Molecular Genetics: Structure, Function, and Expression of Genetic Material

Molecular Genetics

Nucleic Acids

Nucleic acids are the molecules that store and transmit genetic information in living organisms. The two main types are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Genes, which are segments of DNA, encode the primary amino acid sequence of proteins.

• Genes are made from DNA and encode proteins.

• DNA carries information needed to make proteins, but this information is first transcribed into RNA before translation into protein.

• Transcription: DNA → RNA

• Translation: RNA → Protein

Nucleotide Structure and Nucleic Acid Polymers

Nucleic acids are polymers formed from nucleotide monomers. Each nucleotide consists of:

• A five-carbon sugar (deoxyribose in DNA, ribose in RNA)

• A phosphate group

• A nitrogenous base (adenine, guanine, cytosine, thymine in DNA; uracil replaces thymine in RNA)

Nucleotides are linked by dehydration reactions, forming a sugar-phosphate backbone.

DNA and RNA Structure

DNA is typically double-stranded, forming a double helix, while RNA is single-stranded. The backbone of nucleic acids is a repeating sequence of sugar and phosphate groups.

• Nitrogenous bases pair according to specific rules: Adenine (A) pairs with Thymine (T) (or Uracil (U) in RNA), and Cytosine (C) pairs with Guanine (G).

• Base pairs are held together by hydrogen bonds: two between A and T, three between C and G.

• Pyrimidines: Cytosine and Thymine (single ring)

• Purines: Adenine and Guanine (double ring)

DNA Discovery and Structure

DNA was discovered as a double helix by Watson and Crick in 1953. The structure ensures complementary base pairing and genetic stability.

• DNA appears as a ladder: side bars are the sugar-phosphate backbone, middle rungs are paired nitrogenous bases.

• Each strand runs in opposite directions (antiparallel).

• The sequence of nucleotides is random, allowing for genetic variation.

DNA Replication

DNA replication is the process by which DNA is copied before cell division. It is semi-conservative, meaning each new DNA molecule contains one original and one new strand.

• Replication requires separation of the two DNA strands.

• Free nucleotides are used to synthesize new strands, guided by enzymes.

• Replication begins at origins of replication and proceeds in both directions.

• Each DNA strand has a 3' and 5' end; synthesis occurs in the 5' to 3' direction.

• One strand is synthesized continuously (leading strand), the other discontinuously (lagging strand) in fragments joined by DNA ligase.

• DNA polymerase and DNA ligase also repair errors and damage.

Genotype vs Phenotype

Genotype refers to the genetic makeup encoded in DNA, while phenotype refers to observable traits. Proteins link genotype to phenotype.

• DNA directs the formation of RNA, which then directs protein synthesis.

• This flow of information is called the Central Dogma of Biology: DNA → RNA → Protein.

Genetic Information to Amino Acids

DNA contains specific nucleotide sequences that are transcribed into RNA and then translated into proteins.

• RNA is produced from DNA by complementary base pairing.

• Translation uses RNA to assemble amino acids into proteins.

• There are 20 amino acids; each set of three nucleotides (codon) encodes one amino acid.

The Genetic Code

The genetic code matches RNA codons to amino acids in proteins.

• Of 64 possible codons, 61 encode amino acids and 3 are stop codons (UAA, UAG, UGA).

• AUG encodes methionine and acts as the start codon.

• Some amino acids are encoded by more than one codon.

Codon	Amino Acid
AUG	Methionine (Start)
UAA, UAG, UGA	Stop
ACU, ACC	Threonine
Example Sequence	Gly-Pro-Ser-Pro-Phe

Transcription

Transcription is the process of synthesizing RNA from DNA in the nucleus.

• Only one DNA strand is used as a template.

• RNA polymerase adds nucleotides to the growing RNA chain.

• Steps: Initiation (RNA polymerase binds promoter), Elongation (RNA strand grows), Termination (RNA polymerase releases RNA).

• Three forms of RNA: mRNA, tRNA, rRNA.

Processing of RNA

RNA undergoes modifications before translation, especially in eukaryotes.

• Messenger RNA (mRNA) directs protein synthesis.

• In eukaryotes, transcription occurs in the nucleus, translation in the cytoplasm.

• Modifications include addition of a 5' cap (G nucleotide) and a poly-A tail (A nucleotides).

• Non-coding regions (introns) are removed; coding regions (exons) are spliced together.

Transfer RNA (tRNA)

tRNA acts as a translator, converting mRNA codons into corresponding amino acids during protein synthesis.

• tRNA picks up specific amino acids and recognizes codons via its anticodon loop.

• tRNA is about 80 nucleotides long and folds into a characteristic structure.

Ribosomes Build Proteins

Ribosomes are organelles that synthesize proteins from mRNA in the cytoplasm.

• Composed of two subunits: protein and rRNA.

• Prokaryotic and eukaryotic ribosomes have similar functions but differ in size and composition.

Translation: Initiation, Elongation, and Termination

Translation is the process of assembling proteins from mRNA, occurring in three phases.

• Initiation: mRNA binds to ribosome; initiator tRNA binds start codon (AUG).

• Elongation: Amino acids are added one by one via codon recognition, peptide bond formation, and translocation.

• Termination: Stop codon is reached; completed protein is released.

Mutations

Mutations are changes in the DNA sequence that can result in alternative alleles and affect protein function.

• Base substitutions: Replacement of one nucleotide with another; may be silent, missense, or nonsense (truncation).

• Base insertions/deletions: Addition or removal of nucleotides; can cause frameshift mutations, often with severe effects.

• Mutations arise spontaneously or due to mutagens (e.g., X-rays, UV light).

Type of Mutation	Effect
Base Substitution	May be silent, missense, or nonsense
Insertion/Deletion	Frameshift, often severe
Mutagenesis	Spontaneous or induced by mutagens

Additional info: The notes cover key concepts from chapters on nucleic acids, DNA structure and replication, gene expression, and mutations, aligning with General Biology curriculum.