biology 3201 14.2 notes meiosis.notebook · stages of meiosis •meiosis occurs in two sequential...
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biology 3201 14.2 notes meiosis.notebook
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Biology 3201 Unit II Reproduction
14.2 – How Reproductive Cells are Produced
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Asexual vs Sexual
• single parent• offspring identical to parent• parent passes on ALL its genes• results in a clone
• two parents• offspring is unique
• each parent passes on HALF its genes• offspring are different from their parents and siblings
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Basic terms necessary for an understanding of meiosis:
• germ cell: a sex cell that produces sperm or egg • Unlike somatic cells, germ cells are produced by meiosis • somatic cell - any cell other than a sperm or egg
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What is meiosis? • Meiosis: type of cell division that occurs only in reproductive organs producing reproductive cells called gametes (sperm and egg). Meiosis is a form of reductive division. • Reductive division: division of cells that reduces the chromosome number. For example, in meiosis the chromosome number is reduced from diploid to haploid
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Diploid vs. Haploid
• Diploid (2n): cells that contain two copies of every chromosome • Haploid (n): cells that contain only one copy of every chromosome, half the number of a diploid cell
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Human Chromosomes
• In humans, we have 22 pairs of autosomes (chromosomes not involved in determining sex) and 1 pair of sex chromosomes (XX or XY). Males have XY while female are XX. Sperm cells carry either an X chromosome or a Y chromosome and 22 autosomes, while eggs can only carry an X chromosome and 22 autosomes.
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Stages of Meiosis
• Meiosis occurs in two sequential phases; both of which are very similar to mitosis. As well, DNA is replicated in the cell during interphase just like in mitosis. Unlike mitosis, however, meiosis involves 2 rounds of cell division producing 4 daughter cells and each daughter cell contains half of the DNA as the parent cell.
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Stages of Meiosis (see fig. 14.14, p. 472)
• Meiosis I • 1. Prophase I • chromatin condenses to form chromosomes • pairs of homologous chromosomes align to form tetrads. Sometimes chromatids from homologous chromosomes intertwine and crossing over occurs • homologous chromosomes: chromosomes that contain the same gene sequences but may not be made up of the same allele • tetrads: a homologous pair found in prophase I • alleles: alternate form of a gene
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Stages of Meiosis – Prophase I• crossing over: the process where non-sister chromatids exchange genes during prophase I of meiosis (see fig. 14.15, p. 473). Allows for genetic variety • nuclear membrane and nucleolus diasappear • centrioles produced during interphase migrate towards opposite ends of the cell • spindle fibers begin to form between the two centrioles
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biology 3201 14.2 notes meiosis.notebook
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Stages of Meiosis – Metaphase I
• spindle fibers attach to pairs of sister chromatids of tetrads and pull them towards the equator • chromosomes line up at equator in their homologous pairs; one on one side of the equator, the other on the other side of the equator
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Stages of Meiosis - Anaphase I
• homologous chromosomes separate and are pulled towards opposite ends of the cell (sister chromatids are held together)
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Telophase I
• does not occur in all cells. If it does not, the cells go directly to meiosis II • if telophase I occurs, homologous chromosomes begin to uncoil and spindle fibers disappear, cytoplasm divides, nucleolus and nuclear membrane reforms and two cells are formed
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biology 3201 14.2 notes meiosis.notebook
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Stages of Meiosis – Meiosis II
• Prophase II • spindle fibers reform • centrioles are at opposite ends of the cells • chromosomes form
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biology 3201 14.2 notes meiosis.notebook
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Stages of Meiosis - Metaphase II• spindle fibers attach to centromeres of chromosomes and guid them to equator of cell where they line up
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biology 3201 14.2 notes meiosis.notebook
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Stages of Meiosis - Anaphase II
• centromeres split apart and chromatids move to opposite poles of the cell
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Stages of Meiosis - Telophase II• chromatids unravel; spindle disappears; cytokinesis occurs; nuclear membrane and nucleolus reappear.
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Mitosis and Meiosis Compared
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Key differences:• meiosis is a reduction division. Mitosis ensures equal amounts of DNA per daughter cells.
• meiosis produces variation - four daughter cells are different from the parent and from each other. Mitosis makes exact copies
• meiosis is two separate divisions. Mitosis has one division.
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Meiosis I vs Mitosis
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Gamete formation• Gametogenesis – gamete formation where daughter cells, or gametes, are produced at the end of meiosis II resulting in the production of sperm and egg. • Spermatogenesis – the process of male gamete production in animals • Oogenesis – the process of female gamete production in animals
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Spermatogenesis• Spermatogenesis begins with diploid germ cells called spermatogonia. After meiosis I, one spermatigonia divides into two cells called primary spermatocytes. Primary spermatocytes undergo meiosis II to form secondary spermatocytes, which develop into sperm. In the end, from 1 parent spermatigonia 4 sperm cells are produced.
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Oogenesis• Oogenesis begins with a diploid cell called an oogonia. After meiosis I, one oogonia forms one primary oocyte and one polar body (due to unequal division of the cytoplasm). The primary oocyte and polar body undergo meiosis II to form one secondary oocyte and three polar bodies. The secondary oocyte develops into an egg cell, while all the polar bodies die. Only one functional egg cell comes from this process, as the unequal division of the cytoplasm makes the egg cell big (needs extra nutrients).
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Sperm
• Small• numerous • motile• cap called an acrosome that contains enzymes
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Egg
• large• one per month
• non-motile• specialized outer covering that becomes impenetrable to other sperm
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Sperm vs. egg cells
Sperm Egg
small Large
mobile Not mobile
have a cap called an acrosome which contains enzymes used to enter the egg cell
covered by a thick outer coating. After one sperm penetrates the egg, no more can enter
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Sperm vs. Egg cells
Sperm Egg
millions produced continuously ﴾300 million500 million﴿
one egg matures per month from puberty to menopause
50100 mitochondria per cell
about 140, 000 mitochondia per cell
before ejaculation: uses fat for energy after ejaculation: uses sugar ﴾fructose﴿ for energy
can only live for about a day or so with its food supply