Chapter 9 Meiosis

Asexual vs. sexual reproduction asexual: one parent sexual: two parents 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 the cells in your body (muscle, liver, heart, tissue, etc.) EXCEPT for sex cells reproductive (sex) cells: sperm (male) and egg (female) cells

Chromosome Number Pairs of matching chromosomes are homologous chromosomes homologous: a chromosome that has a corresponding chromosome from the opposite sex parent (one from the dad, one from the mom) Homologs have the same genes, in the same order along the chromosome

Karyotype Homologous chromosomes can be arranged into a karyotype Humans have 23 pairs of chromosomes Is this a male or female?

Chromosome Number diploid: “two sets”; both sets of homologous chromosomes - human body cells have 46 chromosomes (diploid = 46) - fruit fly has 8 chromosomes (diploid = 8) haploid: “one set”; a single set of chromosomes (or half the needed DNA) - human sex cells have 23 chromosomes (haploid = 23) - fruit fly sex cells have 4 chromosomes (haploid = 4)

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

Life Cycle Germ (or sex) cells fuse during fertilization to make a zygote

Purpose of Meiosis To produce eggs and sperm with only 23 chromosomes, so fertilization can produce a fertilized egg (zygote) with 46 chromosomes.

Meiosis Specialized type of cell division that produces haploid cells from diploid parent cell

Stages of Meiosis 2 cell divisions after replication: Meiosis I Meiosis II Each division has 4 stages: Prophase Metaphase Anaphase Telophase

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.

“Crossing Over” In meiosis, chromosomes get together in homologous (similar) pairs & crossing over occurs. Pieces from one chromosome break off and rejoin the other!

Genetic Diversity Independent assortment and crossing over create new combinations of genes in gametes

Prophase I Pause Prophase I is longest phase of meiosis Production of proteins and structures in egg In humans, meiosis starts before birth, pauses in prophase I until puberty, then 1 egg/month continues for up to 40 years

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

There is NO Interphase II so there is no DNA replication Now on to Meiosis II … There is NO Interphase II so there is no DNA replication

Separation of Chromatids

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: the new haploid cells will grow flagella and become sperm … a female: only 1 of the 4 haploid cells will survive and become an egg

A zygote with 46 chromosomes! Male + female = A zygote with 46 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 leads to birth defects from non-disjunction.

Nondisjunction Occurs when chromosomes do not separate in either anaphase I or anaphase II Gamete is missing 1 chromosome or has 1 extra chromosome 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.