Chapter 9 Meiosis

Asexual vs. sexual reproduction asexual: one parent creates identical copy sexual: two parents create new, different cell What is MITOSIS? ASEXUAL or SEXUAL? asexual: one parent sexual: two parents 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?

Human Chromosomes

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

Prophase I Pause Prophase I is longest phase of meiosis; crossing over occurs here Meiosis is different in males and females. Human egg production starts before birth and pauses until puberty, then continues until no more eggs

Meiosis 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 Chromatids Meiosis I – Splits the homologous pairs Meiosis II – splits the chromosome

Meiosis II: Prophase II Metaphase II Anaphase II Telophase 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 Prophase II Metaphase II Anaphase II Telophase 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 Prophase II Metaphase II Anaphase II Telophase 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 Prophase II Metaphase II Anaphase II Telophase 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!

If this took place in … … a male: … a female: all four grow flagella and become sperm … a female: only 1 will survive and become an egg

A zygote with the diploid number of chromosomes! Male + female = A zygote with the diploid number of chromosomes!

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.