Biology Chapter 5: Chromosomes and Reproduction

Biology 30 - Cell Division

• Cell division is the process by which a cell divides and creates new daughter cells.
• Two types of cell division:
  • Mitosis: Division of somatic (body) cells.
  • Meiosis: Division of gametes (sex) cells.

Topic 1: Mitosis

• Somatic cells vs. sex cells
• Autosomes vs. sex chromosomes
• Chromosomes vs. chromatin
• Homologous chromosomes vs. sister chromatids
• Haploid (n) vs. diploid (2n)
• Stages of interphase: G1, S, G2
• Stages of mitosis: prophase, metaphase, anaphase, telophase
• Cancer, cloning, and stem cells
• Telomeres & aging
• Textbook pages: 556-571

Why do cells divide?

• Replace damaged cells.
• Growth of an organism.
• Reproduction.

From the moment of fertilization...

• Cells are continuously growing, dying, and being replaced.

Two Types of Cell Division

• Mitosis:
  • Somatic cells (body cells)
  • Produces two identical daughter cells
• Meiosis:
  • Sex cells (gametes)
  • Sperm and egg
  • Produces four haploid (half the number of chromosomes) daughter cells.

Asexual Reproduction (Mitosis)

• One parent passes copies of its genes to offspring without the fusion of gametes.
• The daughter cell is a clone of the parent.
• Genetic differences sometimes arise from mutations.

Sexual Reproduction (Meiosis)

• Two parents produce offspring with unique combinations of genes inherited from both parents.
• Meiosis creates variations, not exact replicas.
• Crossing-over is the shuffling of genes in meiosis when gametes cells are created.
• Higher genetic diversity means higher chances of survival when environmental conditions change.

Chromosomes

• Structure: Condensed DNA organized structure.
• Chromatid: Arm of the chromosome.
• Centromere: Region in the middle of chromosome where sister chromatids attach.
• Chromatin: DNA + proteins (histones)
• Telomeres
• Sister chromatids: Two identical chromatids after replication, attached at the centromere.

Karyotype

• A display of chromosome pairs ordered by size and length.
• In actual karyotypes, it can be difficult to see sister chromatids in each pair.
• A picture of an individual's chromosomes.
• Chromosomes are isolated, stained, and examined under a microscope, often using white blood cells.
• The chromosomes in a karyotype differ in size, location of their centromere, and staining properties.

Chromosome Number

• Chromosome numbers vary from species to species.
• Human cells have 46 chromosomes (23 pairs)
  • 22 pairs of autosomes
  • 1 pair of sex chromosomes (either XX or XY)
• More chromosomes do not necessarily correspond to higher intelligence or complexity.

The Cell Cycle

• The process by which a cell divides and creates new daughter cells.
• Has two main phases:
  • Interphase: Normal cellular functions & DNA replication, growth stage
  • Mitosis (M phase): DNA is divided and cytoplasm is divided to form two new daughter cells. This has multiple phases:
    • Prophase
    • Metaphase
    • Anaphase
    • Telophase
    • Cytokinesis (division of cytoplasm)

Preparing for Cell Division

• Chromatin becomes highly compact as chromosomes become visible under a light microscope.
• Chromosomes duplicate to form two identical sister chromatids.
• Sister chromatids are joined at the centromere, a narrow region.

Haploid vs. Diploid

• Haploid (n): one set of chromosomes. In humans, gametes (sperm and ova) are haploid.
• Diploid (2n): two sets of chromosomes. In humans, all body cells (other than gametes) are diploid.

Division Stage (M-Stage)

• Mitosis: Division of the nucleus
• Cytokinesis: Division of the cytoplasm.

Mitosis stages (summary)

• Prophase: Chromosomes condense, nuclear membrane breaks down.
• Metaphase: Chromosomes line up at the cell's equator.
• Anaphase: Sister chromatids separate and move to opposite poles.
• Telophase: Chromosomes decondense, nuclear membrane reforms.

Mitosis (Amoeba Sisters)

• A video about mitosis.

Mitosis (Summary, Purposes)

• Mitosis: performed by all autosomes
• It is a form of cell division that produces daughter cells.
• Purposes: Growth, Maintenance, and Repair.

Mitosis and Growth

• Example demonstrating how mitosis relates to growth.

Cancer and the Cell Cycle

• Uncontrolled cell division may be caused by mutagens or agents that change an organism's genetic material. These include physical, chemical or biological origins. Mutagens can be classified as carcinogens.

Carcinogens

• Cancer-causing substances that can lead to mutations in DNA.

