Chapter 13 – Campbell’s 6th Edition Meiosis and Sexual Life Cycles

Definitions Heredity: transmission of traits from generation to the next Variation: slight differences in appearance Genetics: scientific study of heredity and hereditary variation

Genes Genes: coded information in the form of hereditary units (lots of genes on each chromosome) Where else can you find DNA beside the nucleus of the cell? Mitochondria Chloroplast

Asexual Reproduction Asexual reproduction single individual is sole parent and passes copies of all its genes to its offspring Clone the offspring of asexual individuals

Sexual reproduction Two parents give rise to offspring with unique combinations of genes; results in more variation

Animals Life Cycle Plants and some algae Fungi and some algae sequence of stages from an organisms conception to production of its own offspring Animals

More Vocab somatic cell: body cells; any cell other than sperm or ovum karyotype: micrograph of the 46 human chromosomes in which they are lined up in pairs starting with the longest homologous chromosomes: same length, centromere position, staining pattern; carry genes governing the same inherited characters sex chromosomes: X and Y chromosomes autosomes: all non-sex chromosomes (1 through 22)

Karyotype

Meiosis and Sexual Reproduction Sexual Reproduction – chromosomes of two parents combine to form offspring 23 23 + (N) + Gametes Fertilization (2N) 46 Zygote

About Chromosomes (2N) = diploid – any cell with 2 complete sets of chromosomes (N) = haploid – a cell with 1 complete set of chromosomes Zygote (2N) – egg cell after fertilzation = egg + sperm Each chromosome in egg has a matching chromosome from sperm Homologous Pairs – matching chromosomes

Homologous pairs chromosomes occur in pairs one set from each parent Diploid or 2N Cells that contain both sets of chromosomes. Haploid =1N Sex cells have only half the chromosome number

Just before Meiosis begins, during interphase identical sister chromatids are attached at the centromere, and centrioles have been duplicated. Centrioles

Meiosis: Prophase I During Prophase I: chromosomes condense nucleus breaks down spindle fibers form Homologous chromosomes form tetrads (fours)

Meiosis: Prophase I the chromosomes condense centrioles move to opposite poles of the parent cell and begin building the spindle microtubules. the nuclear envelope breaks down

Meiosis: Prophase I Homologous chromosomes pair up, forming tetrads.

Meiosis: Prophase I Homologous chromosomes pair up, forming tetrads...

Meiosis: Prophase I …. and crossing over may occur. Crossing over- pieces of DNA are exchanged with the matching homologous chromosome

Meiosis: Prophase I centrioles begin building the spindle fibers.

Meiosis: Prophase I Spindle fibers grab each chromosome at the centromere. Move the chromosomes towards the poles.

Meiosis: Metaphase I During Metaphase I: Homologous chromosomes line up in the middle. Equator = metaphase plate

Meiosis: Metaphase I Each pair is assorted independently; this means that the maternal homolog in each pair randomly faces up or down. And this pair could have assorted like this. So this pair could have assorted like this.

Meiosis: Anaphase I During Anaphase I: Homologous pairs of separate. Sister chromatids remain attached at the centromere.

Meiosis: Anaphase I

Meiosis: Anaphase I

Meiosis: Telophase I During Telophase: spindle fibers break down new nuclear membrane forms

Meiosis: Telophase I the spindle fibers break down

Meiosis: Telophase I new nuclear membrane forms around each group of chromosomes

Cytokinesis: Splitting of the cytoplasm

Now these two haploid cells will begin Meiosis II; in this division, sister chromatids will separate.

Meiosis: Prophase II Meiosis II is very much like Mitosis. During Prophase II: nuclear membrane breaks spindle fibers reform

Meiosis: Prophase II the nuclear membrane breaks down

Meiosis: Prophase II centrioles move to opposite poles of the parent cell and begin building new spindle fibers

Meiosis: Prophase II

Meiosis: Prophase II centrioles begin building the spindle fibers

Meiosis: Prophase II The spindle fibers attach to the chromosomes at the centromere

Meiosis: Metaphase II The spindle fibers move the chromosomes to the center (metaphase plate) of the cell.

Meiosis: Anaphase II Sister chromatids separate.

Meiosis: Anaphase II

Meiosis: Telophase II During Telophase II: the spindle fiber break down nuclear membrane reforms around each group of chromosomes

Meiosis: Telophase II the spindle fibers break down

Meiosis: Telophase II new nuclear membrane form around each group of chromosomes

Cytokinesis: Result: Four non-identical haploid cells are formed.

Mitosis vs. Meiosis

A comparison of mitosis and meiosis Mitosis and Mieosis Chromosomes replicate only once Mitosis Cell divides once Produces 2 identical daughter cells Meiosis Cell divides twice Daughter cells with half the number of chromosomes Daughter cells are not all alike Produces 4 cells (gametes)

Independent Assortment

Crossing Over