Cell Division, Chromosomes, and Reproduction: Study Notes for General Biology

Cell Division and Reproduction

Functions of Cell Division

• Growth: Cell division increases the number of cells, allowing organisms to grow.

• Healing: Damaged tissues are repaired by producing new cells.

• Development: Multicellular organisms develop from a single cell through repeated cell divisions.

• Sexual Reproduction: Involves the production of gametes (sperm and egg cells).

• Asexual Reproduction: Common in single-celled organisms; new individuals are produced without the fusion of gametes.

Methods of Reproduction

• Asexual Reproduction:

  • Produces offspring that are genetically identical to the parent (clones).

  • Common in bacteria, some plants, and some animals.

• Sexual Reproduction:

  • Produces offspring with unique combinations of genes from two parents.

  • Requires fertilization of an egg by a sperm (gametes).

  • Increases genetic diversity in populations.

Chromosomes and the Cell Cycle

Chromosomes

• A chromosome is a single piece of DNA containing many genes.

• Humans have 46 chromosomes (23 pairs) in most cells.

The Cell Cycle

• Interphase: The cell grows and duplicates its contents, including DNA. Sister chromatids are created during this phase.

• Mitotic Division: Consists of two main processes:

  • Mitosis: Division of the nucleus.

  • Cytokinesis: Division of the cytoplasm, resulting in two daughter cells.

• A mitotic spindle forms to move and guide the chromosomes during mitosis.

Phases of Mitosis

• Prophase:

  • Chromosomes condense and become visible.

  • Nuclear envelope breaks up.

  • Spindle forms and attaches to sister chromatids.

• Metaphase:

  • Chromosomes align at the cell's equator (metaphase plate).

  • Sister chromatids are centered at equilibrium.

• Anaphase:

  • Sister chromatids separate and move toward opposite spindle poles.

  • The cell elongates as chromosomes are segregated.

• Telophase:

  • Chromosomes arrive at spindle poles and decondense.

  • New nuclei form; spindle disappears.

  • Nuclear envelopes reform.

• Cytokinesis:

  • Not a phase of mitosis, but follows telophase.

  • The cell divides into two daughter cells.

Types of Chromosomes and Genetic Variation

Karyotype

• A karyotype is an image of an individual's diploid set of chromosomes, used to study chromosome number and structure.

Types of Chromosomes

• Autosomes: Chromosomes that are the same in males and females; each pair has the same size, shape, and genetic information about the same traits.

• Sex Chromosomes: Differ between males and females (XX for females, XY for males in humans).

Alleles

• Alleles are different forms of the same gene, found at the same locus on homologous chromosomes.

• Homologous chromosomes carry the same genes but may have different alleles.

• Sister chromatids are identical copies of a chromosome, connected at the centromere.

Meiosis and Genetic Diversity

Overview of Meiosis

• Meiosis is the process by which gametes (sperm and egg cells) are produced.

• Reduces the chromosome number by half, resulting in haploid cells.

• Increases genetic diversity through independent assortment and crossing over.

Phases of Meiosis I

• Prophase I:

  • Chromosomes condense.

  • Homologous chromosomes pair up by synapsis to form tetrads.

  • Crossing over occurs, exchanging genetic material between homologous chromosomes.

• Metaphase I:

  • Tetrads align along the cell equator.

• Anaphase I:

  • Homologous pairs separate and move toward opposite poles.

  • Chromosome number is reduced from diploid to haploid.

  • Chromatids remain attached.

• Telophase I:

  • Duplicated chromosomes reach the poles.

  • Nuclear envelope reforms.

  • Each nucleus is now haploid.

Genetic Variation in Meiosis

• Independent Assortment: Random distribution of maternal and paternal chromosomes to gametes.

• Crossing Over: Exchange of genetic material between homologous chromosomes during prophase I.

• These processes result in genetically unique gametes and increase variation in offspring.