Meiosis – the basis of sexual reproduction Section 6.1

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)

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

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?

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

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

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

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.

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.

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

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)

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

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

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

Crossing Over

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.

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

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

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

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

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

Meiosis Video http://www.youtube.com/watch?v=rqPMp0 U0HOA

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

Mitosis and Meiosis

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

Gamete formation

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.

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

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.

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.

Normal Male Karyotype Note the small Y chromosme

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

Other genetic disorder chromsome mutation locations

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

Meiosis video Ameba sisters – see handout http://www.youtube.com/watch?v=toWK0fI yFlY