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Chapter 9 Meiosis. Asexual vs. sexual reproduction. asexual: one parent sexual: two parents What is MITOSIS? ASEXUAL or SEXUAL?. asexual: one parent creates identical copy sexual: two parents create new, different cell What is MITOSIS? ASEXUAL or SEXUAL?.
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Asexual vs. sexual reproduction • asexual: one parent • sexual: two parents • What is MITOSIS? ASEXUALor SEXUAL? • asexual: one parent creates identical copy • sexual: two parents create new, different cell • What is MITOSIS? ASEXUAL or SEXUAL?
Somatic (body) vs. Reproductive (sex) • somatic (body) cells: all body cells EXCEPT sex cells (ex: liver, skin, brain, etc) • reproductive (sex) cells: haploid (half the DNA) gametes used for sexual reproduction ONLY; sperm (male) and egg (female)
Chromosome Number • Remember how DNA works in pairs? • Homologous chromosomes: diploid chromosomes that work in pairs; same trait, different versions (one from dad, one from mom) • Homologs have the same genes in the same order along the chromosome, but they may not be EXACTLY the same. Ex- blue eyes vs brown eyes; blonde vs black hair
Chromosome Number • diploid: cell with “two sets”; has homologous pairs - human body cells have 46 chromosomes (diploid number = 46 or 23 pairs) - fruit fly has 8 chromosomes (diploid number = 8 or 4 pair) • haploid: cell with “half set”; no homologous pairs are present; HALF the normal amount of DNA - human sex cells have 23 chromosomes (haploid = 23) - fruit fly sex cells have 4 chromosomes (haploid = 4)
Karyotype • Karyotype: a picture of all the chromosome pairs; checks for extra or missing; last pair = sex pair (XX female, XY male) • To read: make sure no extra or missing and identify sex • Is this a male or female?
What if? • What would happen if two diploid cells (with 2 complete sets of 23 chromosomes or 46 total) came together during reproduction? • How many chromosomes would that new cell have? 92 chromosomes! • Would it be human?
Mitosis • You have already learned how a cell makes a complete copy of its chromosomes in Mitosis
But … • How do sex cells end up with half the chromosomes that body cells have???? Meiosis
Purpose of Meiosis • produce HAPLOID eggs/sperm, so fertilization can produce a diploid zygote (fertilized egg) with new DNA combinations
“Crossing Over” • In meiosis, variety is BEST! The offspring should be DIFFERENT. To mix up the DNA, sometimes crossing over occurs between homologous pairs. • Crossing Over: Homologous pairs trade pieces to create new combinations. “Mom” and “Dad” chromosomes become “Mad” and “Dom”
Genetic Diversity • Independent assortment (all chromosomes distribute randomly) and crossing over create new combinations of genes for DIVERSITY
Stages of Meiosis (See DIAGRAM) • Meiosis starts the same as mitosis with a duplication of the DNA. In order to create half cells, there are TWO divisions. Meiosis I to split the pairs and Meiosis II to split the chromosomes. • Interphase • Meiosis I (Division of the homologous pairs) • Prophase I • Metaphase I • Anaphase I • Telophase I • Cytokinesis • Meiosis II (Division of the chromosomes) • Prophase II • Metaphase II • Anaphase II • Telophase II • Cytokinesis
Prophase I Pause • Prophase I is longest phase of meiosis; crossing over occurs here • Meiosis is different in males and females. Human egg productionstarts before birth and pauses until puberty, then continues until no more eggs
Meiosis I: Interphase I Interphase I Prophase I Metaphase I Anaphase I Cells undergo a round of DNA replication, forming duplicate Chromosomes. Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. Spindle fibers attach to the chromosomes. The fibers pull the homologous chromosomes toward the opposite ends of the cell.
Meiosis I: Prophase I Interphase I Prophase I Metaphase I Anaphase I Cells undergo a round of DNA replication, forming duplicate Chromosomes. Each chromosome pairs in synapsis with its corresponding homologous chromosome to form a tetrad. Crossing over may occur. Spindle fibers attach to the chromosomes. The fibers pull the homologous chromosomes toward the opposite ends of the cell.
Meiosis I: Metaphase I Interphase I Prophase I Metaphase I Anaphase I Cells undergo a round of DNA replication, forming duplicate Chromosomes. Each chromosome pairs in synapsis with its corresponding homologous chromosome to form a tetrad. Crossing over at chiasma. Spindle fibers attach to the chromosomes. The fibers pull the homologous chromosomes toward the opposite ends of the cell.
Meiosis I: Anaphase I Interphase I Prophase I Metaphase I Anaphase I Cells undergo a round of DNA replication, forming duplicate Chromosomes. Each chromosome pairs in synapsis with its corresponding homologous chromosome to form a tetrad. Crossing over at chiasma. Spindle fibers attach to the chromosomes. The fibers pull the homologous chromosomes toward the opposite ends of the cell.
At the end of Meiosis I … • 2 new cells are formed; although each new cell now has 4 chromatids (as it would after mitosis), something is different • neither of the daughter cells has the two complete sets of chromosomes that it would have in a diploid cell • the two cells produced by Meiosis I have sets of chromosomes and alleles that are different from each other and different from the diploid cell that entered Meiosis I
In Between Meiosis I and Meiosis 2… • There is NO Interphase II • There is no DNA replication
Separation of ChromatidsMeiosis I – Splits the homologous pairsMeiosis II – splits the chromosome
Meiosis II: Prophase II Metaphase II Anaphase II Telophase II Prophase II The chromosomes line up in a similar way to the metaphase stage of mitosis. The sister chromatids separate and move toward opposite ends of the cell. Meiosis II results in four haploid (N) daughter cells. Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.
Meiosis II: Metaphase II Metaphase II Anaphase II Telophase II Prophase II The chromosomes line up in a similar way to the metaphase stage of mitosis. The sister chromatids separate and move toward opposite ends of the cell. Meiosis II results in four haploid (N) daughter cells. Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.
Meiosis II: Anaphase II Metaphase II Anaphase II Telophase II Prophase II The chromosomes line up in a similar way to the metaphase stage of mitosis. The sister chromatids separate and move toward opposite ends of the cell. Meiosis II results in four haploid (N) daughter cells. Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.
Meiosis II: Telophase II Metaphase II Anaphase II Telophase II Prophase II The chromosomes line up in a similar way to the metaphase stage of mitosis. The sister chromatids separate and move toward opposite ends of the cell. Meiosis II results in four haploid (N) daughter cells. Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.
At the end of Meiosis II … • You have FOUR (4) daughter cells with the haploid number (N) of chromosomes!
… a male: all four grow flagella and become sperm … a female: only 1 will survive and become an egg If this took place in …
Male + female = A zygote with the diploid number ofchromosomes!
Problems in Mitosis and Meiosis • What happens if something goes wrong during cell division? • In Mitosis, incorrect cell division or uncontrolled cell division leads to cancer. • Cells divide incorrectly or too fast. This can lead to the growth of tumors or abnormal cells that do not do their job correctly. • In Meiosis, incorrect cell division (nondisjunction) leads to birth defects from extra or missing chromosomes
Nondisjunction • chromosomes do not separate in meiosis; 1 extra or 1 missing chromosome in gamete • Fertilization will produce a zygote with 45 or 47 chromosomes
Down’s Syndrome • Trisomy 21 results from nondisjunction of chromosome 21 • These kids have 3 chromosomes at pair 21.