Chromosomes Chromatin – long strands of DNA wrapped around proteins Chromosome – a rod-shaped structure that forms when a single DNA molecule coils tightly before cell division Chromatid – one of two copies of each chromosome Sister Chromatid – two identical copies of the chromosome Centromere – a protein disk that attaches the two chromatids

Single ChromosomeDuplicated Chromosome

Single ChromosomeDuplicated Chromosome Sister Chromatids Centromere Chromatid

The Cell Cycle Video

Meiosis Chromosome Number Chromosomes occur in pairs (23 pairs in humans) One from mom and one from dad Diploid (2n) – 2 of each kind of chromosome (46) Haploid (n) – one of each kind of chromosome (23) All species contain a characteristic number Pea Plant - 14 Apple - 34 Human – 46 Dog – 78

Fig. 10-4, p XX (or XY)

Why Meiosis? When cells divide by mitosis the new cells have exactly the same number and kind of chromosomes as the original cells. If fertilization occurs by mitosis then gametes would have twice the number of chromosomes = 92 Meiosis – cell division that cuts the chromosome number in half

Gamete Formation Germ Cell – a type of cell that goes through meiosis to make gametes Gametes – sex cells (sperm and egg) Sperm – male gametes Egg – female gametes Zygote – union of an egg and sperm Fertilization – the uniting of male and female gametes (sperm and egg) Sexual Reproduction – involves the production and fusion of haploid sex cells

Sperm (n = 23)Egg (n = 23) Zygote (2n = 46)

Phases of Meiosis Meiosis I Prophase I Chromosomes coil up Nuclear envelope breaks down Centrioles form and move to opposite poles Spindle fibers form

Pairs of homologous chromosomes come together and form a tetrad Crossing Over – homologous chromosomes exchange portions of their chromatids There is an average of two to three crossovers for each pair of homologous chromosomes Crossing over results in new combinations of genetic material

Metaphase I Spindle fibers attach to centromere Pairs of chromosomes line up in center

Anaphase I Pairs of chromosomes separate and move to opposite poles

Telophase I Spindle fibers break down Centrioles break down Chromosomes uncoil Nuclear envelope reforms

Cytokinesis Cytoplasm divides into 2 new cells Each cell has half the genetic info of the original cell

Even though we now have 23 chromosomes in each cell, the chromosomes are still in the duplicated state and it needs to divide again.

Meiosis II Prophase II Chromosomes coil Centrioles form and move to opposite poles Spindle fibers form Nuclear envelope breaks down

Metaphase II Spindle fibers attach to centromere Chromosomes line up in center single file

Anaphase II Centromere splits and sister chromatids separate and move to opposite poles Each chromatid is now called a chromosome

Telophase II Spindles fibers break down Centrioles break down Chromosomes uncoil Nuclear envelope reforms

Cytokinesis Cytoplasm divides into two new cells for a total of 4 new cells. Each cell has 23 single chromosomes, they are haploid Video

plasma membrane newly forming microtubules in the cytoplasm spindle equator (midway between the two poles) one pair of homologous chromosomes PROPHASE IMETAPHASE IANAPHASE ITELOPHASE I MEIOSIS I Fig. 10-5, p.158

PROPHASE IIMETAPHASE IIANAPHASE IITELOPHASE II MEIOSIS II there is no DNA replication between the two divisions Fig. 10-5b, p.159

In mitosis, 1 diploid cell (2n) produces 2 diploid cells (2n) In meiosis, 1 diploid cell (2n) produces 4 haploid cells (n) MitosisMeiosis 2n nn nnnn

Male (sperm) Female (egg) 2n n n n n n n nn n nn n 4 Sperm 1 Egg 3 Polar Bodies

Meiosis Provides for Genetic Variation Genetic Recombination – the reassortment of chromosomes and the genetic information they carry by crossing over

3 Places for Variation during Meiosis 1.There are 2 possibilities when homologous chromosomes line up in metaphase I 2 23 = 8 million 2.8 million different kinds of sperm or egg Because any egg can be fertilized by any sperm the number of different possible offspring is 8 million x 8 million trillion 3.Crossing over leads to more variation Crossing over can occur anywhere at random on a chromosome 2 or 3 crossovers per chromosome occur during meiosis

MitosisMeiosis Two new cellsFour new cells New cells are genetically identical to the parent cell and each other New cells are genetically different from the parent cell and each other New cells are diploidNew cells are haploid For growth and replace old somatic cells To make sperm and egg Asexual reproductionSexual reproduction