Chapter 10 Section 10.1

Sexual Reproduction and Genetics

Bio 30 NWRC

Meiosis – A Source of Distinction

Why do you share some but not all characters of each parent?

At one level, the answers lie in meiosis.

Meiosis – A Source of Distinction

Meiosis does two things -

1) Meiosis takes a cell with two copies of every chromosome (diploid) and makes cells with a single copy of every chromosome (haploid).

This is a good idea if you’re going to combine two cells to make a new organism. This trick is accomplished by halving chromosome number.

In meiosis, one diploid cells produces four haploid cells.

Reduction of Chromosome Number in the First Division of Meiosis

Meiosis – A Source of Distinction

The second accomplishment of meiosis:

2) Meiosis scrambles the specific forms of each gene that each sex cell (egg or sperm) receives.

This makes for a lot of genetic diversity. This trick is accomplished through independent assortment and crossing-over.

Genetic diversity is important for the evolution of populations and species.

Keys to Understanding Meiosis

Chromosomes are paired.

Chromosomes carry genes.

The gene forms on a pair of chromosomes may be identical ..

Brown eyes

Brown eyes

Tall

Tall

Brown eyes

Blue eyes

Tall

Short

or different.

One Way Meiosis Makes Lots of Different Sex Cells (Gametes) – Independent Assortment

Independent assortment produces 2n distinct gametes, where n = the number of unique chromosomes.

That’s a lot of diversity by this mechanism alone.

In humans, n = 23 and 223 ≈ 8,000,0000.

Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over

Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.

Between Independent Assortment and Crossing-Over, No Two Gametes Are Identical. Your book calculates the number of possible combinations of children possible to one set of parents as over 70

trillion. (Reading Essentials page 113)

The Key Difference Between Mitosis and Meiosis is the Way Chromosomes Uniquely Pair and Align in Meiosis

Mitosis The first (and distinguishing) division of meiosis

Oogenesis – A Path of Meiosis in Humans

Woman are less busy in meiosis than men – meiosis produces only a few hundred mature eggs over a lifetime.

Spermatogenesis – Another Path of Meiosis in Humans

Men are busy – meiosis produces roughly 250,000,000 sperm per day.

“Putting It All Together” - Fertilization

What Meiosis is About

Meiosis allows the creation of unique individuals through sexual reproduction.

Meiosis

Meiosis

• Sexual reproduction • Formation of new individual by a combination of

two haploid sex cells (gametes). • Fertilization- combination of genetic information

from two separate cells that have one half the original genetic information

• Gametes for fertilization come from separate parents – Female- produces an egg – Male produces sperm

Meiosis

• Both gametes are haploid, with a single set of chromosomes

• The new individual is called a zygote, with two sets of chromosomes (diploid).

• Meiosis is a process to convert a haploid cell to a diploid gamete, and cause a change in the genetic information to increase diversity in the offspring.

Meiosis

• Summary of chromosome characteristics • Diploid set for humans; 2n = 46 • Autosomes; homologous chromosomes, one

from each parent (humans = 22 sets of 2) • Sex chromosomes (humans have 1 set of 2)

– Female-sex chromosomes are homologous (XX)

– Male-sex chromosomes are non-homologous (XY)

Meiosis

• Haploid (n)-- one set chromosomes

• Diploid (2n)-- two sets chromosomes

• Most plant and animal adults are diploid (2n)

• Eggs and sperm are haploid (n)

Meiosis I Before going into PROPHASE 1 the chromosomes double

Meiosis IIn Metaphase 1 the chromosomes line upon the equator

equator

Meiosis IIn Anaphase 1 the chromosomes separate and move to opposite ends

Meiosis I

In Telophase

2 nuclei are formed

Meiosis II

In prophase II the nuclear envelope is again dissolved and the spindle is set up again. Prophase II is identical to prophase of mitosis except that there is half the amount of chromosomes. 

Meiosis II

In metaphase II the chromosomes line up randomly at the equator

Equator

Meiosis II

In Anaphase II the centomeres split-sister chromatids move to opposite poles

Equator

Meiosis II In Telophase II 4 nuclei form around the chromosomes cells divide

4 cells have formed – each cell has a HAPLOID number of chromosomes

Cytokinesis

These cells with half the number of chromosomes will either be eggs or sperm

Importance of Meiosis• The importance of meiosis in promoting genetic variation• 　• Each daughter cell (gamete) receives• only one distinct chromosome of a homologous pair.• The different pairs of homologous chromosomes• separate independently of each other (during anaphase)• so that the daughter cells have different combinations of

chromosomes.• Crossing over of chromatids between homologous

chromosomes• results in an exchange of chromosomal segments• and therefore reshuffling of genes.• Watch this video and then get ready for some group work on

meiosis