1. Meiosis – the basis of sexual reproduction
Section 6.1

2. 6.1 Part 1 Asexual reproduction – involves cell division (mitosis)
Sexual reproduction – involves sex cells joining to form a zygote
Zygote – first cell created when two sex cells join (egg and sperm)

3. Organisms that reproduce sexually have two types of cells:
Somatic cells
Body cells (muscles, skin, bone etc.)
Reproduce by cell division (mitosis) (grow and replace dead cells)
In humans 23 pairs of chromosomes (46 total)
Gametes
Sex cells
½ the chromosomes of the parent cell (23 total in humans)
Sperm: male gametes Ova: Female gametes
Gametes: name for sex cells

4. Sex is basically the joining of genetic material from two individuals.
In humans – we receive one set of 23 chromosomes from our mother and one set of 23 chromosomes from our father.
In our somatic cells we have 23 pairs of chromosomes – how are the chromosome numbers kept down if there is genetic information from both parents? How do the egg and sperm only have one set?

5. Meiosis
Chromosomes need to be reduced so offspring have the correct number of chromosomes.
Meiois – reduces the number of chromosomes in sex cells
produces gametes
½ the chromosomes of the parent cell
only occurs in cells that produce gametes (germ cells)
“reductive” division

6. Cells with half the chromosomes are called haploid (n) – (gametes)
Cells with the full amount of chromosomes are called diploid (2n)

7. In our body cells we have pairs of chromosomes called homologous pairs
In our body cells we have pairs of chromosomes called homologous pairs. Half of the chromosomes come from the mother, half from the father

8. In our body cells 22 chromosomes have a matching chromosome from the other set. The two chromosomes that form the 23rd pair are called the sex chromosomes.
For females – the sex chromosomes appear identical and have an X shape. This pair is referred to as XX.
In Males – the chromosomes are not homologous and do not have the same shape. One appears to have an X shape, and the other smaller chromosome is called the Y chromosome. This pair is called XY.

9. Remember mitosis produces two identical daughter cells from one parent cell – they have exactly the same genetic information as the parent cells.
Meiosis is a different form of cell division – it is the basis for sexual reproduction.
Meiosis ensures variation within a species.
Meiosis produces gametes.

10. Do Questions pg 203 #2, 3, 4, 17
Start Vocabulary Assignment

11. 6.1 Part 2 Meiosis Reminders of terms: Gamete – sex cells
Diploid – cells with a full set of chromosomes (2 sets of 23 in humans)
Haploid – cells with a half set of chromosomes (1 set of 23 in humans)
Male sex cell = sperm
Female sex cell = egg (ova)

12. Phases of Meiosis: Interphase: Before Meiosis begins
each homologous pair of chromosomes is replicated

13. Meiosis I Meiosis II Homologous chromosomes separate into two cells
Meiosis continues with a second division
Chromosomes do not replicate
Sister chromatids separate
Outcome is four cells with haploid set of chromosomes in each cell

15. Ensuring variation
An important step in meiosis 1 is called Crossing Over.
Non –sister chromatids cross over and exchange DNA segments – each chromosome now has new genetic information.
Multiple crossovers can occur between chromosomes

16. Crossing Over

17. Independent Assortment
In Meiosis pairs of homologous chromosomes are divided in half – to form haploid cells
This separation is random.
Not all maternal chromosomes end up in one cell, and not all paternal chromosomes end up in another. There is a mixture of genetic material in cells from both the parents.

18. Meiosis I Prophase 1 Chromosomes condense spindle fibres forms
centrioles move to opposite poles of cell
nuclear envelope disappears
homologous chromosomes pair up
Crossing over occurs

19. Meiosis 1 continues Metaphase 1 Anaphase 1
Paired homologous chromosomes move to the equator of cell
spindle fibers are connected to centromeres of one of each homologous pair
Anaphase 1
spindle fibers move homologous chromosomes away from each other to opposite poles
sister chromatids are attached
each side of cell only has half the chromosomes

20. Meiosis 1 continues Telophase 1 Cytokinesis occurs – two cells formed
spindle fibers disappear
nuclear envelope appears briefly
equator of cell pinches in to create new cells
Cytokinesis occurs – two cells formed
Chromosomes are still duplicated

21. Meiosis 2 begins Prophase 2 Metaphase 2 similar to mitosis
nuclear envelope disappears
spindle fibers appear from centrioles at opposite poles
chromosomes condense
Metaphase 2
non homologous chromosomes move to equator forming a line
spindle fibers are connected to each sister chromatid

22. Meiosis 2 continues Anaphase 2 Telophase 2 spindle fibers pull
sister chromatids separate and move to opposite ends of cell
Telophase 2
chromatids (now chromosomes) at each pole lengthen and decondense
Nuclear membrane reappears around chromatin
Four new cells are created with different genetic components

24. Meiosis Video
U0HOA

25. 6.1 Part 3 Comparing Mitosis and Meiosis, Gamete formation and Mutations
The following diagram shows how meiosis and mitosis fit in to the life cycle of a sexually reproducing organism

26. Mitosis and Meiosis

27. Comparing Meiosis and Mitosis
Keeps the number of chromosomes from doubling each generation by producing haploid sperm or egg cells
Is responsible for growth, tissue repair and some forms of asexual reproduction
Ensure variation in a species because chromosomes from each parent are combined
Ensures all cells produced are identical
Has two cell divisions
Has one cell division
Makes the diploid sex cell become haploid
Makes two exact copies (daughter cells) of a parent cell

28. Gamete formation

29. Four sperm are produced from one “germ” cell in males, whereas only one egg cell is produced in females for every female germ cell.
Germ cells are produced in the gonads of organisms – testicles in males and ovary in females.

30. Chromosome mutations Mutations to chromosomes can occur in meiosis.
pieces of chromosomes may be lost
chromosomes may be duplicated
gene sequences may be moved within chromosomes or to other chromosomes
Mutations may be caused by problems in meiosis or can be caused by mutagens, such as radiation or chemicals

31. In Meiosis 1 , whole chromosomes may not separate
In Meiosis 2, sister chromatids may not separate
One gamete may have two copies of a chromosome and the other none.

32. Karyotypes
Karyotype: is the number and visual appearance of chromosomes in the cell nuclei of a species
Karyotypes can be analyzed to determine genetic abnormalities, or to compare relatedness.
Karyotypes are prepared by staining cells in stages of cell division and basically a picture is taken of chromosomes. They are then compiled into one image.

33. Normal Male Karyotype
Note the small Y chromosme

34. Karyotypes of Males with genetic disorders
Downs Syndrome – extra copy of chromosome 21

35. Other genetic disorder chromsome mutation locations

36. Answer Questions pg 203: 6, 7, 8, 10, 16, 18

37. Meiosis video
Ameba sisters – see handout yFlY