Cell Division, Cell Cycle, and Patterns of Inheritance: Study Notes

Cell Division and Reproduction

Sexual vs. Asexual Reproduction

Reproduction is the biological process by which new individual organisms are produced. It can occur via two main mechanisms: sexual reproduction and asexual reproduction.

• Sexual reproduction: Involves the fusion of gametes (egg and sperm), resulting in offspring with genetic variation.

• Asexual reproduction: Produces offspring genetically identical to the parent, without the fusion of gametes.

Examples and Advantages of Asexual Reproduction

Asexual reproduction occurs in many organisms, including plants and some animals. Common forms include budding, fragmentation, and vegetative propagation.

• Examples:

  • Plants: Many plants reproduce asexually via runners (stolons) or tubers.

  • Animals: Hydra and starfish can reproduce by budding or regeneration.

• Advantages:

  • Rapid population increase

  • No need for a mate

  • Offspring are well-adapted to stable environments

Types of Cell Division

Binary Fission, Mitosis, and Meiosis

Cell division is essential for growth, development, and reproduction. There are three main types:

• Binary Fission: Occurs in prokaryotes; DNA is replicated and the cell splits into two identical cells.

• Mitosis: Occurs in eukaryotes; involves DNA replication, segregation into two daughter nuclei, and cell division. Results in two genetically identical daughter cells.

• Meiosis: Specialized division for sexual reproduction; produces four haploid gametes with genetic diversity.

Summary Table: Types of Cell Division

Type	Process	Result	Main Purpose
Binary Fission	DNA replication, cell splits	2 identical cells	Asexual reproduction
Mitosis	DNA replication, segregation, cell splits	2 identical cells	Growth, development, tissue repair
Meiosis	DNA replication, two rounds of division	4 haploid gametes	Sexual reproduction

Chromosomes and the Eukaryotic Genome

Chromosome Structure and Function

Chromosomes are single molecules of DNA containing many genes. In humans, there are approximately 21,000 genes distributed across 46 chromosomes. Chromatin, a mixture of DNA and protein, makes up chromosomes. During cell division, chromosomes condense and become visible under a microscope.

• Gene: Unit of heredity; sequence of DNA encoding a trait.

• Chromatin: DNA-protein complex forming chromosomes.

The Cell Cycle

Phases of the Cell Cycle

The cell cycle describes the sequence of events in the life of a cell, including growth, DNA replication, and division. It consists of interphase (G1, S, G2) and the mitotic phase (mitosis and cytokinesis).

• G1 phase: Cell growth

• S phase: DNA synthesis (replication)

• G2 phase: Further growth and preparation for division

• Mitotic phase: Mitosis and cytokinesis, resulting in two daughter cells

Cell Cycle Control and Cancer

Checkpoints and Growth Regulation

Cell cycle progression is regulated by checkpoints that ensure proper growth, DNA integrity, and division. Key checkpoints include G1, G2/M, and M (metaphase).

• G1 checkpoint: Requires growth factor signals for cell to proceed.

• G2/M checkpoint: Prevents entry into mitosis if DNA is damaged or cell is too small.

• M checkpoint: Ensures proper chromosome alignment before division.

Cancer Progression

Cancer arises from uncontrolled cell growth due to failures in cell cycle regulation. Tumors can be benign (localized) or malignant (capable of metastasis). Cancer therapies include surgery, radiation, chemotherapy, and immunotherapy.

• Transformation: Cell acquires cancerous properties.

• Metastasis: Spread of cancer cells to distant sites.

Chromosome Number and Sex Determination

Diploid vs. Haploid Cells

Somatic cells are diploid (2n), containing two sets of chromosomes. Gametes are haploid (n), containing one set. Humans have 22 pairs of autosomes and one pair of sex chromosomes (XX in females, XY in males).

Meiosis and Genetic Diversity

Meiosis Overview

Meiosis is a two-step division process that produces four genetically unique haploid gametes. It includes pairing and recombination of homologous chromosomes, increasing genetic diversity.

• Independent assortment: Random alignment of chromosomes during metaphase I.

• Crossing over: Exchange of chromosome segments between homologous chromosomes during prophase I.

• Random fertilization: Further increases genetic variation.

Nondisjunction and Aneuploidy

Nondisjunction is the failure of chromosomes to segregate properly during meiosis, leading to aneuploidy (abnormal chromosome number). Examples include Down syndrome (trisomy 21), Turner syndrome (XO), and Klinefelter syndrome (XXY).

Mendelian Genetics

Law of Segregation

Gregor Mendel's experiments with pea plants established the principles of heredity. The law of segregation states that each parent has two alleles for a gene, which segregate during gamete formation.

• Homozygous: Two identical alleles

• Heterozygous: Two different alleles

Punnett Square, Genotype, and Phenotype

The Punnett square is a tool for predicting genotype and phenotype ratios in offspring. For a monohybrid cross:

• Phenotypic ratio: 3:1 (dominant:recessive)

• Genotypic ratio: 1:2:1 (homozygous dominant : heterozygous : homozygous recessive)

Law of Independent Assortment

Mendel's second law states that genes for different traits assort independently during gamete formation, unless they are linked.

Extensions and Modifications of Mendelian Genetics

Incomplete Dominance and Codominance

Some traits do not follow simple dominance. In incomplete dominance, the heterozygote has an intermediate phenotype. In codominance, both alleles are fully expressed (e.g., ABO blood groups).

Polygenic Traits and Environmental Effects

Most traits are influenced by multiple genes (polygenic) and environmental factors. Examples include height and cholesterol levels.

Genetic Diseases and Testing

Monogenic Diseases

Single-gene disorders include sickle cell anemia, cystic fibrosis, and phenylketonuria (PKU). PKU is a recessive disease caused by deficiency of phenylalanine hydroxylase.

Genetic Testing

Genetic testing is used for diagnosis, carrier screening, and newborn screening for treatable genetic diseases.

The Chromosomal Basis of Inheritance

Gene Linkage and Sex-Linked Traits

Genes located close together on the same chromosome are linked and do not assort independently. Sex-linked traits, such as hemophilia, are often carried on the X chromosome.

DNA: The Genetic Material

Structure and Function of DNA

DNA encodes genetic information and is composed of nucleotides. The double helix structure was discovered by Watson and Crick, with contributions from Rosalind Franklin.

Central Dogma: Transcription and Translation

The central dogma describes the flow of genetic information: DNA is transcribed into RNA, which is translated into protein.

Mutations

Mutations are changes in DNA sequence. Types include nucleotide substitutions (silent, missense, nonsense) and indels (insertions/deletions causing frameshifts).

Summary Table: Key Terms

Term	Definition
Chromosome	DNA molecule containing genes
Gene	Unit of heredity; DNA sequence encoding a trait
Allele	Variant form of a gene
Genotype	Genetic makeup of an organism
Phenotype	Physical traits of an organism
Homozygous	Two identical alleles
Heterozygous	Two different alleles
Mutation	Change in DNA sequence