Mitosis and Sex Determination Quiz

Genetics: From Genes to Genomes - Fourth Edition

• This is a genetics textbook, written by Leland H. Hartwell, Leroy Hood, Michael L. Goldberg, Ann E. Reynolds, and Lee M. Silver.
• The book is in its fourth edition.
• The notes are prepared by Mary A. Bedell, from the University of Georgia.
• The copyright is held by The McGraw-Hill Companies, Inc.

Basic Principles: How Traits Are Transmitted - Chapter 4

• Chapter Outline:
  • Chromosomes: The Carriers of Genes
  • Mitosis: Cell Division That Preserves Chromosome Number
  • Meiosis: Cell Divisions That Halve Chromosome Number
  • Gametogenesis
  • Validation of the Chromosome Theory

Chromosomes and Genetic Information

• Chromosomes are cellular structures that transmit genetic information.
• Breeding experiments and microscopy supported the chromosome theory of inheritance.
• Accurate gene transfer and chromosome maintenance are crucial for proper development.
• The concept of a gene was fundamentally altered by understanding chromosomes as physical carriers of genes.

Evidence That Genes Reside In Chromosomes

• Mitosis: A process creating two identical daughter cells with the same number and type of chromosomes as the parent cell.
• Meiosis: A process producing gametes (egg and sperm) with half the number of chromosomes as the parent cell. In humans, this means a change from 46 chromosomes (parent cell) to 23 chromosomes (gamete).

Diploid vs. Haploid

• Diploid cells contain chromosomes in pairs (one copy from each parent).
• Haploid cells contain one set of chromosomes. In humans, 2n=46 and n=23.
• This difference is critical for sexual reproduction. In Drosophila 2n=8 and n=4.

Fertilization

• Fertilization is the process where haploid gametes (egg and sperm) unite, restoring the diploid chromosome number (2n) in a zygote.

Metaphase Chromosomes

• Metaphase chromosomes can be classified based on their centromere position.
• This classification helps to identify homologous and nonhomologous chromosomes.

Human Karyotype

• The human karyotype represents a photographic display of stained chromosomes arranged in pairs according to their size and shape.
• In humans, there are 22 pairs of autosomes and two sex chromosomes.
  • Autosomes are all chromosomes excluding the sex chromosomes.
  • Sex chromosomes are designated X and Y.

Homologous Chromosomes

• Homologous chromosomes share similar size, shape, and banding patterns.
• Homologous chromosomes carry the same genes, although they can carry different alleles of those genes.

Sex Determination

• The X and Y chromosomes are critical in determining sex in humans.
• Females receive an X chromosome from each parent (XX) and males receive an X from their mother and a Y from their father (XY).
• This produces a 1:1 ratio of females to males

Mechanisms of Sex Determination

• Mechanisms for sex determination differ across species and are complex. Common systems use different combinations of chromosomes (like XX/XY).

Mitosis Stages overview

• Mitosis has 5 distinct stages.
• Interphase - DNA replication
• Prophase - Condensing of chromatin to form chromosomes
• Prometaphase - Spindle forms and sister chromatids attach to spindle fibers
• Metaphase - Chromosomes line up at the metaphase plate
• Anaphase - Sister chromatids separate and move to opposite poles
• Telophase - Chromosomes begin to decondense, nuclear envelope forms, and cytokinesis begins

Cytokinesis

• Cytokinesis is the final step of cell division, where the cytoplasm divides to form two genetically identical daughter cells.
• Animals have a contractile ring that forms the cleavage furrow during cytokinesis, while plants develop a cell plate.

Meiosis Overview

• Meiosis has two rounds of cell division.
• Replication occurs only once before the first division to separate homologous pairs.
• Four haploid daughter cells are produced from two rounds of the process.

Meiosis I Overview

• In Prophase I, chromosomes condense, homologous pairs pair up exchanging genetic material (crossing over).
• In Metaphase I, homologous chromosomes line up on the metaphase plate.
• In Anaphase I, homologous chromosomes separate, but sister chromatids remain together; these move to opposite poles.
• In Telophase I, chromosomes decondense; followed by a second round (Meiosis II)

Meiosis II Overview

• Prophase II: Chromosomes condense.
• Metaphase II: Chromosomes align on the metaphase plate.
• Anaphase II: Sister chromatids separate and move to opposite poles.
• Telophase II: Four haploid daughter cells are produced.

Cell Cycle

• The cell cycle involves interphase (G1, S, and G2 phases) followed by mitosis, with checkpoints for regulation.
• DNA replication is crucial during the S stage of interphase.
• Importantly, checkpoints ensure proper cell division and growth.

Gametogenesis

• Gametogenesis involves specialized events leading to gamete formation.
  • In humans, oogenesis produces one ovum from each primary oocyte.
  • In humans, spermatogenesis produces four sperm from each primary spermatocyte.

Mistakes in Meiosis

• Nondisjunction is a common error in meiosis leading to abnormal chromosome numbers in resulting gametes or embryos.
  • Nondisjunction can lead to conditions like Down syndrome, XXY (Klinefelter syndrome), etc.
  • Importantly, many hybrids between species (like donkey x horse = mule) are often sterile due to mismatched chromosome numbers.

Genetic Diversity

• Crossing over between homologous chromosomes and independent assortment of chromosomes are key processes that promote genetic variation in organisms.