Gr.12 Life Sciences Meiosis

Daughter cells - new cells produced by cell division Definition of Terms Meiosis – cell division whereby a diploid cell(somatic cell)undergoes two cell divisions to form four dissimilar haploid cells( sex cells) Chromosomes - cell parts that determine what traits a living thing will have Chromatid - one strand of DNA; after replication, a chromosome is made up of two identical Chromatids Daughter cells - new cells produced by cell division

The Process

Where does it take place…

THE SIGNIFICANCE OF MEIOSIS The process of meiosis reduces the number of chromosomes by half ( diploid to haploid) Meiosis is the process through which gametes are produced , and are prepared for fertilization Meiosis ensures that the number of chromosomes in species stays constant over generations Meiosis is important to introduce genetic variation though: - Crossing over ( Prophase 1) Random arrangement of chromosomes at the equator ( Metaphase )

How chromosomes are arranged…

ABNORMAL MEIOSIS -Down Syndrome

Spindle fibre fails to separate during meiosis NON -DISJUNCTION Spindle fibre fails to separate during meiosis Members of one pair of homologous chromosomes fail to become separated during Anaphase 1 and may happen in Anaphase 2 2 gametes receive extra copy of affected chromosome 2 gametes lack that chromosome

Non-Disjunction continued…

COMPARISON OF MITOSIS AND MEIOSIS Site where it occurs Somatic cells Ovaries and testis Ovules & anthers Purpose of process Growth Formation of haploid gametes No. of daughter cells produced Two identical cells 4 haploid cells genetically different No. of divisions One nuclear division Two nuclear divisions

Practical activity Meiosis song

ACTIVITY 1 The following diagram represents a phase of meiosis. Answer questions based on this diagram. 1. Write down the term that best describes the paired chromosomes labelled A. (1) 2. Identify structure B (1) 3. How many chromosomes are shown in the diagram above? (1) 4. How many chromosomes would be there in each cell at the end of meiosis? (1) 5. Give two significance of meiosis. (2)

Answers to Activity 1 Homologous √ (1) 2. Spindle threads / fibre √ (1) 3. 4 √ (1) 4. 2 / Half √ (1) 5. -To reduce the number of chromosomes from diploid to haploid √ - for the production of gametes √ - introduce genetic variation - keep the number of chromosomes the same throughout generations any x (2)

Activity 2 Study the following diagram and answer questions. Provide labels for part A - D on the diagrams a alongside. (4) 2. Name the process in meiosis that is illustrated on this diagram. (1) 3. Name ONE importance of the process mentioned in question 2. (1) 4. Draw the structure of the diagram labelled A to show its appearance immediately after the process mentioned in question 2. (2) C B A D

Answers to Activity 2 A- Chromosome (1) B- Centromere (1) C- Chromatid (1) D- Chiasma (1) 2. Crossing over (1) 3. To bring about genetic variation (1) 4. - diagram –(1) - different shades- (1) P

Activity 3 The diagram below represents an animal cell in a phase of meiosis. State which phase of meiosis is represented in the diagram. (1) 2. Give a reason for your answer to question 1. (2) 3. How many chromosomes: (a) were present in the parent cell before it underwent meiosis (1) (b) will be present in each cell after meiosis (1) 4. Where does meiosis take place in : (a) Humans (1) (b) Plants (1) 5. Could the cell represented in the diagram be that of a human. (1) 6. Give a reason for your answer to question 5. (2)

Activity 3 Answers 1. Anaphase 2 √ (1) 2. Chromatids are pulled towards the poles. √ (1) 3. a – 4 √ (1) b – 2 √ (1) 4. (a) – Ovaries and testes √ (1) (b) – Ovaries and anthers √ (1) 5. No √ (1) 6. Humans would have 23√ chromosomes/ 46 chromatids in this phase (2) In this diagram only 4 √chromosome/ 8 chromatids are shown in the diagram.

Activity 4

Activity 4 Answers