GRADE 12 LIFE SCIENCE

SECTION 1 Tissues, cells and molecular studies

PART 2 CELL DIVISION MEIOSIS

MEIOSIS UNIT 1 Introduction and Terms

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

Human Chromosomes Photograph of human chromosomes Can you find another other pair of homologous chromosomes ? Look for a pair of similar looking (homologous) chromosomes 1

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 2

Chromosome Terminology Chromosomes consist of a chromatid and centromere chromatid centromere 3

Chromosome Terminology Chromosome Terminology When cells divide, chromosomes appear as double stranded structures with 2 chromatids and a centromere centromere chromatid chromatid 4

Chromosome Terminology Chromosome Terminology Chromosomes that are similar and occur in pairs are called homologous chromosomes 5

Cell Division When cells divide, the chromosomes divide and separate into the new cells 6

There are 2 types of cell division MITOSIS MEIOSIS For growth To make sex cells (gametes) 7

MEIOSIS UNIT 2 Revision of Mitotic Division Revision of Mitotic Division

Mitosis 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 ? ? 1

Looking at what happens to the chromosomes when the cell divides by MITOSIS 2

Remember the different stages of the division from Grade 10 InterphaseInterphase ProphaseProphase MetaphaseMetaphase AnaphaseAnaphase TelophaseTelophase 3

MITOSIS – Interphase MITOSIS – Interphase - Centrioles Cell Membrane Nucleolus Nuclear Membrane Chromatin network Cell prepares to divide. DNA replication occurs TEST Yourself on cell labels 4

MITOSIS – Interphase MITOSIS – Interphase - Centrioles Cell Membrane Nucleolus Nuclear Membrane Chromatin network LABELSTEST 5

MITOSIS – Prophase MITOSIS – Prophase - Nuclear envelope disperses Centrioles move to poles and spindle fibres form Chromosomes appear as double stranded structures Chromosomes appear as double stranded structures Nucleolus disperses 6

MITOSIS – Metaphase MITOSIS – Metaphase - Chromosomes line up SINGLE FILE at the Equator 7

MITOSIS – Anaphase MITOSIS – Anaphase - Double stranded Chromosomes split to form TWO single stranded chromosomes 8

MITOSIS – Anaphase Single stranded chromosomes move to opposite poles 9

MITOSIS – Telophase Two new cells form Each with TWO single stranded chromosomes 10

INTERPHASE PROPHASE METAPHASE ANAPHASE TELOPHASE CYTOKINESIS

MEIOSIS UNIT 3 Differences from Mitosis and The Human Life Cycle

Meiosis ? ?? In Meiosis, The cell divides TWICE to produce FOUR cells with HALF of the original number of chromosomes. How does MEIOSIS MEIOSIS differ? differ? 1

MEIOSIS is the division of Reproduction. In reproduction there is a fusion of a sperm and egg cell. Sperm Egg 2

When MEIOSIS takes place to make these cells, the chromosome number is halved Sperm Egg 3

That means that the sperm and egg cell will have 23 chromosomes Sperm Egg 4

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 Sperm Egg 5

The Human Life Cycle 6

n n n n 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 in the testes and ovaries The Human Life Cycle 2n 7

n n n n meiosis fertilisation zygote (2n) Sperm and egg cells (n-haploid) fuse to form a (2n-diploid) zygote The Human Life Cycle 8

n n n n meiosis fertilisation zygote (2n) 2n mitosis Zygote divides many times by mitosis to form a child then adult The Human Life Cycle 9

CELL DIVISION MEIOSIS UNIT 4 The Process: Division I

Looking at what happens to the chromosomes when the cell divides by MEIOSIS 1

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 2

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 3

Crossing Over brings Variation Four different types of chromatids Two Instead of 4

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 5

How many possible combinations are there ? With 2 chromosome pairs (2) there are 4 possible combinations 2 2 = 4 This is called independent assortment 6

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!! 7

MEIOSIS – Anaphase I MEIOSIS – Anaphase I - Chromosomes separate and move to opposite poles 8

MEIOSIS – Telophase I MEIOSIS – Telophase I - Two new cells form Each with TWO chromosomes 9

MEIOSIS UNIT 5 The Process: Division II

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 1

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 2

MEIOSIS – Metaphase II MEIOSIS – Metaphase II - Chromosomes line up SINGLE FILE at the Equator 3

MEIOSIS – Anaphase II MEIOSIS – Anaphase II - Double stranded Chromosomes split to form TWO single stranded chromosomes 4

MEIOSIS – Anaphase II MEIOSIS – Anaphase II - Single stranded chromosomes move to opposite poles 5

MEIOSIS – Telophase II MEIOSIS – Telophase II - From each cell in TelophaseI, two new cells form Each with TWO single stranded chromosomes 6

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 7

NOW WATCH THE WHOLE PROCESS CONTINUOUSLY FIRSTDIVISION 8

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One Cell in SECOND DIVISION 9

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MEIOSIS Summary Summary- PARENT CELL with 4 chromosomes FIRST DIVISION TWO cells with 2 chromosomes 2 chromosomes SECOND DIVISION FOUR cells with 4 chromosomes 4 chromosomes 10

CELL DIVISION MEIOSIS UNIT 6 Problems in Meiosis Down’s Syndrome

Down’s Syndrome Can you see any problem with this set of human chromosomes? 1

Down’s Syndrome There is an extra chromosome 21 2

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 3

Down’s Syndrome :Egg cell formation in Meiosis In Down’s syndrome, chromosome pair No. 21 does not separate during Anaphase 4

Telophase I one cell has an extra chromosome 21 and the other has no chromosome 21 Down’s Syndrome 5

Down’s syndrome When the cell with an extra chromosome divides during MEIOISIS II.. 6

Down’s syndrome Metaphase II 7

Down’s Syndrome Anaphase II 8

From each cell in Telophase I, two new cells form, each with an extra chromosome 21 Down’s Syndrome 9

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 10

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 11

Down’s Syndrome: Chromosome recap Showing the extra chromosome 21 12

The following diagram shows an abnormality during a certain phase of meiosis with the number 21 chromosome pair. 4 Give TWO symptoms of this genetic abnormality. (2) 5 People with this condition are often sterile. Explain why you think this is so. (2) (9) 1 Which phase of meiosis does this diagram refer to? (2) 2 What is the specific purpose of this phase of the cell division? (2) 3 What specific genetic condition does this abnormality (shown by A) cause? (1)