Unit 8- Reproduction

8a- Meiosis & Variation

Collect!

• Benchmark 3 Study Guides that were due yesterday

Chromosome Number• Organisms have tens of thousands of genes that

determine individual traits. – The more closely related two organisms are,

the more genes they’ll have in common• Genes are lined up on chromosomes.• In somatic (body) cells of animals & most

plants, chromosomes occur in pairs.– Called homologous chromosomes – each

pair has genes that code for the same proteins.

– 1 chromosome came from the male parent & 1 from the female parent.

• A cell with two of each kind of chromosome is called diploid (2n).

Chromosome Number• Organisms also produce gametes

– Haploid (n) reproductive cells- contain one of each kind of chromosome

• Gametes have one of each kind of chromosome so that when they combine (as egg and sperm do during fertilization), the resulting cell is diploid.

• Each species has a specific number of chromosomes. – Humans have 23 pairs (46 total)– Fruit Flies have 4 pairs (8 total)– Dogs have 39 pairs (78 total)

Human Chromosomes• Humans have 23 pairs

of chromosomes

• 22 pairs of autosomes (chromosomes that do not determine gender)

• 1 pair of sex chromosomes

Karyotype

Meiosis: Why?• Mitosis divides one diploid cell to form two diploid cells

– Example: A human cell with 46 chromosomes divides to form two cells with 46 chromosomes.

• If each parent were to pass on a diploid cell to the offspring, that offspring would then have 4 copies of each chromosome– 46 chromosomes from each parent would yield a 92

chromosome offspring• Meiosis allows for two divisions to divide one diploid

cell into four haploid cells.– Results in a reduction in chromosome number – Aka reduction division

Meiosis: Where and Who?• Meiosis takes place in the

gonads (sexual organs)– For humans, these are the ovaries

& testes– Meiosis produces egg & sperm

cells (gametes)– Humans can produce 223 different

kinds of eggs & sperm….more than 8 million combinations!

• Sexual reproduction requires the fusion of gametes (fertilization)– The haploid sperm and egg join to

form a diploid zygote

Video!

• https://www.youtube.com/watch?v=toWK0fIyFlY

Checkpoint

• Mitosis created ____________ cells, meaning they had the same number of chromosomes, and they were genetically similar.

• This was useful for processes like ________, and __________ of tissues.

• Meiosis created genetically ___________ cells.• This is useful for ______________, the fusion

of egg & sperm cells to form a zygote.

Meiosis Phases• Meiosis occurs in 2 steps:1. Meiosis I

– Results in 2 haploid daughter cells with duplicated chromosomes different from the sets in the original diploid cell.

2. Meiosis II– Results in 4 haploid

daughter cells with single (unduplicated) chromosomes

Interphase- G1, S, G2

• Before meiosis the cell must prepare for division:Cells increase in sizeDNA is replicated– Necessary proteins

& RNA are synthesized

• During this phase, chromosomes are not yet visible.

Prophase I• Nuclear membrane breaks down• Centrosomes head to opposite poles &

spindle forms• Duplicated chromosomes condense

– As in mitosis, each chromosome consists of two identical sister chromatids attached at a point called the centromere.

• (*hint- count centromeres to determine # of chromosomes*)

Prophase I• Synapsis- homologous chromosomes pair

up to form a tetrad– A homologous chromosome pair consists of

two chromosomes containing the same type of genes.

– Because the homologous chromosome pairs are very close to one another, an exchange of genetic material between the pairs occurs in a process called crossing over.

– Crossing over causes the daughter cells to have DNA that is different (different gene combination) from the original parent cell• Genetic recombination

Crossing Over Results in Genetic Recombination

Metaphase I

• Paired homologous chromosomes are aligned along equator of the cell with 1 chromosome of a pair on one side and the other chromosome of a pair on the other side. – Each pair is randomly oriented in terms of whether the

paternal or maternal chromosome is on a given side of the equator.

– Result is that 23 chromosomes, some from the mother & some from the father are lined up on each side of the equator. • This arrangement is called independent assortment • Causes the daughter cells to have DNA that is different from the

original parent cell

Anaphase I• The homologous chromosome pairs separate and

move to opposite poles of the cell. • Each daughter cell will receive only one chromosome

from each homologous chromosome pair. • Sister chromatids remain attached to each other

– Centromeres DO NOT split

Telophase I and Cytokinesis

• Chromosomes gather at the poles and uncoil

• Nuclear membrane reforms• Cytokinesis begins

– Each of the 2 daughter cells contains one chromosome (consisting of 2 sister chromatids) from each parental pair, and are therefore haploid

Meiosis I

Prophase II

• Chromosomes become visible

• Spindle forms• If nuclear membrane

reformed after Telophase I, it will break down now

Metaphase II

• Chromosomes, made up of two sister chromatids, line up across the center of the cell in random order (just as they did during Mitosis)

Anaphase II

• The chromosomes separate so that one chromatid from each chromosome goes to each pole.

Telophase II & Cytokinesis

• Nuclear membrane reforms around each set of chromosomes

• The cell undergoes cytokinesis• The four resulting daughter cells are still haploid

(as they were at the end of meiosis I) because meiosis II is almost identical to mitosis– Each cell contains

one chromosome from each homologous pair

Let’s See it!

Meiosis II

Genetic Variation• The DNA of the daughter cells produced by meiosis is

different from that of the parent cells due to 3 sources of genetic diversity provided by sexual reproduction and meiosis:– Fertilization combines the genetic material of two

genetically unique individuals (the 2 parents)– Crossing over produces new combinations of genes. – Independent assortment allows for the possibility of about 8

million different combinations of chromosomes that could end up in a cell produced by meiosis.