Meiosis!! Chapter 10

Meiosis  Purpose: to make sex cells  Gamete: sex cell  Male gamete= sperm  Female gamete= egg (ovum)

Let’s define the following words…  Meiosis:  Sexual reproduction- production of gametes  Gamete:  Sex Cells (egg and sperm)  Derived from germ cells  Somatic Cells:  Body Cells

 Haploid:  Half the chromosome number (n)  Cells with 1 of each pair of chromosomes (23)  Ex: Gametes  Diploid:  2 of each type of chromosome (2n)  Cells with twice the haploid number (46)  Ex: Somatic cells  Fertilization:  Sperm enters egg  Zygote:  Fertilized egg (diploid)

Chromosomes and Chromosome Number  Homologous chromosomes —set of chromosomes with same genes in same locations:  Get one from each parent  Somatic cells: 46 chromosomes  Gametes: 23 chromosomes

Sister chromatids One duplicated chromosome Centromere Homologous pair of chromosomes

Homologous Chromosomes  Same centromere position  Same length  Carry genes that control the same inherited traits

General Meiosis  The sexual life cycle in animals involves meiosis  Meiosis produces gametes (haploid)  When gametes combine in fertilization, the number of chromosomes is restored (diploid)

DNA Replication PrepareGrowth Division Mitosis Meiosis 2N NNNN

Karyotype

Meiosis  Like mitosis (PMAT), but 2 divisions:  Meiosis I (separation of homologous chromosomes)  Meiosis II (separation of sister chromatids)  Produces 4 daughter cells (NOT genetically identical!!!!!!)

Interphase  Same as Mitosis (G1, S, G2)  DNA- Chromatin  Centrioles- located near the nucleus  Nucleolus is present

Prophase I  Pairing of homologous chromosomes occurs forming a tetrad (Synapsis)  The nuclear envelope breaks down/nucleolus gone  Spindles form  Crossing over takes place

Crossing over  Homologous chromosomes exchange genetic material  Recombination of genes between chromosomes  How is crossing over related to genetic variation?

Metaphase I  Chromosome centromeres attach to spindle fibers  Homologous chromosomes line up at the equator

Anaphase I  Homologous chromosomes separate and move to opposite poles

Telophase I  The spindles break down  Chromosomes uncoil, 2 new nuclei form  The cell divides

Do Now!!  What gets separated in meiosis I? In meiosis II?  What is a tetrad?  What is the point of crossing over?

Prophase II  A second set of phases begins  Chromosomes condense again  Spindles form in each new cell

Metaphase II  Chromosomes line up at metaphase plate of each cell

Anaphase II  The sister chromatids are pulled apart at the centromere by spindle fibers  Move toward the opposite poles of the cell

Telophase II  The chromosomes reach the poles and the nuclear membrane and nuclei reform  Spindles break down

Cytokinesis o Cytokinesis results in four haploid cells (gametes) o Each with n number of chromosomes. *So… if the original diploid cell has 10 chromosomes, how many will each gamete have? *Why are the gametes NOT genetically identical?

Do Now!!  Match the description to the phase of meiosis. 1. Sister chromatids are pulled apart. _____ A. Prophase I 2. Homologous chromosomes line up in the middle of the cell._____ B. Metaphase I 3. 2 daughter cells are formed ____ C. Telophase I 4. Nuclear membrane disappears__ D. Anaphase II 5. Nuclear membrane reforms around 4 cells ____ E. Anaphase I 6. Homologous chromosomes are pulled apart ____ F. Telophase II

Crash course biology!!  YY7o

Spermatogenesis vs oogenesis

Genetic Variability  Depending on how the chromosomes line up at the equator, four gametes with four different combinations of chromosomes can result.  Genetic variation also is produced during crossing over and during fertilization, when gametes randomly combine.

Label the phases of meiosis