Examples of carcinogens (summary)

• UV radiation
• X-rays
• Chemicals in cigarettes
• Unhealthy Drinking/Eating (alcoholic beverages, processed foods, etc.)
• Environmental exposure (car exhaust, fumes, work chemicals)

Types of Tumors

• Benign tumor: Cells remain at the tumor site and are unable to spread elsewhere in the body.
• Malignant tumor: Cells lose their anchorage dependency and can leave the tumor site.

Treatment for cancer

• Chemotherapy
• Surgery
• Radiation therapy
• Immunotherapy

Cloning

• Cloning is a form of asexual reproduction.
• Identical offspring can be formed from a single cell or tissue of a parent organ.
• Monozygotic twins form from a fertilized egg accidentally undergoing mitosis and splitting into two cells.
• Dizygotic twins form from two separate eggs being fertilized by different sperm.

Animal Cloning Technology

• Methods to artificially clone animals.

Stem Cell Research

• Cloning-based research is focused on cloning individual cells, tissues, and organs.
• This research uses cells that have not yet become specialized.

Stem Cell Ethics

• Totipotent cells (a zygote) can develop into a complete organism, but usage is usually considered unethical.
• Pluripotent stem cells can be easily obtained from adult tissues.

Scientists Create Human Eggs Using Stem Cells

• Scientific breakthrough using stem cells to create human eggs.

Mitosis and Aging

• Telomeres are caps at chromosome ends.
• Every cell division results in telomere shortening.
• Short telomeres may trigger cell death.
• Telomere length can be a molecular clock for cell aging

Homozygous vs. Non-homozygous chromosomes

• Homozygous Chromosomes: Same length and centromere position. Same genes, different alleles (e.g. Brown or blue eyes)
• Non-homozygous Chromosomes: Different genes, different alleles

Meiosis overview:

• Reduction of chromosome number from diploid (2n) to haploid (n) in sexual reproduction.
• Two divisions: Meiosis I and Meiosis II
• Results in four haploid cells, genetically distinct from each other.

Stages of Meiosis (Summary)

• Important Note: Meiosis I and Meiosis II each have stages very similar to the individual stages of mitosis.
• Prophase I: Chromosomes condense, homologous chromosomes pair up. Crossing-over occurs
• Metaphase I: Homologous chromosomes line up at the middle of the cell
• Anaphase I: Homologous chromosomes separate and move towards opposite ends of the cell
• Telophase I: Chromosomes begin to uncoil, and cytoplasm divides; two haploid cells are formed.
• Prophase II: Chromosomes condense, nuclear membrane breaks down.
• Metaphase II: Chromosomes align at the cell's equator
• Anaphase II: Sister chromatids separate and move to opposite poles.
• Telophase II: Chromosomes decondense, nuclear membrane reforms, cytoplasm divides.

Alternation of Generations

• Life cycle of some plants consists of two generations: Diploid generation (sporophyte) and Haploid generation (gametophyte)
• The sporophyte produces haploid spores via Meiosis, which develop into the gametophyte. The gametophyte produces male & female gametes.

Reproductive Strategies

• Mitosis is utilized in asexual reproduction
• Meiosis is utilized in sexual reproduction
• Most organisms' life cycles involve both methods

Spermatogenesis

• Meiosis to produce sperm (male gametes).
• Results in four viable sperm cells.

Oogenesis

• Meiosis to produce eggs(female gametes)
• Results in one functional egg and three polar bodies. Cytoplasm is not divided evenly.

Nondisjunction

• Failure of chromosomes to separate during meiosis, leading to an incorrect number of chromosomes in gametes. A possible cause of genetic disorders.

Types of Nondisjunction

• Nondisjunction during meiosis I yields abnormal gametes.
• Nondisjunction during meiosis II yields abnormal gametes.

Monosomy

• A condition with one less than the normal number of chromosomes.
• example: Turner's Syndrome = 45, XO
• Leads to physical and developmental problems.

Trisomy

• A condition with one more than the normal number of chromosomes.
• example: Down syndrome (Trisomy 21).

Cell Division and Twins

• Fraternal twins: Formed from two separate ovum and sperm.
• Identical twins: Formed from one ovum and sperm, but the zygote splits in two early in development.

Mitosis vs Meiosis

• Mitosis: One division; creates two identical diploid cells.
• Meiosis: Two divisions; creates four genetically different haploid cells.

Questions (sample)

• How many chromosomes are present in a human cell at the end of interphase? 46
• How many chromatids are present in a human cell at the end of metaphase? 92
• How many chromosomes are present in a human cell at the end of anaphase? 92
• How many chromosomes are present in a human cell at the end of Telophase? 46
• How many chromatids are present in a human cell at the end of cytokinesis? 0

Test your knowledge on chromosomes, cell cycles, and reproduction processes with this engaging quiz. Explore questions related to diploid cells in humans, the life cycles of various organisms, and the specifics of gametophytes and spores. Ideal for students studying cell biology and genetics.