Chromosomes and Cell Division

Chromosomes and Cell Division

• Chapter 8 explores chromosomes and cell division, explaining different types, how worn-out cells are replaced, and the creation of sperm and eggs through meiosis.
• The material covers sex differences in chromosomes, and the impact of deviations from normal chromosome numbers.

Cell Division and Telomeres

• Normal cell division is essential for organism maintenance.
• In eukaryotic cells, telomeres track cell divisions by shortening with each division.
• Excessively short telomeres lead to the loss of crucial DNA and cell death.

DNA and Reproduction

• DNA relies on the cell for energy and carbon necessary for reproduction.
• DNA, not the cell, is the primary element in life.
• The objective of DNA is to replicate as much as possible, requiring the use of the cell.

Mitosis Review

• Chromosomes replicate before mitosis.
• Sister chromatids then condense.
• The spindle apparatus forms as mitosis starts.
• Spindle fibers link to centromeres to align and move sister chromatids.

Interphase

• Chromosomes replicate during interphase in preparation for mitosis.

Prophase I

• The nuclear membrane breaks down.
• Sister chromatids condense.
• The spindle forms from the cytoskeleton.

Metaphase

• Sister chromatids align along the center of the cell

Anaphase and Beyond

• Sister chromatids are pulled apart by the spindle fibers during anaphase.
• Chromosomes begin to uncoil, and the nuclear membrane reforms during telophase.
• Cytokinesis begins, pinching the cell into two.

Genetic Inheritance

• All organisms inherit traits from a common ancestor, with modifications accumulated over time.
• Each branch of the tree of life has a distinct evolutionary history.

Homology

• Homology reveals common ancestry.
• The lancelet represents the earliest chordates, possibly resembling our common ancestor.
• Homologous structures, like forelimbs, evolve differently from the same bones.

Ploidy

• Ploidy refers to the number of homologous chromosomes.
• Having more than a haploid set is seen as typically redundant

Homologous Chromosomes

• Homologous chromosomes share the same genes but may have different forms of those genes.

Recombination

• Recombination moves genes between evolutionary lineages.

Horizontal Gene Transfer

• Horizontal gene transfer is a genetic recombination process among bacteria.
• Viruses can facilitate horizontal gene transfer
• Plasmids can "hop" between bacteria, or by cells taking in floating DNA.

Evolution and Endosymbiosis

• Evolution isn't always linear; endosymbiosis represents horizontal gene transfer as cells acquire new genomes.

Cancer

• Cancer includes unrestrained cell growth that can harm adjacent tissues.
• It is a major health concern and a leading cause of death in the United States.
• It arises from DNA disruptions affecting cell division control

Cancer Cells vs Normal Cells

• Cancer cells differ from normal cells in contact inhibition, cell division, and "stickiness."
• Normal cells stop when they touch other cells, while cancer cells pile up.
• Normal somatic cells have a limited number of divisions, while cancer cells divide indefinitely.
• Cancer cells also exhibit reduced adhesiveness.

Tumors

• Benign tumors are masses that don't spread and can be safely removed.
• Malignant tumors shed and spread cancer cells through metastasis.

Cancer Treatments and Causes

• Common cancer treatments are chemotherapy and radiation.
• Mutations in genes that regulate cell growth are the main cause of cancer.

Sexual vs Asexual Reproduction

• Asexual reproduction involves a single parent producing identical offspring.
• Sexual reproduction combines reproductive cells (gametes) in fertilization.
• Some species utilize both methods.

The Details of Meiosis

• Meiosis is required for sexual reproduction, and creates special cells.
• Meiosis produces haploid gametes.
• Diploid cells have two copies of each chromosome.
• Fertilization is the fusion of two haploid cells, creating a diploid individual.

Outcomes of Meiosis

• Meiosis generates two important outcomes
• Turns diploid cells to haploid
• Produces varied gametes with different combinations of alleles.

Meiosis

• Meiosis is the process by which reproductive cells are produced in sexually reproducing organisms
• It results in gametes that differ in the combinations of alleles they carry and contain half the genetic material of the parent cells

Sperm and Egg Production by Meiosis

• Meiosis occurs only in the gonads.
• It starts with a cell containing 46 chromosomes, which includes both maternal and paternal copies plus two sex chromosomes.

Meiosis Chromosome Reduction

• Meiosis reduces ploidy from diploid (2) to haploid (1).

