Cell Division and Genetics Lecture 12

Lecture 12: Cell Division & Intro to Genetics

• Today's Lecture Topics: Familiarizing with terminology, Importance of Cell Division, Human Life Cycle, Mitosis, Meiosis, Chromosomal disorders, Introduction to genetics.
• Reading Assignment: Chapter 19
• Chromosomes and DNA: Chromosomes are individual DNA molecules located within a cell's nucleus. Specific regions on chromosomes are called genes. Genes are instructions that determine traits.
• Chromosome Number in Cells:
  • Tissue cells (e.g., muscle, bone, skin) have 46 chromosomes (two sets of 23 unique chromosomes, one from each parent)
  • Gametes (sperm and egg cells) have 23 chromosomes (one unique set).
• Karyotype: A profile of a person's chromosomes arranged from largest to smallest. Karyotypes are used to diagnose chromosomal disorders. This example is from a tissue cell, not a gamete.
• Autosomal & Sex Chromosomes: There are 22 unique autosomal chromosomes and the sex chromosomes. A tissue cell has two copies of each autosomal chromosome and two sex chromosomes (can be the same or different, determining sex).
• Homologous Chromosomes: Pairs of chromosomes that share the same genes are termed homologous. Tissue cells have 2 sets (one from each parent) of 23 unique chromosome pairs (total of 46 chromosomes).
• Male vs Female Karyotype: A tissue cell karyotype for a male has one X and one Y chromosome, while a female's has two X chromosomes.

Cell Division

• Reasons for Cell Division:
  • Reproduction
  • Growth and development
  • Repair and renewal of tissues.
• Mitosis: Cell division that results in two genetically identical daughter cells.
  • Crucial in growth, development, renewal, and repair of tissues.
  • DNA replication occurs before, creating identical copies of each chromosome that remain attached.
  • Replicated chromosomes align, separating, and moving to opposite ends of the cell.
  • After cell division, each daughter cell gets a copy of the original chromosomes.
• Stem Cells: Cells in the body that continually undergo mitosis to replenish damaged or lost cells, such as in skin.
  • Some specialized cells, like mature brain neurons don’t undergo mitosis.

Meiosis

• Gamete Production: Meiosis is a cell division process important for producing gametes (sperm and egg cells) needed for sexual reproduction.
• Meiosis and Chromosome Number: Meiosis reduces the number of chromosomes in each daughter cell, to half the number of the original stem cell, from 46 to 23.
• Conception: One sperm cell with 23 chromosomes fertilizes one egg cell with 23 chromosomes, forming a zygote (new cell).
• Zygote Growth: The zygote grows through repeated cell divisions, increasing the number of cells and copying the chromosomes in each new cell precisely.
• Why Meiosis is Essential: If eggs and sperm had 46 chromosomes, the child would have 92 (a doubling of the number of chromosomes) in each cell, and the chromosome numbers would rapidly increase through successive generations.
• Job of Meiosis: Meiosis converts cells with two sets of chromosomes (46) into gametes (egg or sperm) with only one set (23).

Chromosomal Disorders

• Nondisjunction: Errors in meiosis can cause nondisjunction, where chromosomes don't separate properly.
• Consequences of Nondisjunction: These abnormalities in gametes (egg or sperm cells) result in a zygote with the incorrect number of chromosomes.
• Down Syndrome: A common genetic disorder caused by an extra copy of chromosome 21 resulting from nondisjunction. People with Down syndrome often have distinctive physical features and developmental delays.
• Turner Syndrome: A genetic disorder occurring in females, caused by having only one X chromosome. This causes the development of underdeveloped internal sex organs and sterility.
• Klinefelter Syndrome: A genetic condition in males where there are extra (one or more) X chromosomes which results in underdeveloped testes, sterility, and some secondary female sex characteristics at puberty.

Genetics

• What is Genetics?: The study of heredity; the chromosomes we inherit from our parents contain genes that determine traits.
• Genes: The basic unit of heredity.
  • Each gene is a section of a chromosome that gives instructions for making proteins. Proteins that affect structure or function in the body.
• Alleles: Different variations of a gene (different instructions for the same trait). For example, different eye colors.
• Genotype: The combination of alleles an individual has for a particular gene. Example: EE (homozygous dominant), Ee (heterozygous), or ee (homozygous recessive).
• Phenotype: The observable trait, the result of the genotype. Examples: brown eyes, freckles.
• Dominant vs Recessive Alleles: Some alleles are dominant, and their traits are shown in the phenotype, as long as one is present, while others are recessive, and their traits are expressed only when two recessive copies are present.