Molecular Biology - Protein Synthesis Quiz

Biology Chapter 12: Molecular Biology of the Gene

• Chapter Overview: This chapter details the molecular biology of the gene, encompassing the genetic material, DNA replication, gene expression (transcription and translation), and the accuracy of replication. It explores historical experiments and the fundamental mechanisms underlying genetic information flow.

12.1 The Genetic Material

• Genetic Material Requirements: Genetic material must store developmental, structural, and metabolic information, be accurately replicated, be transmittable between generations, and allow for mutations creating genetic variability.

• Nucleic Acids Discovery: Scientists identified deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) as major components within the nucleus.

12.2 Replication of DNA

• DNA Replication: The process of copying a DNA molecule.

• Semiconservative Replication: Each new DNA molecule contains one old strand and one newly synthesized strand.

• Meselson-Stahl Experiment: Demonstrated the semiconservative nature of DNA replication using isotopes. This experiment provided evidence showing the parent strands act as a template for building new strands.

Steps of DNA Replication

• Unwinding: Helicase unwinds the DNA double helix at the origin of replication. SSB proteins stabilize the open strands.

• Complementary Base Pairing: Primase adds an RNA primer where DNA polymerase can bind. Complementary nucleotides are added and paired by DNA polymerase. DNA polymerase has a proofreading capability to avoid mistakes.

• Joining: Ligase joins the Okazaki fragments, forming new strands.

Prokaryotic vs. Eukaryotic Replication

• Prokaryotic Replication: Occurs in two directions from a single origin of replication within a circular loop in bacteria.

• Eukaryotic Replication: Begins at numerous origins of replication forming replication bubbles, with replication forks moving outward in two directions.

12.3 Gene Expression: RNA & the Genetic Code

• One Gene-One Enzyme Hypothesis: Proposed that a gene codes for a single enzyme.

• Pauling's Sickle Cell Anemia Discovery: Suggested that a mutation in a gene could lead to a change in protein structure.

RNA Types

• mRNA (messenger RNA): Carries the genetic message from DNA in the nucleus to ribosomes in the cytoplasm.

• rRNA (ribosomal RNA): With proteins, forms ribosomes where polypeptide synthesis occurs

• tRNA (transfer RNA): Transfers the appropriate amino acid to the ribosomes for protein synthesis.

The Central Dogma of Molecular Biology

• Flow of Genetic Information: DNA → mRNA → Protein

• Transcription: Production of mRNA from a DNA template within the nucleus.

• Translation: Synthesis of proteins from an mRNA template at ribosomes in the cytoplasm.

12.4 Gene Expression: Transcription

• RNA polymerase: Attaches to a promoter region, which signals the start of a gene, and initiates transcription.
• mRNA synthesis: mRNA is formed based on the DNA template (with uracil replacing thymine), reading the DNA in a 3' to 5' direction.
• Transcription termination: RNA polymerase stops when it reaches a termination sequence in the DNA.
• mRNA processing: Primary mRNA transcripts in eukaryotes are modified before leaving the nucleus to mature mRNA:
  • 5' cap: added to the 5' end, aiding ribosome binding
  • poly-A tail: added to the 3' end, stabilizing the transcript
  • splicing: removal of introns (non-coding regions) and joining of exons (coding regions).
• Spliceosomes: Complex molecules involved in removing introns and joining exons during mRNA processing.
• snRNA: Small nuclear RNAs are components of spliceosomes.

12.5 Gene Expression: Translation

• tRNA: Transfers amino acids to the ribosomes according to the mRNA codons. Carries a specific amino acid and contains an anticodon to match to the mRNA codon.

• Ribosomes: Cellular structures where amino acids are linked together to form polypeptide chains. Contains tRNA binding sites (A, P, E sites).

• Translation steps:

  • Initiation: Start codon (AUG) recognized. Initiator tRNA is bound to the start codon. Small and large ribosomal subunits assemble.

  • Elongation: tRNA carrying amino acids bind to codons. Peptide bonds form between amino acids. Ribosome moves along mRNA.

  • Termination: Stop codon encountered. Release factor binds. Polypeptide chain released. Ribosomal subunits dissociate.

• Protein Folding: Polypeptide chain folds into its functional 3D structure assisted by chaperone proteins.

Accuracy of Replication

• DNA Polymerase Accuracy: DNA polymerase is quite precise, often making one error for every 100,000 base pairs. Polymerases have proofreading capabilities.
• Genetic Mutations: Permanent changes in the DNA sequence, which can arise from DNA replication errors or DNA damage. DNA repair enzymes can fix some of these errors.

12.6 Gene Expression: Summary

• Genome Properties: Degenerative, unambiguous, and universal in the genetic code. The codons that code for an amino acid could be more than one.
• Prokaryotic vs. Eukaryotic Gene Processing: Prokaryotes lack a nucleus. Their transcription and translation can occur concurrently. Eukaryotes have a nucleus in which transcription is isolated from the cytoplasm during which translation happens.