form homologous pairs A cell with only one of each type of chromosome is called a HAPLOID cell. Sex cells or gametes. No homologous pairs ½ the chromosomes found in body cells

LET’S APPLY… How many chromosomes in human body cells? How many chromosomes does MITOSIS ensure are in each new cell that forms? How many chromosomes would be in human offspring if the egg & sperm have the same number of chromosomes as all other cells? 46 46 92

Therefore… It is important that sex cells be formed by a different manner of cell division… One that will ensure the chromosome number is reduced to half the normal number of all other cells.

Meiosis Meiosis is the form of cell division that produces these haploid sex cells. The chromosome number is reduced to half, each sex cell gets one chromosome from each homologous pair In males this is sperm and females it is an egg. They combine during sexual reproduction/ fertilization. The resulting zygote will have the diploid number of chromosomes.

LET’S PRACTICE… A skin cell has 24 chromosomes… how many in a blood cell of the same organism? How many in a sperm cell of the same organism? How many chromosomes will be in the zygote (offspring) of this organism Now work through the practice worksheet 24 12 24

Stages of Meiosis Interphase Prophase I Metaphase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II Prepare your flipbook in order for notes New info will be highlighted in red

Interphase The period between cell division Cell is growing & metabolizing Cell replicates its chromosomes NO CHROMOSOMES VISIBLE

Prophase I DNA coils into chromosomes which are now visible Centrioles separate & Spindles form Nucleus disappears Homologous chromosomes line up next to each other

Metaphase I Spindle is complete Centromeres become attached to a spindle fiber. The homologous chromosomes (tetrad) line up in the middle of the cell.

Anaphase I Homologous chromosomes separate One chromsomes from each pair moves to opposite ends of the cell

Telophase I Chromosomes have reached opposite sides of cell Spindles break down Nucleus reforms Cytoplasm divides to form two new cells.

Prophase II New spindles form Chromosomes are still made up of sister chromatids (replicated copy of DNA)

Metaphase II Chromosomes line up at the equator This time, they are single file as in mitosis

ANAPHASE II Sister chromatids separate and move to opposite ends of each cell.

Telophase II Cytoplasm divides Meiosis produces four haploid cells. Are they identical to the chromosomes that began? What is different? There are two chromosomes instead Of 4 The chromosomes do not have a matched (homologous) pair

IN MALES… These 4 cells would be SPERM cells Sperm cells have tails so they can swim to find the egg Made in the TESTES

IN FEMALES… One cell would be very large…receive most of the cell material so that if fertilized it can support developing offspring The other 3 (polar bodies) are so small they die The egg doesn’t move Made in the OVARIES

In Conclusion… Meiosis is the process by which sex cells are produced. Meiosis reduces the number of chromosomes in sex cells to half the normal number Meiosis keeps the number of chromosomes from doubling when offspring are produced.