chapter 8

Key Definitions

1. Genetics – The study of heredity, including how genes are passed from one generation to another.

2. Genome – The complete set of genetic material (DNA) in an organism.

3. Chromosome – A structure made of DNA that carries genes; bacteria typically have a single circular chromosome.

4. Gene – A segment of DNA that codes for a functional product, usually a protein.

5. Genetic Code – The set of rules by which nucleotide sequences (codons) in mRNA are translated into amino acids to form proteins.

6. Genotype – The genetic makeup of an organism, including all its genes.

7. Phenotype – The observable traits or characteristics of an organism, influenced by both genotype and environment.

8. Genomics – The study of entire genomes, including gene sequencing, function, and regulation.

How DNA Serves as Genetic Information

• Structure: DNA is a double-stranded helix made of nucleotides (A, T, C, G).

• Storage: The sequence of nucleotides in DNA encodes genetic information.

• Expression: DNA directs protein synthesis via transcription and translation.

• Replication: Before cell division, DNA is copied so each daughter cell receives identical genetic material.

DNA Replication Process

• Purpose: Ensures genetic material is copied before cell division.

• Type: Semiconservative replication (each new DNA molecule contains one original and one new strand).

Steps of DNA Replication

1. Initiation: Helicase unwinds the DNA helix at the origin of replication.

2. Priming: Primase synthesizes RNA primers to provide a starting point for DNA polymerase.

3. Elongation:

• DNA polymerase adds nucleotides to the growing strand (5’ → 3’ direction).

• Leading strand: Synthesized continuously.

• Lagging strand: Synthesized in short Okazaki fragments, later joined by DNA ligase.

4. Termination: DNA polymerase proofreads and corrects errors; replication stops when the entire DNA molecule is copied.

Protein Synthesis: Transcription, RNA Processing, and Translation

Transcription (DNA → mRNA)

1. Initiation: RNA polymerase binds to the promoter region of DNA.

2. Elongation: RNA polymerase synthesizes a complementary RNA strand (A-U, G-C).

3. Termination: Transcription stops at the terminator sequence, and mRNA is released.

RNA Processing (in Eukaryotes only)

• Introns (non-coding regions) are removed, and exons (coding regions) are spliced together.

• A 5’ cap and poly-A tail are added for stability.

Translation (mRNA → Protein)

1. Initiation: Ribosome binds to mRNA at the start codon (AUG – methionine).

2. Elongation: tRNA molecules bring amino acids to the ribosome, matching codons with anticodons. Peptide bonds form between amino acids.

3. Termination: The ribosome reaches a stop codon (UAA, UAG, UGA), and the protein is released.

Comparison: Protein Synthesis in Prokaryotes vs. Eukaryotes