Chapter 13 – Campbell’s 6th Edition

Meiosis and Sexual Life Cycles

Definitions

1) Heredity: transmission of traits from generation to the

next

2) Variation: slight differences in appearance

3) 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?

MitochondriaChloroplast

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

Life Cyclesequence of stages from an organisms

conception to production of its own offspring

Animals

Fungi and some algaePlants and some algae

More Vocab

1) somatic cell:body cells; any cell other than sperm or ovum

2) karyotype:micrograph of the 46 human chromosomes in which they are lined up in pairs starting with the longest

1) homologous chromosomes:same length, centromere position, staining pattern; carry genes governing the same inherited characters

2) sex chromosomes:X and Y chromosomes

3) 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

+

46

Gametes

Fertilization

Zygote

(2N)

(N) + (N)

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 interphaseidentical sister chromatidsare 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.

Tetra =4

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 IEach pair is assortedindependently; this means that the maternal homolog in each pair randomly faces up or down.

So this pair could have assorted like this.

And 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

the spindle fibers break down

Meiosis: Telophase I

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 II is very much like Mitosis.

Meiosis: Prophase II

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 fibersattach to the chromosomes at the centromere

Meiosis: Metaphase II

The spindle fibersmove 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