Chromatin - Loose, uncoiled DNA found in the nucleus during interphase.
Chromatid - One of two identical halves of a duplicated chromosome.
Sister Chromatids - Two identical chromatids joined together at a centromere.
Chromosome - Condensed, coiled DNA that carries genetic information.
Centromere - The central region where sister chromatids are joined.
Chromatid - One half of a duplicated chromosome.
Telomere - Protective ends of chromosomes that prevent degradation.
Long Arm (q arm) - The longer section of a chromosome.
Short Arm (p arm) - The shorter section of a chromosome.
Why are telomeres important?
They prevent the loss of genetic information during DNA replication.
Shortening of telomeres is linked to aging and cell death.
Relationship between sister chromatids?
Sister chromatids are identical copies of the same chromosome, held together at the centromere.
Interphase (G1, S, G2)
G1: Cell growth and preparation for DNA replication.
S: DNA replication occurs.
G2: Final preparation for mitosis, organelles are duplicated.
Mitosis - Division of the nucleus (see stages below).
Cytokinesis - Division of the cytoplasm, forming two daughter cells.
G1 Checkpoint - Checks cell size and DNA integrity.
G2 Checkpoint - Ensures DNA replication was successful.
M Checkpoint - Verifies chromosome alignment during mitosis.
Apoptosis:
Programmed cell death to remove damaged or unneeded cells.
Mitosis in Single vs. Multicellular Organisms
Single-celled organisms: Mitosis is used for reproduction.
Multicellular organisms: Mitosis is used for growth, repair, and maintenance.
Prophase - Chromatin condenses into chromosomes; nuclear envelope breaks down.
Metaphase - Chromosomes align at the metaphase plate.
Anaphase - Sister chromatids separate and move to opposite poles.
Telophase - Nuclear envelope reforms; chromosomes decondense.
Recognizing the Stages:
Use images to identify chromosome alignment and nuclear envelope status.
Purpose of Meiosis
Creates gametes (sperm and egg) for sexual reproduction.
Introduces genetic variation.
Gene vs. Allele
Gene - A section of DNA that codes for a trait.
Allele - A variant of a gene (e.g., blue vs. brown eye color).
Sister Chromatids vs. Homologous Chromosomes
Sister Chromatids - Identical copies of a chromosome.
Homologous Chromosomes - Chromosome pairs (one from each parent) with similar genes but possibly different alleles.
Chromosome Count in Humans
46 total chromosomes (23 pairs), inherited from both parents.
Diploid vs. Haploid Cells
Diploid (2n) - Two sets of chromosomes (body cells).
Haploid (n) - One set of chromosomes (gametes).
Sources of Genetic Variation
Crossing Over - Exchange of DNA between homologous chromosomes in Prophase I.
Independent Assortment - Random separation of chromosomes during Metaphase I.
Tetrad Formation and Breakdown
Tetrad - A pair of homologous chromosomes (4 chromatids) that forms during Prophase I.
Tetrads separate in Anaphase I, while sister chromatids separate in Anaphase II.
DNA Duplication in Meiosis
DNA is duplicated once, but the cell divides twice.
Process | Number of Gametes Produced |
---|---|
Oogenesis | 1 egg cell, 3 polar bodies |
Spermatogenesis | 4 sperm cells |
Description | Mitosis | Meiosis |
Produces gametes | ❌ | ✅ |
Growth & repair | ✅ | ❌ |
Genetic variation | ❌ | ✅ |
One division | ✅ | ❌ |
Two divisions | ❌ | ✅ |
Identical daughter cells | ✅ | ❌ |
Synapsis of homologous chromosomes | ❌ | ✅ |
Asexual reproduction | ✅ | ❌ |
Four haploid gametes produced | ❌ | ✅ |
Also called reduction division | ❌ | ✅ |
Nondisjunction:
Failure of chromosomes to separate properly.
Can occur in Meiosis I (homologous chromosomes fail to separate) or Meiosis II (sister chromatids fail to separate).
Karyotype:
A visual representation of chromosomes used to detect abnormalities.
Can reveal sex (XX or XY) and chromosomal disorders (e.g., Down syndrome - Trisomy 21).
Chromosomal Abnormalities:
Autosomal disorders (e.g., Down syndrome, Trisomy 18).
Sex chromosome disorders (e.g., Turner syndrome - XO, Klinefelter syndrome - XXY).