Chapter 6: Protein synthesis and Nucleic acids

DNA structure

Double-stranded antiparallel polynucleotide with complementary base pairing (A–T, C–G) forming a double helix

DNA nucleotide

Monomer with deoxyribose sugar, phosphate group, and nitrogenous base (A, T, C, G)

Phosphodiester bond

Covalent bond linking phosphate of one nucleotide to sugar of the next

Antiparallel strands

One DNA strand runs 5’→3’ and the other 3’→5’

Complementary base pairing

A pairs with T (2 H-bonds) and C pairs with G (3 H-bonds)

Purines

Double-ring bases (adenine, guanine)

Pyrimidines

Single-ring bases (cytosine, thymine)

Double helix

Two polynucleotide strands twisted into a helical structure

RNA structure

Single-stranded polynucleotide with ribose sugar, phosphate, and bases A, U, C, G

mRNA

RNA that carries genetic code from DNA to ribosomes

tRNA

Cloverleaf-shaped RNA with anticodon and amino acid attachment site

rRNA

RNA that forms ribosomes and catalyses peptide bond formation

Uracil

Pyrimidine base in RNA that pairs with adenine

DNA replication

Semi-conservative process producing two identical DNA molecules

Semi-conservative replication

Each daughter DNA contains one original and one new strand

DNA helicase

Enzyme that breaks hydrogen bonds between bases

DNA polymerase

Enzyme that adds complementary DNA nucleotides in the 5’→3’ direction

Leading strand

Strand synthesised continuously

Lagging strand

Strand synthesised discontinuously as Okazaki fragments

Okazaki fragments

Short DNA fragments formed on the lagging strand

DNA ligase

Enzyme that joins Okazaki fragments with phosphodiester bonds

Replication fork

Y-shaped region where DNA is unwound

Transcription

Process of forming mRNA from a DNA template

Promoter region

DNA sequence where RNA polymerase binds to begin transcription

RNA polymerase

Enzyme that unwinds DNA locally and synthesises mRNA

Template strand

DNA strand used to build mRNA

Coding strand

DNA strand with the same sequence as mRNA except T replaces U

Terminator sequence

DNA sequence that stops transcription

Pre-mRNA

Initial mRNA transcript containing introns and exons (eukaryotes)

Splicing

Removal of introns and joining of exons

Translation

Process where ribosomes read mRNA to build a polypeptide

Codon

Three-base sequence on mRNA coding for one amino acid

Anticodon

Complementary triplet on tRNA

Ribosome

Structure of rRNA and proteins where translation occurs

Amino acid activation

Attachment of amino acid to tRNA using ATP

Initiation (translation)

Ribosome assembles on mRNA and first tRNA binds to start codon

Elongation (translation)

Ribosome moves along mRNA forming peptide bonds

Peptide bond

Covalent bond between amino acids

Termination (translation)

Stop codon reached and polypeptide released

Polyribosome

Several ribosomes translating the same mRNA simultaneously

Genetic code

System where mRNA codons determine amino acids

Degenerate code

Many amino acids have multiple codons

Non-overlapping code

Each base is read only once in sequence

Universal code

Codons code for the same amino acids in almost all organisms

DNA vs RNA differences

DNA has deoxyribose, T, double strands; RNA has ribose, U, single strand

Meselson–Stahl experiment

Demonstrated semi-conservative replication using 15N/14N isotope labelling