Meiosis and Sexual Life Cycles

Chapter 13

Genetics

• Genetics is the scientific study of heredity and hereditary variation.

• Heredity is the transmission of traits from one generation to the next.

• Variation is how offspring differ from their parents and siblings.

Inheritance of Genes

• Genes are segments of DNA. Copies of genes (with some differences due to crossing over) are passed from parents to children.

• Gametes are sex cells, eggs and sperm, that carry genes from one generation to the next.

• During fertilization, gametes unite to form a zygote, which develops into an embryo, then a fetus, and then a newborn.

Types of Reproduction• Asexual: a single

individual is the sole parent, offspring are identical to each other AND the parent (clones).

• Sexual: two parents produce offspring that are different from each other and the parents.

Karyotypes• A karyotype is a picture of all 23 pairs of chromosomes

(duplicated to be 46) arranged in order from 1 23.• Homologous chromosomes are pairs; one from Mom

and one from Dad

Types of Chromosomes

• 22 pairs of our chromosomes are autosomes, non-sex chromosomes.

• 1 pair of our chromosomes are called sex chromosomes, and determine gender.

• Women = XX• Men = XY

Diploid vs. Haploid

• Diploid Cells:• 2n• 46 chromosomes• Somatic cells (body cells)• 2 sets of chromosomes• 2n = 46• Skin, nerve, muscle...all

body cells• Zygotes

• Haploid Cells:• n• 23 chromosomes• Gametes only• Egg and sperm• 1 set of unduplicated

chromosomes• n=23• Sex cells

Meiosis• Meiosis is cell division that reduces the number

of sets of chromosomes from two to one, creating gametes (eggs and sperm).

• It ONLY happens in the ovaries and testes.

Meiosis• It’s like double mitosis –

cells divide TWICE so that gametes have half as many chromosomes.

• That way, after fertilization, the zygote has the right number of chromosomes: 23 pairs

• 4 daughter cells are made instead of 2

The process of meiosis

• Meiosis happens in two steps, Meiosis I and Meiosis II.

Stages of Meiosis I:Prophase IMetaphase IAnaphase ITelophase I and Cytokinesis

Stages of Meiosis II:Prophase IIMetaphase IIAnaphase IITelophase II and Cytokinesis

Interphase

• Chromosomes replicate during S phase and are two genetically identical sister chromatids.

Prophase I

• 90% of meiosis• Chromosomes condense• Crossing over occurs:

DNA in non-sister chromatids mix and match; resulting in genetic variation of offspring

• Tetrads are held together at chiasmata

Crossing Over• Duplicated homologous

chromosomes connect, this is called synapsis.

• Pieces of DNA swap, called crossing over.

• All 4 chromatids together make a tetrad.

• Each tetrad has at least one site of chiasma, where crossing over occurs.

Metaphase I

• Tetrads line up on the metaphase plate

Anaphase I

• The chromosomes begin to move to the poles.• Sister chromatids remain attached!

Telophase I and Cytokinesis

• 2 new cells form.• Each has a complete

haploid set of chromosomes (23), each chromosome is still composed of two sister chromatids.

Prophase II

• A new spindle apparatus forms, and the 2 new cells get ready to divide again.

Metaphase II

• Chromosomes line up on the metaphase plate.

• The two sister chromatids are NOT genetically identically (because crossing over happened in Prophase I)

Anaphase II

• Centromeres of each chromosome separate, and sister chromatids start moving apart.

Telophase II and Cytokinesis

• Nuclei form, 4 new daughter cells are formed.• Each of the 4 daughter cells are genetically

different.

Mitosis vs. Meiosis

Mitosis• Cells divide once• No crossing over• Two daughter cells made• Daughter cells are identical

to each other and parents• Daughter cells are 2n• Occurs in somatic cells

Meiosis• Cells divide twice• Crossing over (prophase I)• Four daughter cells made• Daughter cells are all

different from each other and parents

• Daughter cells are n• Occurs in ovaries/testes• Makes gametes

Why is crossing over important?• During Prophase I, crossing over occurs and

produces genetic variation.• This produces recombinant chromosomes, that

carry genes (DNA) from two different parents• Powers natural selection/evolution: all

individuals are different and the most fit survive to reproduce

• Makes species more “hardy,” if bad things happen, at least some will have adaptations that help them survive.

Random Fertilization

• Each egg or sperm represents 8 million possible chromosome combinations

• This is because during Metaphase I, sometime the male chromosome and sometimes the female chromosome points towards a pole

• So, each new cell has some genes from Mom and some from Dad

• Each zygote has 64 trillion possible diploid combinations, increasing genetic variation.