The Stages of Meiosis

Meiosis I

• Homologous chromosomes are separated.
  • Prophase I: Replicated chromosomes condense, spindles are formed, nuclear membranes disintegrate, and crossing over occurs to generate variation
  • Metaphase I: Homologues line up at the center of the cell -Random assortment of maternal and paternal sister chromatids.
  • Anaphase I: Homologues separate and are pulled to opposite poles so that the sister chromatids going to each side are a mix of maternal and paternal genetic material.
  • Telophase I: Sister chromatids arrive at the cell poles, nuclear membranes reassemble, and cells pinch into two daughter cells.

Meiosis II

• Sister chromatids are separated in Meiosis II.
  • Prophase II: Chromosomes in daughter cells condense, spindles form, and nuclear membranes disintegrate
  • Metaphase II: Sister chromatid pairs line up at the center of the cell
  • Anaphase II: Sister chromatids are pulled apart by the spindle fibers toward opposite cell poles.
  • Telophase II: Nuclear membranes reassemble around the chromosomes, and the cells divide, resulting in four haploid daughter cells.

Genetic Dissimilarity

• The final product of meiosis in a diploid organism is four genetically dissimilar haploid gametes.

Plant Meiosis

• Meiosis changes the ploidy from haploid (1N) to diploid (2N).

Gamete Development in Females

• Female and male gametes are produced differently.
• In the first division, one cell gets nearly all the cytoplasm.
• The result is one large egg and smaller cells that are called polar bodies which degrade

Number of Chromosomes in Organisms

• Different organisms have different quantity of chromosomes

Female vs Male Gametes

• In two-sex species, the female produces the larger gamete, and the male produces the smaller one.
• Each gamete has just one copy of each chromosome.

Genetic Recombination

• Meiosis is distinct to different forms of genetic recombination.
• Crossing over occurs when homologous chromosomes interchange genetic information.
• Homologous chromosomes are similar enough in structure that they have interchangeable parts.

Independent Assortment

• Independent assortment means that the paternal and maternal lineages are "shuffled", where each of us is a randomized assortment of grandparents' chromosomes.

Recombination

• Recombination occurs so that the DNA can try to do something.

Crossing Over Importance

• Crossing over creates gametes with unique combinations of alleles, and helps promote evolutionary variance.

Sexual Reproduction

• There are multiple reasons why offspring are genetically different from their parents and from one another:
• Each parent donates his or her own set of genetic material.
• Crossing over occurs to produce a mixture of maternal and paternal genetic material
• The homologues and sister chromatids are a random mix of maternal and paternal genetic material

Bacteria

• Bacteria reproduce asexually, and most plants and animals reproduce sexually.
• Asexual reproduction leads to genetically identical offspring.
• Sexual reproduction creates genetically varying offspring from one another and either parent.

Sex Determination

• Human sex chromosomes are X and Y.
• Males are XY, and females are XX.

Sex Chromosomes

• No information on the Y chromosome is needed for development of a functioning human.
• Females lack a Y chromosome and live normal, healthy lives.

Sex Determination Other species

• Hermaphrodites produce both male and female gametes.
• Birds are determined when the females determine sex.
• Ants, bees and wasps have sex determined based on how many sets of chromosomes there are.
• Turtles are determined by the environment of their offspring

Turtles and Temperature

• In 1966, Madeleine Charnier observed that sex ratio of offspring was influenced by the environment.
• Warmer temperatures resulted in more eggs hatched as females.
• Cooler temperatures resulted in more eggs hatched as males.
• Rising tempurate can have a negative impact on the earth.

Genetic Testing

• A karyotype is a visual display of chromosomes.
• Chromosomal disorders can be detected before birth.
• Trisomy 21 (Down Syndrome) includes:
  • First described by Dr. John Langdon Down in 1866
  • The presence of an extra copy of the chromosome 21

Chromosomal Disorders and Nondisjunction

• Chromosomal disorders can be tested for before birth.
• Unequal chromosome division is called nondisjunction.
• Cases of trisomy tend to involve chromosomes with the fewest genes.
  • Chromosomes 13, 15, 18, 21, and 22
• Problems associated with reproduction occur as women age

Sex Chromosomes

• Turner Syndrome are only females with short height
• Klinefelter Syndrome are only males that are underdeveloped and have low testosterone levels

Telomeres

• Telomeres contain repetitive, noncoding regions of DNA, acting as protective caps on chromosomes.
• Every time a cell divides, the telomere gets shorter.
• When telomeres become too short it results in cell death
• Telomere rebuilding and excessive cell division lead to cancer.