Unit 4

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 and asexual reproduction.

• Asexual reproduction: Involves a single parent and produces genetically identical offspring (clones). Common in unicellular organisms and some plants and animals.

• Sexual reproduction: Involves two parents and the fusion of gametes, resulting in genetically diverse offspring.

Examples of Asexual Reproduction

• Budding in hydra

• Regeneration in starfish

• Vegetative propagation in plants (e.g., runners/stolons in strawberries)

Advantages of Asexual Reproduction

• Rapid population increase

• No need for a mate

• Preservation of successful genotypes in 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:

Type	Process	Result	Purpose
Binary Fission	DNA replication; cell splits in two	Two identical cells	Asexual reproduction (prokaryotes)
Mitosis	DNA replication; segregation; cell splits	Two identical daughter cells	Growth, development, tissue repair (eukaryotes)
Meiosis	DNA replication; two rounds of division	Four haploid gametes	Sexual reproduction (eukaryotes)

The Cell Cycle and Mitosis

The Cell Cycle

The cell cycle is the ordered sequence of events that a cell goes through between one division and the next. 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: Preparation for mitosis

• Mitotic phase: Mitosis (nuclear division) and cytokinesis (cytoplasmic division)

Mitosis: An Overview

Mitosis is the process by which a eukaryotic cell separates its duplicated chromosomes into two identical sets, resulting in two genetically identical daughter cells.

• Prophase: Chromosomes condense, spindle forms

• Metaphase: Chromosomes align at the cell equator

• Anaphase: Sister chromatids separate

• Telophase: Nuclear envelopes reform

• Cytokinesis: Division of the cytoplasm

Chromosomes and Chromatin

Chromosomes are structures made of DNA and proteins (chromatin) that carry genetic information. In humans, there are 46 chromosomes (23 pairs).

• Chromatin: DNA-protein complex that is diffuse in non-dividing cells

• Chromosome: Condensed form of chromatin visible during cell division

• Gene: Segment of DNA encoding a trait

Cell Cycle Regulation and Cancer

Cell Cycle Checkpoints

Cell cycle checkpoints ensure that cells only proceed to the next stage when conditions are favorable and DNA is undamaged.

• G1 checkpoint: Checks for cell size, nutrients, growth factors, and DNA damage

• G2 checkpoint: Checks for DNA replication completion and damage

• M checkpoint: Ensures chromosomes are properly attached to the spindle

Cancer and the Cell Cycle

Cancer results from uncontrolled cell division due to failures in cell cycle regulation. Tumors can be benign (localized) or malignant (capable of metastasis).

• Transformation: Process by which a normal cell becomes cancerous

• Metastasis: Spread of cancer cells to other parts of the body

• Cancer therapies: Surgery, radiation, chemotherapy (targets dividing cells)

Chromosome Number and Structure

Diploid and Haploid Cells

Organisms have two sets of chromosomes (diploid, 2n) in somatic cells and one set (haploid, n) in gametes. Humans have 23 pairs of chromosomes: 22 pairs of autosomes and 1 pair of sex chromosomes (XX or XY).

Meiosis and Genetic Diversity

Overview of Meiosis

Meiosis is a specialized form of cell division that reduces the chromosome number by half, producing four genetically unique haploid gametes. It consists of two sequential divisions: meiosis I and meiosis II.

• Meiosis I: Homologous chromosomes separate

• Meiosis II: Sister chromatids separate

Sources of Genetic Variation

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

• Crossing over: Exchange of genetic material between non-sister chromatids during prophase I

• Random fertilization: Any sperm can fertilize any egg

Nondisjunction and Aneuploidy

Nondisjunction is the failure of chromosomes to separate properly during meiosis, leading to gametes with abnormal chromosome numbers (aneuploidy).

• Down syndrome: Trisomy 21 (three copies of chromosome 21)

• Turner syndrome: XO (single X chromosome)

• Klinefelter syndrome: XXY (extra X chromosome in males)

Mendelian Genetics

Gregor Mendel and the Laws of Inheritance

Gregor Mendel's experiments with pea plants established the basic principles of heredity, including the law of segregation and the law of independent assortment.

• Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation.

• Law of Independent Assortment: Genes for different traits assort independently during gamete formation.

Monohybrid Crosses and Punnett Squares

Punnett squares are used to predict the genotypic and phenotypic ratios of offspring from genetic crosses.

• Genotype: Genetic makeup (e.g., PP, Pp, pp)

• Phenotype: Observable trait (e.g., purple or white flowers)

Dihybrid Crosses and Probability

Dihybrid crosses involve two traits and demonstrate independent assortment. Probability rules (product and addition) are used to calculate genotype and phenotype ratios.

Extensions and Exceptions to Mendel's Laws

• Incomplete dominance: Heterozygotes show an intermediate phenotype (e.g., pink flowers from red and white parents)

• Codominance: Both alleles are expressed (e.g., ABO blood groups)

• Polygenic inheritance: Traits influenced by multiple genes (e.g., height, skin color)

• Linkage: Genes located close together on the same chromosome tend to be inherited together

DNA: The Genetic Material

Structure and Function of DNA

DNA (deoxyribonucleic acid) is the hereditary material in all living organisms. It encodes genetic information using four nucleotide bases: adenine (A), thymine (T), cytosine (C), and guanine (G).

• Double helix: Two strands held together by complementary base pairing (A-T, C-G)

• Central Dogma: DNA → RNA → Protein

Gene Expression

• Transcription: Synthesis of RNA from a DNA template

• Translation: Synthesis of protein from an mRNA template at the ribosome

Mutations

Mutations are changes in the DNA sequence that can affect gene function. Types include:

• Substitution: One base is replaced by another

• Insertion/Deletion (indels): Addition or loss of bases, potentially causing frameshifts

Example: Sickle-cell anemia is caused by a single nucleotide substitution in the hemoglobin gene.

Summary Table: Types of Cell Division

Type	Organisms	Purpose	Result
Binary Fission	Bacteria, Archaea	Asexual reproduction	2 identical cells
Mitosis	Eukaryotes	Growth, repair, asexual reproduction	2 identical cells
Meiosis	Eukaryotes	Sexual reproduction	4 genetically unique gametes