Study Notes: Cell Cycle, Mendelian Genetics, DNA Replication, Transcription, and Translation

Topic 9: Cell Cycle

Overview 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 (G1, S, G2 phases) and the mitotic phase (mitosis and cytokinesis).

• Cell Division: The process by which a parent cell divides into two or more daughter cells.

• Cell Cycle Stages: Includes G1 (cell growth), S (DNA synthesis), G2 (preparation for mitosis), and M (mitosis).

• Cell Growth Factors: Proteins that regulate cell division and growth.

• Chromosome Structure: DNA is packaged into chromosomes, which are duplicated before cell division.

• Mitosis: Division of the nucleus, resulting in two genetically identical daughter cells.

• Cytokinesis: Division of the cytoplasm, completing cell division.

• Cell Cycle Regulation: Controlled by checkpoints (G1, G2, M) and proteins such as cyclins and cyclin-dependent kinases (CDKs).

• Apoptosis: Programmed cell death, important for development and homeostasis.

Example: Human skin cells undergo the cell cycle to replace dead or damaged cells.

Topic 10: Mendelian Genetics & Heritability

Principles of Mendelian Genetics

Mendelian genetics explains how traits are inherited through discrete units called genes. Gregor Mendel's experiments with pea plants established the laws of inheritance.

• Phenotype vs. Genotype: Phenotype is the observable trait; genotype is the genetic makeup.

• Segregation: Each parent contributes one allele for each gene to their offspring.

• Independent Assortment: Genes for different traits can segregate independently during gamete formation.

• Mendel's Laws: Law of Segregation and Law of Independent Assortment.

• Complex Inheritance: Includes incomplete dominance, codominance, multiple alleles, polygenic inheritance, and epistasis.

• Chromosomal Basis of Inheritance: Genes are located on chromosomes.

• Meiosis: Cell division that reduces chromosome number by half, producing gametes.

• Sex-linked Genes: Genes located on sex chromosomes, often showing unique inheritance patterns.

• Linkage & Recombination: Linked genes tend to be inherited together; crossing over can separate linked genes.

Example: The inheritance of flower color in pea plants follows Mendel's laws.

Topic 11: DNA Replication

Mechanisms and Regulation of DNA Replication

DNA replication is the process by which a cell copies its DNA before cell division. It ensures that each daughter cell receives an identical set of genetic information.

• DNA as Genetic Material: Experiments by Griffith, Avery, Hershey & Chase established DNA as the hereditary material.

• Structure of DNA: Double helix model proposed by Watson & Crick, based on Chargaff's rules.

• Semiconservative Replication: Each new DNA molecule consists of one old strand and one new strand.

• Enzymes in Replication: DNA polymerase synthesizes new DNA; helicase unwinds the helix; primase synthesizes RNA primers.

• Replication Origins: Specific sequences where replication begins.

• Leading and Lagging Strands: Leading strand synthesized continuously; lagging strand synthesized in Okazaki fragments.

• Telomeres: Protective ends of chromosomes; telomerase extends telomeres in some cells.

Equation:

$\text{DNA Polymerase:} \quad (dNTP) + (DNA\ strand) \rightarrow (DNA\ strand\ extended) + (PP_i)$

Example: DNA replication in E. coli starts at a single origin and proceeds bidirectionally.

Topic 12A: RNA Synthesis - Transcription

Transcription and Gene Expression

Transcription is the process by which RNA is synthesized from a DNA template. It is the first step in gene expression.

• Transcription Definition: Synthesis of RNA from DNA by RNA polymerase.

• One Gene-One Enzyme Hypothesis: Each gene encodes a specific enzyme (now updated to one gene-one polypeptide).

• Mutations: Changes in DNA sequence can affect gene function and expression.

• RNA Types: mRNA (messenger), tRNA (transfer), rRNA (ribosomal).

• Genetic Code: Specifies which amino acids are added during translation; degenerate and redundant.

• Transcription in Prokaryotes vs. Eukaryotes: Eukaryotes have more complex regulation and RNA processing (splicing, capping, polyadenylation).

• Splicing: Removal of introns from pre-mRNA; exons are joined to form mature mRNA.

Example: In eukaryotes, the primary transcript (pre-mRNA) undergoes splicing to become mature mRNA.

Topic 12B: Protein Synthesis - Translation

Translation and the Genetic Code

Translation is the process by which the sequence of an mRNA molecule is used to produce a polypeptide (protein).

• tRNA: Transfers amino acids to the ribosome during protein synthesis.

• Ribosome Structure: Composed of rRNA and proteins; site of translation.

• Genetic Code: Triplet codons specify amino acids.

• Stages of Translation: Initiation, elongation, and termination.

• Polyribosomes: Multiple ribosomes translating a single mRNA simultaneously.

• Mutations: Can affect protein structure and function; types include point mutations, insertions, deletions.

Equation:

$\text{Amino Acid} + \text{tRNA} \xrightarrow{Aminoacyl-tRNA\ synthetase} \text{Aminoacyl-tRNA}$

Example: Sickle cell anemia is caused by a point mutation in the gene encoding beta-globin.

Process	Main Enzyme	Product
DNA Replication	DNA Polymerase	Double-stranded DNA
Transcription	RNA Polymerase	Single-stranded RNA
Translation	Ribosome	Polypeptide (Protein)

Additional info: These topics are foundational for understanding cell biology, genetics, and molecular biology, and are directly relevant to introductory college biology courses.