Sexual Reproduction Science 9- Mr. Klasz
Why sexual reproduction? Imagine a world where everyone was IDENTICAL. If we reproduced asexually, that would be our world Boring!
Asexual Reproduction vs. Sexual Reproduction Asexual Reproduction Produces offspring identical to the parent Requires little energy Requires only one partner Sexual Reproduction Produces unique offspring; genetic variation Requires more energy Requires two partners
Somatic Cells Somatic Cells are all cells in our bodies except for our sexual cells Our somatic cells all reproduce via mitosis I.E. Our skin cells come from asexual reproduction, and are all identical.
Gametes Gametes are specialized cells produced by specialized organs in our bodies. Sperm are the male gamete Produced in testicles. Ovum (eggs) are the female gametes Produced in the ovaries.
The zygote When an egg and a sperm fuse, they create the first cell of a new organism! This first cell is called the zygote.
The human chromosomes Humans have 46 chromosomes arranged into 23 pairs of chromosomes. These pairs of chromosomes are called homologous chromosomes. Homologous chromosomes are of similar size and function One chromosome in each pair came from your mom. One chromosome in each pair came from your dad.
Haploid vs. diploid Our somatic cells contain all 46 chromosomes This is called diploid We represent this with 2n Our gametes contain only 23 chromosomes This is called haploid We represent this with 1n
How do we get the haploid cells? Our gametes have to under go a SECOND division, to have only HALF the number of chromosomes as our somatic cells. If our gametes didn t under go this second division, when the sperm and egg fuse there would be 92 chromosomes in the zygote, not 46!
Recap questions! What is the difference between haploid and diploid cells? Be specific to the human body. What is meant by homologous chromosomes?
Meiosis! Meiosis is mitosis for sex cells. It ensures that each gamete has only a half set of chromosomes. It ensures that each gamete has a DIFFERENT combination of chromosomes Not all gametes are identical! This is why you are your siblings are not identical!
In a nut shell The cell cycle for sexual reproduction involves several steps: Interphase Meiosis I Cytokinesis I Meiosis II Cytokinesis II Both stages of meiosis are very similar to mitosis. The biggest difference? Interphase doesn t happen before the second division, i.e. 46 chromosomes are split into two cells of 23 chromosomes. They are not replicated to keep the number at 46!
Cells Produced Via. Meiosis After Interphase 2n=46 n=23 4n=92 n=23 4 gametes haploid (n) 2n=46 n=23 meiosis I diploid (2n) meiosis II n=23
An overview of meiosis http://www.sumanasinc.com/webcontent/animations/co ntent/meiosis.html
Interphase (4n=92 chromosomes) Much like mitosis, the cell prepares for meiosis by undergoing interphase. During this period the cell prepares to divide. The cell grows, and the chromosomes are duplicated. Each chromosome is duplicated.
Prophase 1 (4n=92 chromosomes) During prophase 1: The chromosomes condense, and homologous pairs of chromosomes pair up. These pairs of homologous chromosomes form a tetrad. (Form of xx) The centrioles move to the poles of the cell The spindle fibers begin to form from the centrioles, and attach to the centromeres. And crossing over happens
Tetrads During interphase, each chromosome is duplicated The homologous chromosomes (now each double stranded chromosomes) pair up to form a tetrad during prophase 1.
Tetrads. Homologous Chromosomes Chromatides Tetrad Chromatides
Crossing Over Crossing over ensures that not all gametes are the same. Chromosomes switch pieces so that they are different then the originals. These pieces are swapped within tetrads during prophase 1.
Spindle Fibers centrioles
Metaphase 1 (4n=92 chromosomes) During metaphase 1: Spindle fibres shorten. This tugging action causes the tetrads to line up in the middle of the cell. Random assortment also occurs. This is the second way that meiosis ensures that gametes are unique.
Random Assortment During metaphase I, there are two different ways that the tetrads can line up on the metaphase plate.
Metaphase I (4n = 92)
Anaphase 1 (4n=92 chromosomes) During Anaphase 1: Spindle fibers shorten. This pulls the tetrads apart, and the homologous chromosomes to opposite poles of the cell. The homologous chromosomes are still attached in the middle by their centromeres.
Telophase 1 (2n = 46 chromosomes) Each pole has a diploid number of chromosomes. These are homologous chromosomes. Very similar to mitosis (the spindle fibres disappear, nuclear membranes reform)
Cytokinesis 1 Not officially part of meiosis I, it is a wrapping up phase. The cell splits into two diploid cells.
Cytokinesis
Recap Questions! What is a tetrad? What happens in meiosis 1 that ensures that all gametes are unique?
Meiosis II Interphase II Non-existent!
Prophase II (2n = 46) Same as prophase during mitosis
Metaphase II (n = 46) Same as metaphase during mitosis
Anaphase II (2n = 46) Same as anaphase during mitosis. At this point the homologous chromosomes are separated into chromatid (or single stranded chromosomes)
Telophase II/Cytokinesis II (n = 23) Same as telophase in mitosis, except each cell has only 23 chromosomes! The spindle fibers dissolve, and the nuclear membrane reforms. Cytokinesis Each of the cells created through Meiosis I divide. Important: 4 haploid gametes are formed!
Telophase II/ Cytokinesis (n = 23)
Recap animation http://www.cellsalive.com/meiosis.htm
Wait! There s more! How many cells are produced by meiosis? In males there are four, functional haploid sperm cells produced In females there is only one haploid egg produced, and three polar bodies produced. This happens because egg cells need to be bigger, i.e. require more cytoplasm be functional!