DNA Replication and Protein Synthesis: Study Notes for Anatomy & Physiology

Chapter 3: DNA Replication and Protein Synthesis

Overview

This chapter covers the essential molecular processes of DNA replication and protein synthesis, which are fundamental to cell division and function in human anatomy and physiology. Understanding these mechanisms is crucial for grasping how genetic information is maintained and expressed in living organisms.

DNA Replication

Introduction to DNA Replication

DNA replication is the process by which a cell copies its DNA prior to cell division, ensuring that each daughter cell receives a complete set of genetic instructions.

• Purpose: To duplicate the cell's genetic material before division.

• Location: Occurs in the nucleus of eukaryotic cells.

• Key Enzymes: DNA polymerase, helicase, ligase, and primase.

Steps in DNA Replication

• Initiation: DNA helicase unwinds the double helix, creating replication bubbles with replication forks at each end.

• Template Strands: Each original DNA strand serves as a template for the synthesis of a new complementary strand.

• Primer Formation: RNA primer is laid down by primase to provide a starting point for DNA polymerase.

• Elongation: DNA polymerase adds nucleotides to the growing DNA strand, starting from the RNA primer.

Directionality and Strand Synthesis

• DNA polymerase only works in one direction (5' to 3').

• Leading strand: Synthesized continuously toward the replication fork.

• Lagging strand: Synthesized discontinuously away from the replication fork in short segments called Okazaki fragments.

• DNA ligase: Joins Okazaki fragments to form a continuous strand.

Semiconservative Replication

• End result: Two identical DNA molecules, each composed of one old (parental) strand and one newly synthesized strand.

• Semiconservative replication: Each daughter DNA molecule retains one strand from the original DNA and one new strand.

• Significance: Ensures genetic consistency during cell division.

Protein Synthesis

Introduction to Protein Synthesis

Protein synthesis is the process by which cells build proteins based on genetic instructions encoded in DNA. This process involves two main steps: transcription and translation.

• Gene: A segment of DNA that contains the blueprint for one polypeptide.

• Triplet: Three sequential DNA bases that specify a particular amino acid.

Structure of Genes

• Exons: Coding regions that specify amino acids.

• Introns: Noncoding regions that are removed during RNA processing.

Role of RNA in Protein Synthesis

• Messenger RNA (mRNA): Carries instructions from DNA to ribosomes.

• Ribosomal RNA (rRNA): Structural component of ribosomes; helps translate mRNA.

• Transfer RNA (tRNA): Brings amino acids to the ribosome and matches them to the mRNA codon.

• RNA vs. DNA: RNA contains uracil (U) instead of thymine (T).

Transcription

Transcription is the process by which DNA information is copied into mRNA.

• Initiation: RNA polymerase binds to the promoter region and unwinds DNA.

• Elongation: RNA polymerase adds complementary RNA nucleotides (A, C, G, U).

• Termination: Transcription ends when a termination signal is reached (e.g., UAA, UAG, UGA).

• RNA Processing: Introns are removed, and exons are spliced together to form mature mRNA.

Translation

Translation is the process by which the mRNA sequence is decoded to build a polypeptide (protein).

• Codon: Three-base sequence on mRNA that specifies an amino acid.

• Anticodon: Three-base sequence on tRNA complementary to the mRNA codon.

• Ribosome: Site of protein synthesis; contains A (aminoacyl), P (peptidyl), and E (exit) sites.

Phases of Translation

• Initiation: Small ribosomal subunit binds to mRNA and initiator tRNA; large subunit joins to form a functional ribosome.

• Elongation: tRNAs bring amino acids to the ribosome, where peptide bonds are formed and the ribosome moves along the mRNA.

• Termination: When a stop codon is reached, the polypeptide is released, and the ribosome disassembles.

Genetic Code Table (Sample)

The genetic code is a set of rules by which information encoded in mRNA is translated into proteins. Each codon specifies a particular amino acid.

Codon	Amino Acid
AUG	Methionine (Start)
UAA, UAG, UGA	Stop
UUU, UUC	Phenylalanine
GGC	Glycine
Additional info: Many amino acids are specified by more than one codon.	Additional info: This redundancy is called degeneracy of the genetic code.

Polyribosomes

• Polyribosome: Multiple ribosomes simultaneously translating a single mRNA molecule, producing multiple copies of the same protein.

Role of Rough Endoplasmic Reticulum (ER)

• Rough ER: Proteins synthesized by ribosomes on the rough ER are processed and transported for secretion or use in the cell membrane.

• Signal Recognition Particle (SRP): Directs the ribosome-mRNA complex to the rough ER.

• Protein Processing: Proteins may be modified and packaged into vesicles for transport to the Golgi apparatus.

Summary: From DNA to Proteins

• Complementary base pairing ensures accurate transfer of genetic information from DNA to mRNA and from mRNA to tRNA during protein synthesis.

• Central Dogma: DNA → RNA → Protein

• Key Equations:

$\text{DNA triplet} \rightarrow \text{mRNA codon} \rightarrow \text{tRNA anticodon} \rightarrow \text{Amino acid}$

$\text{Rate of DNA synthesis} = k[\text{DNA polymerase}][\text{Nucleotides}]$

Example: If the DNA sequence is TAC, the mRNA codon will be AUG, and the tRNA anticodon will be UAC, coding for methionine.