Cell Division, DNA Replication, and Gene Expression: Study Notes

Cell Division: Mitosis and Meiosis

Phases of Mitosis

Mitosis is the process by which a eukaryotic cell divides to produce two genetically identical daughter cells. It consists of several ordered phases, each with distinct events and functions.

• Prophase: Chromatin condenses into visible chromosomes; the mitotic spindle begins to form; the nuclear envelope starts to break down.

• Prometaphase: The nuclear envelope fragments completely; spindle fibers attach to kinetochores on chromosomes.

• Metaphase: Chromosomes align at the metaphase plate (cell equator).

• Anaphase: Sister chromatids separate and are pulled toward opposite poles of the cell.

• Telophase: Chromosomes decondense; nuclear envelopes reform around each set of chromosomes.

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

Key Events: Accurate chromosome segregation, spindle formation, and cytokinesis ensure genetic stability.

Phases of Meiosis

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

• Meiosis I:

  • Prophase I: Homologous chromosomes pair (synapsis) and exchange genetic material (crossing over).

  • Metaphase I: Homologous pairs align at the metaphase plate.

  • Anaphase I: Homologous chromosomes separate to opposite poles.

  • Telophase I and Cytokinesis: Two haploid cells form; chromosomes are still duplicated.

• Meiosis II: Similar to mitosis; separates sister chromatids.

  • Prophase II, Metaphase II, Anaphase II, Telophase II, and Cytokinesis occur, resulting in four haploid cells.

Key Events: Crossing over, independent assortment, and reduction of chromosome number.

Comparison of Mitosis and Meiosis

Feature	Mitosis	Meiosis
Number of divisions	1	2
Number of daughter cells	2	4
Genetic identity	Identical to parent	Genetically unique
Chromosome number	Diploid (2n)	Haploid (n)
Role	Growth, repair, asexual reproduction	Sexual reproduction (gametes)

Cell Cycle: Gap 0, Gap 1, Synthesis, and Checkpoints

Phases of the Cell Cycle

• Gap 0 (G0): A resting phase where cells exit the cycle and do not divide (e.g., neurons).

• Gap 1 (G1): Cell grows, carries out normal functions, and prepares for DNA replication.

• Synthesis (S): DNA is replicated, resulting in two sister chromatids per chromosome.

• Gap 2 (G2): Further growth and preparation for mitosis.

Importance of Checkpoints

• G1 Checkpoint: Ensures cell is ready for DNA synthesis; checks for DNA damage.

• G2 Checkpoint: Verifies DNA replication is complete and undamaged before mitosis.

• M Checkpoint (Spindle Checkpoint): Ensures all chromosomes are properly attached to the spindle before anaphase.

Significance: Checkpoints maintain genomic integrity and prevent uncontrolled cell division (cancer).

Reproductive Strategies: Sexual vs. Asexual

Advantages and Disadvantages

Strategy	Advantages	Disadvantages
Asexual	Rapid reproduction; no mate needed; offspring identical to parent	Lack of genetic diversity; vulnerable to environmental changes
Sexual	Genetic variation; adaptability to changing environments	Slower; requires finding a mate; energetically costly

Genetic Variation in Offspring

Mechanisms

• Crossing Over: Exchange of genetic material between homologous chromosomes during Prophase I of meiosis, creating new allele combinations.

• Independent Assortment: Random orientation of homologous pairs during Metaphase I leads to varied combinations of maternal and paternal chromosomes in gametes.

• Random Fertilization: Any sperm can fertilize any egg, further increasing genetic diversity.

Example: These mechanisms explain why siblings (except identical twins) are genetically unique.

DNA Replication

Process Overview

DNA replication is the process by which a cell copies its DNA before cell division. It is semiconservative and occurs in the S phase of the cell cycle.

• Origin of Replication: Specific sequence where replication begins.

• Replication Fork: Y-shaped region where new DNA strands are synthesized.

Key Enzymes and Terms

• Helicase: Unwinds the DNA double helix.

• Single-Strand Binding Proteins (SSBs): Stabilize unwound DNA.

• Primase: Synthesizes RNA primers to initiate DNA synthesis.

• DNA Polymerase III: Adds nucleotides to the growing DNA strand in the 5' to 3' direction.

• DNA Polymerase I: Removes RNA primers and replaces them with DNA.

• Ligase: Joins Okazaki fragments on the lagging strand.

• Okazaki Fragments: Short DNA segments synthesized discontinuously on the lagging strand.

Antiparallel and Semiconservative Replication

• Antiparallel: The two DNA strands run in opposite directions (5' to 3' and 3' to 5').

• Semiconservative Replication: Each new DNA molecule consists of one parental and one newly synthesized strand.

Equation:

$\text{Parental DNA} \rightarrow \text{Daughter DNA 1 (old + new)} + \text{Daughter DNA 2 (old + new)}$

Mutations: Types and Causes

Types of Mutations

• Point Mutation: Change in a single nucleotide (e.g., substitution).

• Insertion: Addition of one or more nucleotides.

• Deletion: Loss of one or more nucleotides.

• Frameshift Mutation: Insertion or deletion that alters the reading frame.

• Silent, Missense, Nonsense Mutations: Affect the resulting protein differently (no change, amino acid change, or premature stop codon, respectively).

Causes of Mutations

• Spontaneous: Errors during DNA replication.

• Induced: Caused by mutagens such as chemicals, radiation, or viruses.

Gene Expression: Transcription and Translation

Transcription

Transcription is the synthesis of RNA from a DNA template. It occurs in the nucleus of eukaryotic cells.

• Initiation: RNA polymerase binds to the promoter region.

• Elongation: RNA polymerase synthesizes the RNA strand in the 5' to 3' direction.

• Termination: RNA polymerase reaches a terminator sequence and releases the RNA transcript.

Translation

Translation is the process by which ribosomes synthesize proteins using mRNA as a template. It occurs in the cytoplasm.

• Initiation: Ribosome assembles around the start codon of mRNA.

• Elongation: tRNAs bring amino acids to the ribosome, which are joined to form a polypeptide chain.

• Termination: The ribosome reaches a stop codon and releases the completed protein.

Comparison of Transcription and Translation

Feature	Transcription	Translation
Template	DNA	mRNA
Product	mRNA	Protein
Location (Eukaryotes)	Nucleus	Cytoplasm
Enzyme	RNA polymerase	Ribosome

Example: The gene for hemoglobin is transcribed into mRNA, which is then translated into the hemoglobin protein.

Additional info: These processes are fundamental to the central dogma of molecular biology: $\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$