Presentation on theme: "Meiosis – the basis of sexual reproduction"— Presentation transcript:
1Meiosis – the basis of sexual reproduction Section 6.1
26.1 Part 1 Asexual reproduction – involves cell division (mitosis) Sexual reproduction – involves sex cells joining to form a zygoteZygote – first cell created when two sex cells join (egg and sperm)
3Organisms that reproduce sexually have two types of cells: Somatic cellsBody cells (muscles, skin, bone etc.)Reproduce by cell division (mitosis) (grow and replace dead cells)In humans 23 pairs of chromosomes (46 total)GametesSex cells½ the chromosomes of the parent cell (23 total in humans)Sperm: male gametes Ova: Female gametesGametes: name for sex cells
4Sex 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?
5MeiosisChromosomes need to be reduced so offspring have the correct number of chromosomes.Meiois – reduces the number of chromosomes in sex cellsproduces gametes½ the chromosomes of the parent cellonly occurs in cells that produce gametes (germ cells)“reductive” division
6Cells with half the chromosomes are called haploid (n) – (gametes) Cells with the full amount of chromosomes are called diploid (2n)
7In 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
8In 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.
9Remember 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.
116.1 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 = spermFemale sex cell = egg (ova)
12Phases of Meiosis: Interphase: Before Meiosis begins each homologous pair of chromosomes is replicated
13Meiosis I Meiosis II Homologous chromosomes separate into two cells Meiosis continues with a second divisionChromosomes do not replicateSister chromatids separateOutcome is four cells with haploid set of chromosomes in each cell
15Ensuring variationAn 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
17Independent Assortment In Meiosis pairs of homologous chromosomes are divided in half – to form haploid cellsThis 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.
18Meiosis I Prophase 1 Chromosomes condense spindle fibres forms centrioles move to opposite poles of cellnuclear envelope disappearshomologous chromosomes pair upCrossing over occurs
19Meiosis 1 continues Metaphase 1 Anaphase 1 Paired homologous chromosomes move to the equator of cellspindle fibers are connected to centromeres of one of each homologous pairAnaphase 1spindle fibers move homologous chromosomes away from each other to opposite polessister chromatids are attachedeach side of cell only has half the chromosomes
20Meiosis 1 continues Telophase 1 Cytokinesis occurs – two cells formed spindle fibers disappearnuclear envelope appears brieflyequator of cell pinches in to create new cellsCytokinesis occurs – two cells formedChromosomes are still duplicated
21Meiosis 2 begins Prophase 2 Metaphase 2 similar to mitosis nuclear envelope disappearsspindle fibers appear from centrioles at opposite poleschromosomes condenseMetaphase 2non homologous chromosomes move to equator forming a linespindle fibers are connected to each sister chromatid
22Meiosis 2 continues Anaphase 2 Telophase 2 spindle fibers pull sister chromatids separate and move to opposite ends of cellTelophase 2chromatids (now chromosomes) at each pole lengthen and decondenseNuclear membrane reappears around chromatinFour new cells are created with different genetic components
27Comparing Meiosis and Mitosis Keeps the number of chromosomes from doubling each generation by producing haploid sperm or egg cellsIs responsible for growth, tissue repair and some forms of asexual reproductionEnsure variation in a species because chromosomes from each parent are combinedEnsures all cells produced are identicalHas two cell divisionsHas one cell divisionMakes the diploid sex cell become haploidMakes two exact copies (daughter cells) of a parent cell
29Four 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.
30Chromosome mutations Mutations to chromosomes can occur in meiosis. pieces of chromosomes may be lostchromosomes may be duplicatedgene sequences may be moved within chromosomes or to other chromosomesMutations may be caused by problems in meiosis or can be caused by mutagens, such as radiation or chemicals
31In Meiosis 1 , whole chromosomes may not separate In Meiosis 2, sister chromatids may not separateOne gamete may have two copies of a chromosome and the other none.
32KaryotypesKaryotype: is the number and visual appearance of chromosomes in the cell nuclei of a speciesKaryotypes 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.