Continuity and variety Lectures by Mark Manteuffel, St. Louis Community College Chapter 6: Chromosomes and Cell Division Insert new photo (Jackson 5)

6.4 There is a time for everything in the eukaryotic cell cycle.

Take-home message 6.4  Eukaryotic somatic cells alternate in a cycle between cell division and other cell activities.  The cell division portion of the cycle is called the mitotic phase.

Take-home message 6.4  The remainder of the cell cycle, called interphase, consists of two gap phases (during which cell growth and other metabolic activities occur) separated by a DNA synthesis phase during which the genetic material is replicated.

6.5 Cell division is preceded by replication. Persistence and propagation

Replication  The process of DNA duplication

Complementarity  The characteristic that in the double- stranded DNA molecule the base on one strand always has the same pairing- partner (called the complementary base) on the other strand

Complementarity  Every “A” (adenine) pairs with “T” (thymine) and vice-versa.  Every “G” (guanine) pairs with “C” (cytosine) and vice-versa.

Errors sometime occur when DNA duplicates itself. Why might that be a good thing?

Mutation  A variety of errors can occur during replication.  Several DNA repair processes occur after replication.  If an error remains, however, the sequences in a replicated DNA molecule (including the genes) can be different from those in the parent molecule.

Take-home message 6.5  Every time a cell divides, the cell’s DNA must duplicate itself so that both new cells have all the DNA of the parent cell.  This process of DNA duplication is called replication.  Errors in replication can lead to changes in the DNA sequence called mutations.

6.6–6.9 Mitosis replaces worn-out old cells with fresh new duplicates.

6.6 Most cells are not immortal: Mitosis generates replacements. What is dust? Why is it your fault?

Mitosis has just one purpose:  To enable cells to generate new, genetically identical cells.  There are two different reasons for this need: 1. Growth 2. Replacement

6.7 Overview Mitosis leads to duplicate cells. Parent cells  daughter cells

Take-home message 6.7  Mitosis is the process by which cells duplicate themselves.  Mitosis follows chromosome replication and leads to the production of two daughter cells from one parent cell.

6.8 The Details Mitosis is a four-step process.

Preparation for Mitosis: The Chromosomes Replicate

Animal chromosomes are linear. So why do they look like the letter “X” in pictures?

Sister Chromatids A chromosome and its identical replicated copy, joined at the centromere.

Take-home message 6.8  The ultimate result of mitosis and cytokinesis is the production of two genetically identical cells.

6.10–6.14 Meiosis generates sperm and eggs and a great deal of variation.

6-10 Sexual reproduction requires special cells made by meiosis.

Meiosis  Gametes- sex cells (haploid)  Diploid – 2n (2 full sets of chromosomes)- 1 set from each parent  Haploid –n (1 full set of chromosomes)- [mother gives one set and father gives one set]  Maintains a stable genome size in a species

 Meiosis achieves more than just a reduction in the amount of genetic material in gametes.  You have two copies of every gene!

Meiosis has two important features: 1. It reduces the amount of genetic material in gametes. 2. It produces gametes that all differ from each other with respect to the combinations of alleles they carry.

Take-home message 6.10  In sexually reproducing organisms, gametes are produced through meiosis.  Gametes have half as much genetic material as the parent cell.  Gametes carry different combinations of alleles.

6.11 Sperm and egg are produced by meiosis: the details, step-by-step. Mitosis occurs almost everywhere in an animal’s body. Meiosis only occurs in one place. Where?

Meiosis starts with a diploid cell.  One of the specialized diploid cells in the gonads

Meiosis starts with a diploid cell.  A homologous pair, or homologues The maternal and paternal copies of a chromosome

Chromosomes are duplicated.  Sister chromatids Each strand and its identical duplicate, held together at the centromere

Cells undergoing meiosis divide twice. There are two major parts to meiosis: 1.The homologues are separated. 2.Each of the two new cells divides again, separating the sister chromatids into two even newer cells.

Meiosis Division 1 Separating the homologues

1. Prophase I  The most complex of all of the phases of meiosis  Crossing over

2. Metaphase I  Each pair of homologous chromosomes moves to the equator of the cell.

3. Anaphase I  Beginning of the first cell division that occurs during meiosis  The homologues are pulled apart toward opposite sides of the cell.  The maternal and paternal sister chromatids are pulled to the ends of the cell in a random fashion.

3. Anaphase I

4. Telophase I and Cytokinesis  This phase is marked by the chromosomes arriving at the two poles of the cell.  The cytoplasm then divides and the cell membrane pinches the cell into two daughter cells.

4. Telophase I and Cytokinesis

Meiosis Division 2 Separating the sister chromatids

5. Prophase II  The genetic material once again coils tightly making the chromatids visible under the microscope.  It is important to note that in the brief interphase prior to prophase II, there is no replication of any of the chromosomes.

6. Metaphase II  The sister chromatids (each appearing as an X) move to the center of the cell.

7. Anaphase II  The fibers attached to the centromere begin pulling each chromatid in the sister chromatid pair toward opposite ends of each daughter cell.

8. Telophase II  The cytoplasm then divides, the cell membrane pinches the cell into two new daughter cells, and the process comes to a close.

Outcome of Meiosis  The creation of four haploid daughter cells, each with just one set of chromosomes which contains a completely unique combination of traits

Take-home message 6.11  Meiosis occurs only in gamete-producing cells.  It occurs after DNA replication and consists of two rounds of cellular division.

Take-home message 6.11  In the first round, homologous pairs of sister chromatids separate and in the second round, sister chromatids separate.  The final product of meiosis in a diploid organism is four haploid gametes.

6.12 Male and female gametes are produced in slightly different ways. How do you distinguish a male from a female?

Take-home message 6.12  In species with two sexes, females produce the larger gamete and males produce a smaller gamete.  Male and female gametes both end up with just one copy of each chromosome.

6.13 Crossing over and meiosis are important sources of variation.

Take-home message 6.13  Although it doesn’t create any new traits, crossing over creates gametes with unique collections of traits.  This variation is important for evolution.