Meiosis, Fertilization, and Zygote Development

• Meiosis and fertilization are cell division types in human reproduction
• Gametes are produced, a zygote is formed, and an embryo develops

Gamete Production

• Sperm-producing cells are somatic cells found in the testes
• These cells have 46 chromosomes (2n = 46)
• Ovum-producing cells are somatic cells in the ovaries
• They also contain 46 chromosomes (2n = 46)
• Sperm and ovum undergo meiotic cell division

Fertilization

• Sperm contains 23 chromosomes (n = 23)
• Ovum contains 23 chromosomes (n = 23)
• Fertilization restores the diploid number of chromosomes

Zygote Development

• Zygote has 46 chromosomes (2n = 46)
• The zygote undergoes mitotic cell division repeatedly
• Growth and development occurs
• The resulting cells maintain 46 chromosomes (2n = 46)

Chromosome Structure

• DNA carries an organism's genetic information, residing in a eukaryotic cell's nucleus
• DNA molecules coil with proteins to form chromosomes
• Chromatin refers to uncondensed DNA fibers during non-division
• Chromosomes become visible, shorter, and thicker when cell division starts

Packing DNA

• Compacting DNA makes it fit within the nucleus
• After replication, a each chromosome is duplicated
• A duplicated chromosome consists of two identical chromatids
• Sister chromatids are joined at the centromere

Karyotypes

• Karyotypes can differ by sex chromosome combinations
• Female karyotype: XX
• Male karyotype: XY

Chromosome Numbers

• Body cells within a species have consistent chromosome numbers
• Chromosome numbers vary across different species
• 'n' represents one set of chromosomes
• 2n represents two sets of chromosomes
• Humans have 23 chromosomes per set, totaling 46 in somatic cells

Intelligence and Chromosomes

• No direct link exists between intelligence/complexity and chromosome count
• Fruit flies have 8 chromosomes (2n = 8)
• Rats have 40 (2n = 40)
• Shrimp have 254 (2n = 254)

Chromosome Pairing

• Chromosomes exist in pairs in somatic cells, termed homologous chromosomes
• One from the male (paternal) and one from the female (maternal) parent

Diploid vs. Haploid

• Diploid (2n) cells have two chromosome sets, formed via mitosis.
• Haploid (n) cells possess one set, created through meiosis.
• Humans contain 23 pairs (46) chromosomes, including sex chromosomes

Chromosome Identification

• Homologous chromosomes are pairs numbered 1 and 2 in diagrams
• Sister chromatids include 3 & 4/5 & 6
• Non-sister chromatids are 4 & 5/4 & 6 / 3 & 6/3 & 5

Cell Cycle Stages

• Interphase:
  • Cell growth, occupying ~90% of cycle
• Mitotic Cell Division:
  • Mitosis (nuclear division) and cytoplasmic division
  • Takes ~10% of cycle

Daughter Cells

• Daughter cells from mitosis either re-enter the cycle to divide or differentiate

Interphase Prep

• Interphase involves synthesis of new organelles/proteins
• DNA replication, resulting in duplicated chromosomes composed of identical DNA molecules
• Cell growth to maximum size

Mitotic Division: Prophase

• Chromosomes become visible
• Each made of two chromatids linked at the centromere
• Nuclear envelope breaks down
• The cell remains in a diploid state

Mitotic Division: Metaphase

• Chromosomes align at the cell's equator

Mitotic Division: Anaphase

• Sister chromatids separate, moving to opposite poles
• Cytoplasm starts dividing

Mitotic Division: Telophase

• New nuclear membranes form around chromosome sets
• Chromosomes uncoil back into chromatin
• The cell remains in a diploid state

Summary of Mitosis

• Mitosis separates sister chromatids
• Separated chromatids distribute equally between two daughter nuclei

Cytoplasmic Division (Cytokinesis)

• Cytoplasm divides into two equal halves, forming daughter cells

Animal vs. Plant Cells

• In animal cells, the cell membrane constricts
• In plant cells, a cell plate forms

Cell Cycle Stages and DNA Content

• Cell at stage C (preparing for mitosis) has double the DNA of stage A
• Mitotic division separates chromosomes into two equal groups
• Mitosis reduces DNA content at stage A to half of stage C

Chromosome and Centromere Count

• Centromeres: 46 in interphase, prophase, metaphase, 92 in anaphase and telophase
• Chromosomes: 46 in interphase, prophase, metaphase, anaphase, telophase
• Chromatids: 0 in interphase, 92 in prophase and metaphase, 0 in anaphase and telophase
• n/2n: 2n in interphase, prophase and metaphase, 4n in anaphase and telophase
• DNA content: x and then 2x in Interphase, 2x in prophase, metaphase, anaphase and telophase, then x

Interphase Note

• DNA replication occurs during interphase
• Chromosomes are in a duplicated state

Cell Cycle Timing

• Cell cycle stage time is estimated by the proportion of cells
• A higher proportion means longer time

Mitosis Timing Calc

• Time taken up by mitosis = (number of cells in mitosis stages / total number of cells) * time taken by a cell cycle
• Using, (22 + 12 + 10 + 20) / 400 * 800 min = 128 min

Interphase Root Cell

• If a part of the root is in interphase, the cells cannot divide.

Purpose of Mitosis

• Mitosis creates genetically identical daughter cells
• Important for growth, repair, and asexual reproduction

Meiosis

• Meiosis creates genetic variation

Gametes

• Meiotic division produces haploid gametes for sexual reproduction
• Fertilization forms a zygote and restores the diploid chromosome number

Mules and Sterility

• Mitotic cell division possible because homologous chromosome pairing is unnecessary
• Meiotic cell division is not possible
• No homologous chromosome pairing during prophase I

Genetic Variation

• Genetic variation increases survival chance
• Independent assortment of chromosomes leads to genetic variation
• Crossing over also causes genetic variation

Meiosis vs. Mitosis

Prophase

• Mitosis: no pairing of homologous chromosomes.
• Meiosis I: Pairing of homologous chromosomes and crossing over may occur

Metaphase

• Mitosis: chromosome lines up along the equator
• Meiosis I: Pairs of homologous chromosomes line up along equator

Anaphase

• Mitosis: sister chromatids separate
• Meiosis I: homologous chromosomes separate

End Result

• Mitosis: produces two diploid daughter cells that are genetically the same
• Meiosis: four haploid daughter cells are produced are genetically different from the parent cell and each other