CELL DIVISION MEIOSIS

Learning outcomes Describe want happens to chromosomes during the process Explain the importance of meiosis Compare mitosis and meiosis

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

There are 22 pairs of autosomes .. and one pair of sex chromosomes 23 pairs 22 + 1 There are 46 human chromosomes found in 23 pairs of similar (homologous) chromosomes Human Chromosomes arranged in homologous pairs. How many pairs are there ?

Chromosome Terminology chromatid Chromosomes consist of a chromatid and centromere centromere

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

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

Cell Division When cells divide, the chromosomes divide and separate into the new cells

There are 2 types of cell division To make sex cells (gametes) MITOSIS MEIOSIS To make sex cells (gametes) For growth

Mitosis 46 In Mitosis, the cell divides ONCE and TWO identical cells with the SAME original number of chromosomes are produced What will be the result of this cell dividing by MITOSIS ? ? 46 46 10

Looking at what happens to the chromosomes when the cell divides by MITOSIS 11

Remember the different stages if the division Interphase Prophase Metaphase Anaphase Telophase 12

Cell prepares to divide. DNA replication occurs MITOSIS – Interphase- TEST Yourself on cell labels Centrioles Cell prepares to divide. DNA replication occurs Cell Membrane Nucleolus Chromatin network Nuclear Envelope

MITOSIS – Interphase- Centrioles LABELS Cell Membrane TEST Nucleolus Chromatin network Nuclear Envelope

MITOSIS – Prophase - Chromosomes appear as double stranded structures Centrioles move to poles and spindle fibres form Nucleolus disperses Nuclear envelope disperses

Chromosomes line up SINGLE FILE at the Equator MITOSIS – Metaphase - Chromosomes line up SINGLE FILE at the Equator

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

Single stranded chromosomes move to opposite poles MITOSIS – Anaphase Single stranded chromosomes move to opposite poles

Two new cells form Each with TWO single stranded chromosomes MITOSIS – Telophase Two new cells form Each with TWO single stranded chromosomes

Cytokinesis The shortest stage of the cell cycle is called cytokinesis (division of the cytoplasm). In cytokinesis, the cytoplasm and its organelles divide into two daughter cells. – Each daughter cell contains a nucleus with an identical set of chromosomes. The two daughter cells then start their own cycles, beginning again with the interphase stage.

In animal cells, a cleavage furrow forms and separates Daughter Cells The first sign of cleavage is the appearance of a cleavage furrow, a shallow groove in the cell surface. The cleavage furrow deepens and eventually pinches the parent cell in two, producing two completely separate cells, each with its own nucleus and share of cytoplasm.

Cytokinesis in plant cells is different because a cell plate forms. During telophase, vesicles containing cell wall material collect at the middle of the parent cell. The vesicles fuse, forming a membranous cell plate. The cell plate grows outward, accumulating more cell wall materials as more vesicles fuse with it.

Cytokinesis in plant cells Eventually, the outer edges of the cell plate fuse with the plasma membrane, and the cell plate’s contents join the parental cell wall. The result is two daughter cells, each bounded by its own plasma membrane and cell wall.

Differences from Mitosis and The Human Life Cycle MEIOSIS Differences from Mitosis and The Human Life Cycle 24

How does MEIOSIS differ? Meiosis In Meiosis, The cell divides TWICE to produce FOUR cells with HALF of the original number of chromosomes. 46 ? 23 23 ? ? 23 23 23 23 25

MEIOSIS is the division of Reproduction. Sperm Egg MEIOSIS is the division of Reproduction. In reproduction there is a fusion of a sperm and egg cell. 26

When MEIOSIS takes place to make these cells, the chromosome Sperm Egg When MEIOSIS takes place to make these cells, the chromosome number is halved 27

That means that the sperm and egg cell will have 23 chromosomes 28

Sperm Egg 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 chromosomes. 23 from mom and 23 from dad 29

The Human Life Cycle

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

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

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

The Process: Division I CELL DIVISION MEIOSIS The Process: Division I 34

Looking at what happens to the chromosomes when the cell divides by MEIOSIS 35

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

MEIOSIS – Prophase I Crossing Over This mixes genetic material and brings variety This happens when partner chromosomes swop pieces of chromatid As homologous pairs line up, crossing over occurs Chromatids from partner chromosomes cross over Pieces of chromosome are swopped 37

Crossing Over brings Variation Four different types of chromatids Instead of Four different types of chromatids Two 38

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

How many possible combinations are there ? With 2 chromosome pairs (2) there are 4 possible combinations 22 = 4 This is called independent assortment

How many possible combinations are there ? What possible combinations are there with 23 pairs? 223 = ? 8 388 608 Remember this is without crossing over and just in a sperm or egg cell!!

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

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

The Process: Division II MEIOSIS The Process: Division II 44

MEIOSIS – Second division 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 way 45

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

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

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

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

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 1st 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 FIRST DIVISION

One Cell in SECOND DIVISION

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

Problems in Meiosis Down’s Syndrome CELL DIVISION MEIOSIS Problems in Meiosis Down’s Syndrome 62

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

There is an extra chromosome 21 Down’s Syndrome There is an extra chromosome 21

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

Down’s Syndrome :Egg cell formation in Meiosis In Down’s syndrome, chromosome pair No. 21 does not separate during Anaphase

Down’s Syndrome Telophase I one cell has an extra chromosome 21 and the other has no chromosome 21

When the cell with an extra chromosome divides during MEIOISIS II .. Down’s syndrome When the cell with an extra chromosome divides during MEIOISIS II ..

Down’s syndrome Metaphase II

Down’s Syndrome Anaphase II

Down’s Syndrome From each cell in Telophase I, two new cells form, each with an extra chromosome 21

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

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

Down’s Syndrome: Chromosome recap Showing the extra chromosome 21