Cell Division & Intro to Genetics Notes

Behavioural Biology: Cell Division & Intro to Genetics

Terminology and Importance of Cell Division

  • Chromosomes: Individual DNA molecules stored in the cell nucleus.

  • Genes: Specific regions on chromosomes that contain instructions for traits (e.g., eye color, height).

  • Nucleus: The structure within cells that stores chromosomes.

Chromosome Number in Human Cells

  • Tissue Cells (Somatic Cells): Contain 46 chromosomes (two sets of 23, one from each parent).

  • Gametes (Sperm and Egg Cells): Contain 23 chromosomes (all unique).

Karyotype

  • A karyotype is a profile of a person's chromosomes, arranged from largest to smallest.

  • Karyotypes of tissue cells have 46 chromosomes, while gametes have 23.

Autosomal and Sex Chromosomes

  • Humans have 22 unique autosomal chromosomes and sex chromosomes.

  • Tissue cells have two copies of each autosomal chromosome and two sex chromosomes (XX for female, XY for male).

  • A chromosome pair consists of one copy from each parent.

Homologous Chromosomes

  • Tissue cells contain homologous chromosomes: 23 unique kinds in two sets (one from each parent).

  • Homologous chromosomes share the same genes.

Determining Sex from Karyotype

  • A karyotype with one X and one Y chromosome indicates a male tissue cell.

  • Females have two X chromosomes.

Cell Division and the Continuity of Life

  • Cell division is essential for:

    • Reproduction.

    • Growth and development.

    • Tissue renewal and repair.

Human Life Cycle and Cell Division Types

  • Two types of cell division drive the human life cycle:

    • Mitosis: For growth, development, renewal, and repair.

    • Meiosis: For creating gametes (sperm and eggs) for reproduction.

Conception and the Zygote

  • A sperm cell (23 chromosomes) fertilizes an egg cell (23 chromosomes).

  • The nuclei fuse, forming a zygote with 46 chromosomes (23 from each parent).

Growth of the Zygote

  • The zygote grows through repeated cell divisions (mitosis).

  • The zygote divides into 2 cells, then 4, then 8, and so on.

  • Chromosomes must be copied exactly before each cell division to ensure each new cell has a complete set.

Chromosome Replication

  • Each of the 46 chromosomes is copied before cell division.

  • Identical copies remain attached at a midpoint, forming a replicated chromosome.

  • Replicated chromosomes are split, and copies are placed into two daughter cells.

Mitosis Explained

  • Mitosis creates two daughter cells genetically identical to the parent cell.

  • Process:

    • DNA replication creates identical copies of each chromosome.

    • Replicated chromosomes align, and identical copies are separated to opposite ends of the cell.

    • The cell divides, forming two daughter cells with a copy of all original chromosomes.

Mitosis in Adult Cells

  • Skin stem cells undergo mitosis to replace sloughed-off cells.

  • Mature neurons typically do not undergo mitosis after brain development is complete.

Meiosis: Making Gametes

  • Gametes (sperm and egg) are necessary for sexual reproduction.

Sexual Reproduction and Genetic Variation

  • Sexual reproduction requires genetic material from two individuals.

  • Each child has a unique assortment of chromosomes despite sharing traits with parents and siblings.

Necessity of Meiosis

  • If eggs and sperm had 46 chromosomes, fertilization would result in offspring with 92 chromosomes per cell, doubling each generation.

  • Meiosis reduces the chromosome number in gametes.

Job of Meiosis

  • Meiosis converts stem cells with 46 chromosomes in ovaries or testes into gametes with 23 chromosomes.

Outcome of Meiosis

  • Meiosis produces 4 daughter cells, each with half the number of chromosomes as the original mother cell.

Meiosis in Males vs. Females

  • In females, unequal division of cytoplasm results in only one viable egg instead of four.

Mitosis vs. Meiosis: Summary

Feature

Mitosis

Meiosis

Cell Divisions Involved

1

2

Daughter Cells Produced

2, each with 46 chromosomes

4, each with 23 chromosomes

Genetic Similarity

Genetically identical

Genetically different

Purpose

Growth, repair, development, renewal

Gamete production for sexual reproduction

Chromosomal Disorders

  • Chromosomal disorders result from errors during meiosis.

  • Nondisjunction: Chromosomes fail to separate properly, leading to gametes with the wrong number of chromosomes.

  • Fertilization involving abnormal gametes results in a zygote with an abnormal chromosome number.

Down Syndrome

  • Individuals with Down syndrome have an extra copy of chromosome 21 (Trisomy 21) due to a nondisjunction error during meiosis.

  • It is the most common genetic disorder (approximately 1 in 800 births).

Sex Chromosome Disorders in Females

  • XXX (Triple X): Generally healthy.

  • XO (Turner Syndrome): Only known viable monosomy in humans; underdeveloped internal organs result in sterility.

Sex Chromosome Disorders in Males

  • OY: Not viable.

  • XYY: Male sexual development; taller than usual.

  • XXY (Klinefelter Syndrome): Male sex organs, but small testes and often sterile; may develop secondary female sex characteristics at puberty.

Introduction to Genetics

  • Genetics is the study of heredity.

  • Chromosomes inherited from parents contain genes that determine our traits.

Genes

  • Humans have approximately 25,000 genes.

  • A gene is a section of a chromosome that carries instructions to make specific proteins.

  • Proteins have structural or functional effects in our bodies.

  • Genes determine physical appearance, traits like eye color, height, hair texture.

Homologous Chromosomes Revisited

  • Human somatic cells have 23 unique chromosomes in 2 sets.

  • Copies of the same chromosome are called homologous chromosomes because they share the same genes.

Non-Identical Homologous Chromosomes

  • Homologous chromosomes are not genetically identical; they have the same genes but may have variations.

  • Variations of genes are called alleles.

  • Example: Gene for eye color, but alleles for different colored eyes.

Dominance of Alleles

  • Individuals have two alleles for every gene because they have two copies of each chromosome.

  • If the two alleles for a gene are different (e.g., one for brown eyes, one for blue eyes), one may be dominant.

Dominant and Recessive Alleles Explained

  • Dominant Alleles: Always expressed as a trait when present; denoted with a capital letter (e.g., E for brown eye color).

  • Recessive Alleles: Only expressed when an individual has two copies of it; denoted with a lowercase letter (e.g., e for blue eye color).

Genotype

  • The genotype is the combination of alleles an individual has for a particular gene.

  • Possible genotypes for eye color (E = brown, e = blue): EE, Ee, ee.

Homozygous vs. Heterozygous

  • Homozygous: An individual's genotype has two identical alleles (e.g., EE or ee).

  • Heterozygous: An individual's genotype has two different alleles (e.g., Ee).

Phenotype

  • The phenotype is the trait that is expressed in the individual (e.g., blue eyes or brown eyes).

  • Genotype: EE (Brown eyes), Ee (Brown eyes), ee (Blue eyes).

Freckles Example

  • Dominant allele (F) causes pigment deposition in the skin (freckles).

  • Recessive allele (f) does not cause freckles.

Additional Examples of Genetic Traits

  • Freckles: FF or Ff

  • No Freckles: ff

  • Widow's Peak: WW or Ww

  • Straight Hairline: ww

  • Unattached Earlobes: TT or Tt

  • Attached Earlobes: tt

  • Tongue Rolling: EE or Ee