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Cell Cycle, Genetics, and Molecular Biology: Comprehensive Study Notes

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Cell Division and the Cell Cycle

Stages of the Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide. It consists of interphase and the mitotic phase (mitosis and cytokinesis).

  • G1 phase: Cell growth occurs. A checkpoint ensures DNA is undamaged.

  • G0 phase: Cells enter a non-dividing state if they do not pass the G1 checkpoint.

  • S phase: DNA replication occurs.

  • G2 phase: Preparation for cell division; another checkpoint checks DNA integrity.

  • M phase (mitosis): Division of the nucleus and chromosomes.

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

Example: Skin cells regularly undergo the cell cycle to replace dead or damaged cells.

Mitosis and Cytokinesis

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

  • Prophase: Chromosomes condense and become visible; spindle fibers form.

  • Metaphase: Chromosomes align at the cell's equator.

  • Anaphase: Sister chromatids are pulled apart to opposite poles.

  • Telophase: Nuclear envelopes reform around each set of chromosomes.

  • Cytokinesis: Cytoplasm divides, completing cell division.

Example: Mitosis is essential for growth, repair, and asexual reproduction in multicellular organisms.

Meiosis and Sexual Reproduction

Stages of Meiosis

Meiosis reduces the chromosome number by half, producing four genetically unique haploid cells (gametes).

  • Prophase I: Homologous chromosomes pair and crossing-over occurs.

  • Metaphase I: Homologous pairs align at the center.

  • Anaphase I: Homologous chromosomes separate to opposite poles.

  • Telophase I and Cytokinesis: Two haploid cells form.

  • Prophase II: Chromosomes condense in each haploid cell.

  • Metaphase II: Chromosomes align at the center.

  • Anaphase II: Sister chromatids separate.

  • Telophase II and Cytokinesis: Four unique haploid cells are produced.

Example: Meiosis produces sperm and egg cells in animals.

Comparison of Mitosis and Meiosis

Feature

Mitosis

Meiosis

Number of divisions

1

2

Number of daughter cells

2

4

Genetic composition

Identical

Unique

Chromosome number

Diploid

Haploid

Function

Growth, repair

Gamete production

Mendelian Genetics and Inheritance

Patterns of Inheritance

Genetics is the study of heredity and variation in organisms. Mendelian genetics describes how traits are inherited through dominant and recessive alleles.

  • Dominant allele: Expressed if present (e.g., AA or Aa).

  • Recessive allele: Expressed only if both alleles are recessive (e.g., aa).

  • Homozygous: Two identical alleles (AA or aa).

  • Heterozygous: Two different alleles (Aa).

  • Genotype: Genetic makeup of an organism.

  • Phenotype: Observable traits.

Example: In pea plants, purple flower color (P) is dominant to white (p).

Non-Mendelian Inheritance

  • Incomplete dominance: Heterozygotes show an intermediate phenotype.

  • Codominance: Both alleles are expressed (e.g., AB blood type).

  • Multiple alleles: More than two alleles exist for a gene (e.g., blood types).

  • Pleiotropy: One gene influences multiple traits.

  • Epistasis: One gene affects the expression of another gene.

  • Polygenic inheritance: Multiple genes influence a single trait.

DNA Structure, Replication, and Biotechnology

DNA and RNA Structure

  • DNA: Double-stranded, deoxyribose sugar, bases A-T and G-C, found in the nucleus.

  • RNA: Single-stranded, ribose sugar, bases A-U and G-C, found in nucleus and cytoplasm.

DNA Replication

DNA replication is the process by which DNA makes a copy of itself during cell division.

  • Enzymes involved: Helicase (unwinds DNA), DNA polymerase (synthesizes new strands), ligase (joins fragments).

  • Semiconservative replication: Each new DNA molecule contains one old and one new strand.

Biotechnology Tools

  • Restriction enzymes: Cut DNA at specific sequences.

  • Gel electrophoresis: Separates DNA fragments by size.

  • Polymerase chain reaction (PCR): Amplifies DNA sequences.

  • Cloning: Producing genetically identical organisms or cells.

Gene Expression and Regulation

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information: DNA → RNA → Protein.

  • Transcription: DNA is transcribed into messenger RNA (mRNA).

  • Translation: mRNA is translated into a protein at the ribosome.

Equation:

Mutations

Mutations are changes in the DNA sequence that can affect protein structure and function.

  • Silent mutation: No change in amino acid sequence.

  • Missense mutation: Changes one amino acid (e.g., sickle cell anemia).

  • Nonsense mutation: Converts a codon to a stop codon, truncating the protein.

  • Duplication mutation: A segment of DNA is duplicated.

Genomics and the Human Genome Project

Purpose of the Human Genome Project

The Human Genome Project mapped and sequenced the entire human genome, identifying all human genes and their functions. This project advanced our understanding of genetics, disease, and biotechnology.

  • Mapped ~20,000 genes

  • Enabled advances in medicine, biotechnology, and genomics

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