MEIOSIS Introduction and Terms

Human Chromosomes Photograph of human chromosomes Can you find another other pair of homologous chromosomes ? Look for a pair of similar looking (homologous) chromosomes

23 pairs Human Chromosomes arranged in homologous pairs. How many pairs are there ? There are 22 pairs of autosomes.. and one pair of sex chromosomes pair of sex chromosomes There are 46 human chromosomes found in 23 pairs of similar (homologous) chromosomes

Chromosome Terminology Chromosomes consist of a chromatid and centromere chromatid centromere

Chromosome Terminology Chromosome Terminology When cells divide, chromosomes appear as double stranded structures with 2 chromatids and a centromere centromere chromatid chromatid

Chromosome Terminology Chromosome Terminology Chromosomes that are similar and occur in pairs are called homologous chromosomes

All the cells DNA is found in the cell nucleus in structures called chromosomes Cell Division

When cells divide, the chromosomes appear, divide and separate

Two new cells form with identical chromosomes to the original cell Cell Division

There are 2 types of cell division MITOSIS MEIOSIS For Growth To make sex cells for Reproduction

MEIOSIS

MEIOSIS is the division of Reproduction. In reproduction there is a fusion of a sperm and egg cell. Sperm cell Egg

When MEIOSIS takes place to make these cells, the chromosome number is halved from 46 to 23 Sperm cell Egg

That means that the sperm and egg cell will have 23 chromosomes Sperm cell 23 Egg 23

When they fuse the resulting cell (zygote) will then have 46 chromosomes. This cell will grow into a baby with a new full set of 23 PAIRS of chromosomes. 23 from mom and 23 from dad Sperm cell Egg

Meiosis: an outline of the process ? ?? In Meiosis, The cell divides TWICE to produce FOUR cells with HALF of the original number of chromosomes.

Mitosis: outline of the process In Mitosis, the cell divides ONCE and TWO identical cells with the SAME original number of chromosomes are produced 46 What will be the result of this cell dividing by MITOSIS ? ?

How Meiosis differs from Mitosis MEIOSIS 46 MITOSIS

The importance of Meiosis in The Human Life Cycle

n n nnmeiosis meiosis Sperm and egg cells have half the number of chromosomes (n=23) than their parent cells which have 46 chromosomes (2n) Sperm and egg cells are produced by MEIOSIS The Human Life Cycle

n n n n meiosis meiosis fertilisation zygote (2n) Sperm and egg cells (n-haploid) fuse to form a (2n-diploid) zygote The Human Life Cycle

n n n n meiosis fertilisation zygote (2n) zygote (2n zygote (2n) 2n2n mitosis mitosis Zygote divides many times by mitosis to form a child then adult The Human Life Cycle

CELL DIVISION MEIOSIS The Process: Division I

Looking at what happens to the chromosomes when the cell divides by MEIOSIS

MEIOSIS – Prophase I MEIOSIS – Prophase I - Nuclear envelope disperses Centrioles move to poles and spindle fibres form Then homologous (similar) Chromosomes pair up Chromosomes appear as double stranded structures Chromosomes appear as double stranded structures Nucleolus disperses

MEIOSIS – Prophase I Crossing Over As homologous pairs line up, crossing over occurs This happens when partner chromosomes swop pieces of chromatid Chromatids from partner chromosomes cross over Pieces of chromosome are swopped This mixes geneticmaterial and brings variety

Crossing Over brings Variation Four different types of chromatids Two Instead of

MEIOSIS – Metaphase I MEIOSIS – Metaphase I - Homologous chromosomes line up IN PAIRS at the equator The The chromosome chromosome pairs can line up in different combinations – combinations –thisbringsvariety

How many possible combinations are there ? With 2 chromosome pairs (2) there are 4 possible combinations 2 2 = 4 This is called independent assortment

How many possible combinations are there ? What possible combinations are there with 23 pairs? 2 23 = ? Remember this is without crossing over and just in a sperm or egg cell!!

MEIOSIS – Anaphase I MEIOSIS – Anaphase I - Chromosomes separate and move to opposite poles

MEIOSIS – Telophase I MEIOSIS – Telophase I - Two new cells form Each with TWO chromosomes

MEIOSIS The Process: Division II

MEIOSIS – Second division Each of these two cells now divides a second time.Each of these two cells now divides a second time. We will look at only ONE cell dividing but BOTH cells divide in the same wayWe will look at only ONE cell dividing but BOTH cells divide in the same way

MEIOSIS – Prophase II MEIOSIS – Prophase II - Nuclear envelope disperses Centrioles move to poles and spindle fibres form at 90 o Centrioles move to poles and spindle fibres form at 90 o to first division Chromosomes re-arrange

MEIOSIS – Metaphase II MEIOSIS – Metaphase II - Chromosomes line up SINGLE FILE at the Equator

MEIOSIS – Anaphase II MEIOSIS – Anaphase II - Double stranded Chromosomes split to form TWO single stranded chromosomes

MEIOSIS – Anaphase II MEIOSIS – Anaphase II - Single stranded chromosomes move to opposite poles

MEIOSIS – Telophase II MEIOSIS – Telophase II - From each cell in TelophaseI, two new cells form Each with TWO single stranded chromosomes

TELOPHASE II Remember, Two cells divided again after the 1 st division So the end result of MEIOSIS is FOUR cells with HALF the number of chromosomes of the parent cell

NOW WATCH THE WHOLE PROCESS CONTINUOUSLY FIRSTDIVISION

One Cell in SECOND DIVISION

MEIOSIS Summary Summary- PARENT CELL with 4 chromosomes FIRST DIVISION TWO cells with 2 chromosomes 2 chromosomes SECOND DIVISION FOUR cells with 2 chromosomes 2 chromosomes

CELL DIVISION MEIOSIS Problems in Meiosis: Down’s Syndrome

Down’s Syndrome Can you see any problem with this set of human chromosomes?

Down’s Syndrome There is an extra chromosome 21 which results in Down’s syndrome

Down’s Syndrome: Symptoms Single crease on palm Skin fold on the eye lid Mental retardation Wide gap between large and second toe Affectionate and can lead fairly independent lives

Down’s Syndrome : Origin in Egg cell formation during Meiosis In normal cells chromosome pair No. 21 separates during Anaphase

Down’s Syndrome :Egg cell formation in Meiosis In normal cells chromosome pair No. 21 separates during Anaphase

Down’s Syndrome :Egg cell formation in Meiosis Look at what happens to chromosome pair No. 21 on Down’s syndrome.

Down’s Syndrome :Egg cell formation in Meiosis Chromosome pair No. 21 does not separate.

Telophase I One cell has an extra chromosome 21 The other cell has no chromosome 21 Down’s Syndrome :Egg cell formation in Meiosis

When the cell with an extra chromosome divides during MEIOISIS II.. Down’s Syndrome :Egg cell formation in Meiosis

Metaphase II Down’s Syndrome :Egg cell formation in Meiosis

Anaphase II Down’s Syndrome :Egg cell formation in Meiosis

The resulting two egg cells have an extra chromosome 21 Telophase II Down’s Syndrome :Egg cell formation in Meiosis

E E E In normal fertilisation, the sperm and egg cell have a single chromosome 21 Fertilisation results in two chromosome 21’s in the child

E E E In Down’s syndrome, the egg cell has TWO chromosome 21’s Fertilisation results in THREE chromosome 21’s in the child