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

Learning Objectives  Understand and be able to describe the different types of cell division  Understand and be able to explain how through mitosis worn out old cells are replaced with fresh new duplicates

Learning Objectives  Understand and be able to explain how sperm and eggs are generated through meiosis  Describe the sex differences in the chromosomes  Discuss the consequences of deviations from the normal chromosome number

6.1–6.5 There are different types of cell division.

6.1 Immortal cells can spell trouble: cell division in sickness and health.

Telomeres  The telomere is like a protective cap at the end of the DNA.  Every time a cell divides, the telomere gets a bit shorter. Insert new fig 6-1

“Cancer cells are those which have forgotten how to die.” —Harold Pinter

Take-home message 6.1  Cell division is an ongoing process in most organisms and their tissues; disruptions to normal cell division can have serious consequences.  In eukaryotic cells, a protective section of DNA called the telomere, at each end of every chromosome, plays a role in keeping track of cell division, getting shorter every time the cell divides.

Take-home message 6.1  If telomeres become too short, additional cell divisions cause the loss of essential DNA and cell death.  Cells that rebuild their telomeres with each division can become cancerous.

6.2 Some chromosomes are circular, others are linear.

Take-home message 6.2  Eukaryotes have much more DNA.  In eukaryotes, genetic information is organized into linear chromosomes.  Eukaryotic chromosomes float freely in the nucleus.

6.3 Prokaryotes divide by binary fission.

Take-home message 6.3 Bacteria divide by a type of asexual reproduction called binary fission:  First, the circular chromosome duplicates itself.  Then the parent cell splits into two new, genetically identical daughter cells.

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

Apoptosis  The pre-planned process of cell suicide  Certain cells are targeted for apoptosis.

Mitosis  The number of (somatic) cells that must be replaced by mitosis every day is huge.  The rate at which mitosis occurs varies dramatically.

Take-home message 6.6  Cells use mitosis to generate new, genetically identical cells.  This makes it possible for organisms to grow and to replace cells that die.

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.

6.9 Cell division out of control means cancer.

Cancer  Unrestrained cell growth and division…  …can lead to tumors…  …the second leading cause of death in the United States!

Tumor Growth  Unregulated cell division

Cancer cells have several features that distinguish them from normal cells, including…

Benign and Malignant Tumors

What is cancer? How does it usually cause death?

Why is the treatment for cancer often considered as bad as the disease?

Take-home message 6.9  Cancer is unrestrained cell growth and division.  Cancer can lead to large masses of cells called malignant tumors that can cause serious health problems.  Treatment focuses on killing or slowing the division of the cells using chemotherapy and/or radiation.

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  Diploid  Haploid  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.

6.14 What are the costs and benefits of sexual reproduction?

Sexual reproduction advantages?  Sexual reproduction leads to offspring that are all genetically different from each other and from either parent in three different ways: 1.Combining alleles from two parents at fertilization 2. Crossing over during the production of gametes 3. Shuffling and reassortment of homologues during meiosis

Bacteria reproduce asexually, while most plants and animals reproduce sexually. Which is a better method?

Sexual reproduction disadvantages?

With asexual reproduction, the advantages and disadvantages are more or less reversed.

Take-home message 6.14  There are two fundamentally different ways that cells and organisms can reproduce: 1.Mitosis and asexual reproduction via binary fission 2.Meiosis and sexual reproduction

Take-home message 6.14  Asexual reproduction can be fast and efficient.  But, asexual reproduction leads to genetically identical offspring.

Take-home message 6.14  Sexual reproduction leads to offspring that are genetically different from one another and either parent.  But, sexual reproduction takes more time and energy and can be risky.

6.15–6.16 There are sex differences in the chromosomes.

Which parent determines the sex of their baby? Why? How is sex determined in humans?

Sex Chromosomes

Take-home message 6.15  In humans, the sex chromosomes carry information that directs a growing fetus to develop as a male or a female. Male if the Y chromosome is present Female if there is no Y chromosome  Sex determination depends on the sex chromosome inherited from the father.