Chromosomes Review Every organism has traits (eye color, hair color, height, etc) passed on to them by their parents. The instructions for each trait is found within a gene that is found on chromosomes

Review! Humans somatic cells (body cells) have how many total chromosomes? How many pairs? Half of your chromosomes come from your father Half of your chromosomes come from your mother 46 23

46 Total Chromosomes 23 pairs of chromosomes

Homologous Chromosomes A pair of chromosomes (one from mom and one from dad) are called homologous chromosomes. They are the same length and have the centromere in the same place. They also carry genes for the same traits at the same place They “read” the same genes in the same places

Chromosome Number The number of chromosomes does not change from generation to generation You have the same number of chromosomes as your parents Gametes are sex cells (sperm and egg) that contain half the number of chromosomes Since you get half from mom and half from dad this guarantees each generation has the same number of chromosomes

Chromosomes The symbol n represents the number of chromosomes. A cell with n number of chromosomes is called haploid In humans n = 23 Gametes are haploid

Chromosomes The process in which one haploid gamete joins with another haploid gamete is called fertilization. After fertilization the cell has 2n chromosomes n from the female parent plus n from the male parent

A cell with 2n chromosomes is called a diploid A fertilized egg is called a zygote In humans the haploid number is? In humans the diploid number is? How many chromosomes in a human gamete? How many chromosomes in a human zygote? 23 46 23 46

LETS MAKE A BABY!!!! X Haploid (n) Y Haploid (n)

LETS MAKE A BABY!!!! X Haploid (n) Y Haploid (n)

LETS MAKE A BABY!!!! X Y Haploid (n) Haploid (n)

LETS MAKE A BABY!!!! X Y Haploid (n) Haploid (n)

LETS MAKE A BABY!!!! Y X Haploid (n) Haploid (n)

LETS MAKE A BABY!!!! Y X Haploid (n) Haploid (n)

LETS MAKE A BABY!!!! Y X Haploid (n) Haploid (n)

LETS MAKE A BABY!!!! Y X

LETS MAKE A BABY!!!! Y X

LETS MAKE A BABY!!!! Y X

LETS MAKE A BABY!!!! Y X

LETS MAKE A BABY!!!! Y X

LETS MAKE A BABY!!!! Y X

Y X LETS MAKE A BABY!!!!

Y X LETS MAKE A BABY!!!! Diploid (2n)

Y X Awww! WE MADE A ZYGOTE! Diploid (2n)

Cell Division Recall that somatic cells are formed by mitosis In mitosis the chromosome number stays the same However, sex cells need HALF the chromosomes so a different type of cell division is needed Gametes are formed during meiosis

Meiosis Meiosis is a type of cell division that produces gametes This produces sex cells, which are not genetically identical, through a series of cell divisions. In Meiosis the nucleus divides twice Important because it creates genetic variation (a variety in genes/traits) Because the nucleus divides twice meiosis creates four daughter cells

Stages of Meiosis There are 2 divisions of the nucleus in meiosis Interphase Prophase I Metaphase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II Meiosis I Meiosis 2

Review Challenge! Interphase Prophase I Metaphase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II Meiosis I Meiosis 2 Which stage does DNA Replication occur? Does Meiosis replicate DNA twice? During Interphase (the S phase) No! That is why is meiosis you end up with HALF of the original chromosome number

Genetic Variation Meiosis allows for new genetic combinations The ways this can occur Independent assortment: of the chromosomes. About 8 million combinations can be made from the original cell Random Fertilization: Zygotes form by random joining of gametes. B) Crossing-over: DNA is exchanged

Genetic Variation Independent assortment—homologous chromosomes are randomly sorted/distributed during meiosis Leads to several possible combinations of genes (why siblings aren’t always identical)

Genetic Variation Random-fertilization—the fertilization of an egg and sperm is random Each sperm can have a different combination of genes (from independent assortment) It is random which combination of genes will fertilize the egg

Genetics Variation Crossing-over- the exchange of genetic material that can occur between homologous chromosomes during Prophase I Part of one chromatid is broken and exchanged with the corresponding chromatid of the other homologous chromosome Crossing over provides new combinations of genes that are different from either parent

How is Meiosis Different from Mitosis? There are 2 divisions in meiosis Meiosis I and meiosis II The result is 4 cells instead of 2 In meiosis II, the DNA is not replicated again. (No interphase) The ending number of chromosomes is 23 in humans (egg has 23 and sperm has 23)This is haploid (n).

Mitosis Vs. Meiosis Mitosis Meiosis 2 divisions = 4 cells Daughter cells different (crossing over; independent assortment) Haploid cells (n)=23 Gametes (sex cells—eggs or sperm) Sexual reproduction Mitosis 1 division=2 cells Daughter cells identical Diploid cells (2n)=46 Body/Somatic cells Asexual reproduction

Meiosis Animation http://www.youtube.com/watch?v=D1_-mQS_FZ0