DNA Replication and Protein Synthesis

Overview

This section explores the molecular mechanisms by which genetic information is stored, replicated, and expressed in living cells. It covers the structure of DNA, the processes of DNA replication, transcription, translation, and the impact of mutations on genetic information.

DNA Structure and Replication

DNA: The Information Center of the Cell

• DNA (deoxyribonucleic acid) is the hereditary material in all living organisms.

• In eukaryotes, DNA is located in the nucleus; in prokaryotes, it is found in the nucleoid region.

• Genetic information is stored in the sequence of nucleotides.

• Genes are segments of DNA that code for proteins.

• Before proteins are made, genes must be transcribed into RNA.

Nucleic Acid Structure

• DNA and RNA are polymers of nucleotides, each consisting of a phosphate group, a five-carbon sugar, and a nitrogenous base.

• Nitrogenous bases are either single-ringed (pyrimidines: cytosine, thymine, uracil) or double-ringed (purines: adenine, guanine).

DNA vs. RNA

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

• The sugar in RNA is ribose; in DNA, it is deoxyribose.

• RNA is generally single-stranded and can fold into complex structures.

• Three main types of RNA: mRNA (messenger), tRNA (transfer), rRNA (ribosomal).

Discovery of DNA Structure

• Rosalind Franklin's X-ray diffraction images revealed DNA's helical structure and uniform width.

• Watson and Crick proposed the double helix model, with a sugar-phosphate backbone and specific base pairing.

Watson-Crick Model of DNA

• DNA is a double-stranded helix with antiparallel strands.

• Strands are held together by hydrogen bonds between complementary bases:

  • Adenine (A) pairs with Thymine (T)

  • Guanine (G) pairs with Cytosine (C)

• Published in 1953; Nobel Prize awarded in 1962.

DNA Replication

• DNA must be copied before cell division (mitosis or meiosis).

• Each strand serves as a template for a new complementary strand.

• Nucleotides are added to the 3' end by complementary base pairing.

• New covalent bonds are formed by the enzyme DNA polymerase.

From Chromosomes to Genes to Phenotype

Organization of Genetic Material

• Chromatin: DNA-protein complex that packages DNA in the nucleus.

• Gene: A sequence of DNA that encodes a functional product (usually a protein).

Central Dogma: DNA to Protein

• Genetic information flows from DNA to RNA to protein.

• Transcription: DNA is used as a template to synthesize RNA.

• Translation: RNA is used as a template to synthesize protein.

Transcription: DNA to RNA

Overview of Transcription

• Transcription is the process by which a gene's DNA sequence is copied into RNA.

• Occurs in the nucleus of eukaryotic cells.

• Relies on the enzyme RNA polymerase.

Steps of Transcription

• Initiation: RNA polymerase binds to the promoter region of the gene.

• Elongation: RNA polymerase synthesizes the RNA strand by adding complementary RNA nucleotides.

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

Transcription: Molecular Details

• The DNA double helix unwinds in the region of the gene.

• RNA is synthesized in the 5' to 3' direction, using one DNA strand as a template.

• The completed RNA transcript leaves the nucleus for translation in the cytoplasm.

Translation: RNA to Protein

Types of RNA Molecules

• mRNA (messenger RNA): Carries the genetic code from DNA to the ribosome.

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

• rRNA (ribosomal RNA): Forms the core of the ribosome's structure and catalyzes protein synthesis.

The Genetic Code

• The genetic code is a set of rules by which information encoded in mRNA is translated into proteins.

• Each codon (three-nucleotide sequence) specifies a particular amino acid.

• The start codon is AUG (codes for methionine).

• Stop codons signal the end of translation.

Ribosomes and Translation

• Ribosomes are composed of rRNA and proteins; they facilitate the linking of amino acids into polypeptides.

• Consist of a small and a large subunit.

Players and Steps in Translation

• Initiation: mRNA binds to the small ribosomal subunit; initiator tRNA binds to the start codon; large subunit joins.

• Elongation: tRNAs bring amino acids to the ribosome; peptide bonds form between amino acids; ribosome moves along mRNA.

• Termination: Stop codon is reached; polypeptide and mRNA are released; ribosomal subunits separate.

Key Details of Translation

• Translation always begins with the AUG codon (methionine).

• The ribosome moves along the mRNA in units of three nucleotides (codons).

• The genetic code is redundant (multiple codons for most amino acids) but not ambiguous (each codon specifies only one amino acid).

Mutations: Changes in Genetic Information

Definition and Types of Mutations

• Mutation: A heritable change in the DNA sequence.

• Mutations can occur in somatic cells or in cells undergoing meiosis (germ cells).

Base Substitution Mutations

• One nucleotide is replaced by another, which may result in a different amino acid in the protein.

Frameshift Mutations

• Caused by insertions or deletions of nucleotides that are not multiples of three.

• Shift the reading frame, often resulting in nonfunctional proteins.

Causes of Mutations (Mutagens)

• Physical mutagens: e.g., radiation (UV, X-rays)

• Chemical mutagens: e.g., chemicals that alter DNA bases

• Viruses: can insert their genetic material into host DNA

Summary Table: DNA, RNA, and Protein Synthesis

Key Equations and Concepts

• Base pairing in DNA:

• Central Dogma: