Can you….. Compare and contrast different types of chromosomes. Distinguish between chromatin, chromatids, and chromosomes. Explain the difference between haploid and diploid cells.

DNA (the instructions) can be found in two forms: Chromatin: loose, uncoiled form. Chromosome: tightly wound, condensed form.

What is a chromosome? Chromosome: tightly wound form of DNA that contains the information about an organism’s traits. Gene: one section of a DNA strand that codes for one trait.

Two Types of Chromosomes: AUTOSOMAL CHROMOSOMES Body chromosomes. Codes for traits that do not depend on your gender. EX. Hair, eye color, ear shape. SEX CHROMOSOMES Make a person male or female. Codes for gender specific traits. EX. Baldness, hemophilia, colorblindness.

Parts of a Chromosome Chromatid: one of the parallel strands of a chromosome. Centromere: where sister chromatids are held together and spindle fibers attach.

A Closer Look at Chromatids Sister Chromatids: two identical pieces of DNA joined at the centromere. Made during DNA Replication. Sisters are exact copies of each other. A duplicated chromosome is two sister chromatids.

Homologous Chromosomes Homologous Chromosomes: pairs of unattached chromosomes with the same genes in the same place. One comes from Mom, one comes from Dad. Humans have 23 pairs.

Diploid vs. Haploid DIPLOID Describes cells when chromosomes are found in pairs. There is two sets of each chromosome. All body cells. HAPLOID Describes cells when chromosomes are NOT in pairs. There is only one set of chromosomes. All sex cells (sperm and egg cells).

Can you…. Describe the events that take place during each stage of meiosis. Discuss differences and similarities between mitosis and meiosis.

What is meiosis? Meiosis: A special type of cell division that makes SEX CELLS (egg or sperm cells). Involves two cell divisions. Makes 4 cells. New cells are haploid.

Before meiosis, DNA replication occurs during INTERPHASE. MEIOSIS I PROPHASE I: Chromosomes coil. Homologous chromosomes pair up, forming tetrads. Sometimes these pairs will exchange DNA.

Crossing over occurs when chromosomes from mom and dad are close together (tetrad) and trade information. This explains how a person can have traits from the same chromosome but from different parents. “I have my dad’s nose, but my mom’s hair!”

METAPHASE I: Homologous chromosomes line up (still as tetrads) on the equator. ANAPHASE I: Spindle fibers pull homologous chromosomes to poles. Sister chromatids are still attached—centromeres do not split!

TELOPHASE I: Chromosomes arrive at poles and unwind. Nuclear membrane reforms. Cell divides into 2. END PRODUCTS OF MEIOSIS I: 2 cells (each cell only has one of the homologous chromosomes). Cells are haploid because they only have half amount of DNA. (1 of 2 copies)

How is this different from mitosis? MITOSIS Division separates sister chromatids. Cells are DIPLOID…have one chromosome from mom and one from dad. Cells are identical to the mother cell. MEIOSIS I Sister chromatids stay together. Cells are HAPLOID…have either the maternal OR the paternal copy of the chromosome…not both! Cells have random combinations of chromosomes from each parent.

Reviewing Meiosis I

MEIOSIS II PROPHASE II: Spindle fibers reform. Cells quickly enter Meiosis II. There is NO DNA Replication before Meiosis II. The steps of Meiosis II are going to be the same as what we saw with Mitosis…

METAPHASE II: Chromosomes line up in single file on the equator. ANAPHASE II: Sister chromatids are pulled apart. Homologous chromosomes were pulled apart in Anaphase I.

TELOPHASE II: Cells pinch in and divide cytoplasm. Cytokinesis occurs, dividing cytoplasm and organelles. At the end of MEIOSIS II, we have… 4 cells—each with a haploid nucleus. (Humans = 23 chromosomes)

Meiosis II Review Movie 1

MEIOSIS SUMMARY 2 divisions 4 cells 1/2 DNA Genetic recombination START INTERPHASE DNA Replication Meiosis I Meiosis II

What if it doesn’t work correctly? Nondisjunction: When chromosomes do not separate normally. This results in egg/sperm cells with more or less of the “normal” amount of DNA. CONDITIONS CAUSED BY NONDISJUCTION… Down Syndrome: extra chromosome #21. Pautau Syndrome: extra chromosome #13. Kleinfelter Syndrome: extra X chromosome. Edward Syndrome: extra chromosome #18. Jacob Syndrome: extra Y chromosome. Turner Syndrome: missing X chromosome.

How is meiosis different in men and women? Meiosis only occurs in the reproductive organs. Males = testes Females = ovaries Male sperm production is called spermatogenesis. Female egg production is called oogenesis.

In meiosis, cell division is unequal in females, but equal in males!

How else are they different? Men make 4 sperm for every cell that goes through meiosis. Men start making sperm at puberty and continue making them into old age. Men make MILLIONS of sperm every day. Women make only 1 egg for every cell that goes thorough meiosis…3 die. Women make all their eggs before they’re born and only use about 500 during their life. Women use 1 egg each month.