The Structure of the Genetic Material: DNA, RNA, and the Flow of Genetic Information

Chapter 10: The Structure of the Genetic Material

DNA and RNA are Polymers of Nucleotides

Molecular biology studies the molecular basis of heredity. DNA and RNA are nucleic acids composed of long chains (polymers) of nucleotides.

• Nucleotide: The monomer unit of nucleic acids, consisting of:

  • a nitrogenous base,

  • a five-carbon sugar, and

  • a phosphate group.

• DNA is a polynucleotide (a nucleotide polymer).

• Nucleotides are joined by a sugar-phosphate backbone.

• Each DNA nucleotide has a different nitrogen-containing base:

  • Adenine (A)

  • Cytosine (C)

  • Thymine (T)

  • Guanine (G)

• RNA (ribonucleic acid) differs from DNA:

  • Uses ribose sugar (instead of deoxyribose in DNA)

  • Has uracil (U) instead of thymine (T)

Example: The sequence of nucleotides in DNA encodes genetic information.

DNA is a Double-Stranded Helix

DNA's structure is a double helix, discovered by Watson and Crick in 1953, based on X-ray crystallography data from Rosalind Franklin.

• DNA consists of two polynucleotide strands wound around each other.

• The strands are held together by hydrogen bonds between complementary bases:

  • A pairs with T

  • C pairs with G

• The two strands are antiparallel (run in opposite directions).

Additional info: The double helix model explained how DNA could replicate and store genetic information.

DNA Replication

DNA Replication Depends on Specific Base Pairing

DNA replication is semiconservative: each new DNA molecule consists of one old strand and one new strand.

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

• Base pairing ensures accurate copying: A with T, C with G.

DNA Replication Proceeds in Two Directions at Many Sites Simultaneously

• Replication begins at origins of replication (specific DNA sequences).

• Proteins involved:

  • Initiator proteins bind to origins and separate DNA strands.

  • DNA polymerases add nucleotides to the 3' end of the new strand.

  • DNA ligase joins DNA fragments (Okazaki fragments) on the lagging strand.

• Replication is continuous on the leading strand, discontinuous on the lagging strand.

Equation:

$\text{DNA}_{\text{parent}} \rightarrow \text{DNA}_{\text{daughter}}$

Proteins and the Flow of Genetic Information

Proteins

Proteins are the link between genotype and phenotype. Genes (DNA) provide instructions for making proteins.

• Transcription: Synthesis of RNA from DNA.

• Translation: Synthesis of proteins from RNA.

Genetic Information Written in Codons is Translated into Amino Acid Sequences

The sequence of nucleotides in DNA is used to construct proteins via a code of three-nucleotide sequences called codons.

• Transcription: DNA code is rewritten into RNA.

• Translation: RNA code is converted into an amino acid sequence.

• Each codon specifies one amino acid.

• There are 64 possible codons (43 combinations).

• The code is redundant (more than one codon per amino acid) but not ambiguous (each codon codes for only one amino acid).

Example: The codon AUG codes for methionine and signals the start of translation.

The Genetic Code Dictates How Codons are Translated into Amino Acids

Codon Type	Function
Start codon (AUG)	Signals start of translation; codes for methionine
Sense codons	Code for amino acids (61 codons)
Stop codons (UAA, UAG, UGA)	Signal end of translation

Additional info: The genetic code is nearly universal among all organisms.

RNA Processing and Translation

Eukaryotic RNA is Processed Before Leaving the Nucleus as mRNA

• Messenger RNA (mRNA): Encodes amino acid sequences and conveys genetic messages from DNA to ribosomes.

• In eukaryotes, mRNA is processed before export:

  • Introns (noncoding sequences) are removed.

  • Exons (coding sequences) are joined.

  • A cap and tail are added for stability and export.

Transfer RNA Molecules Serve as Interpreters During Translation

• Transfer RNA (tRNA): Brings amino acids to the ribosome and matches them to the mRNA codons using its anticodon.

• Each tRNA is specific for one amino acid.

Ribosomes Build Polypeptides

• Ribosomes are the sites of protein synthesis.

• Composed of rRNA and proteins.

• Have binding sites for mRNA and tRNA.

• Prokaryotic and eukaryotic ribosomes are similar in function but differ in size and structure.

An Initiation Codon Marks the Start of an mRNA Message

• Translation is divided into three phases:

  1. Initiation: mRNA, the first tRNA, and ribosomal subunits assemble.

  2. Elongation: Amino acids are added one by one to the growing polypeptide chain.

  3. Termination: A stop codon is reached, ending translation.

• The start codon (AUG) signals the beginning of translation.

Elongation Adds Amino Acids to the Polypeptide Chain Until a Stop Codon Terminates Translation

• Each cycle of elongation adds one amino acid to the chain.

• When a stop codon is reached, the completed polypeptide is released.

Review: The Flow of Genetic Information in the Cell is DNA → RNA → Protein

• Transcription: DNA → RNA

• RNA processing (in eukaryotes): pre-mRNA → mature mRNA

• Translation: RNA → Protein

Equation:

$\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$

Mutations

Mutations Can Affect Genes

A mutation is any change in the nucleotide sequence of DNA. Mutations can involve large chromosomal regions or single nucleotide pairs.

• Base-pair substitution: Replacement of one base by another.

• Insertions or deletions: Addition or loss of one or more nucleotides.

• Frameshift mutation: Insertions or deletions that alter the reading frame, potentially changing every amino acid downstream.

Example: Sickle-cell disease results from a single base substitution in the gene for hemoglobin.

Causes of Mutations

• Spontaneous errors during DNA replication or recombination.

• Mutagens: Physical or chemical agents (e.g., X-rays, UV light, chemicals).

Additional info: Some mutations are silent (no effect), while others can be harmful or beneficial, contributing to evolution.

Summary Table: Types of Mutations

Type	Description	Effect
Base-pair substitution	One nucleotide replaced by another	May change one amino acid
Insertion	One or more nucleotides added	May cause frameshift
Deletion	One or more nucleotides lost	May cause frameshift
Frameshift	Insertion/deletion alters reading frame	Changes all downstream amino acids