Molecular Biology Primer for CS and engineering students

Alan QiJan. 10, 2008

Central Dogma

DNA

RNA

Protein

Genes control the making of cell parts

• The gene is a fundamental unit of inheritance– DNA molecule contains tens of thousands of genes– Each gene governs the making of one functional element, one

“part” of the cell machine– Every time a “part” must be made, a piece of the genome iscopied, transported, and used as a blueprint

• RNA is a temporary copy– The medium for transporting genetic information from the DNAinformation repository to the protein-making machinery is anRNA molecule– The more parts are needed, the more copies are made– Each mRNA only lasts a limited time before degradation

RNA: messager

From pre-mRNA to mRNA: Splicing• In some species (e.g. eukaryotes), not every part

of a gene is coding– Functional exons interrupted by non-translated introns– During pre-mRNA maturation, introns are spliced out– In humans, primary transcript can be 106 bp long– Alternative splicing can yield different exon subsets for the same gene, and hence different protein products

RNA can be functional

• Single Strand allows complex structure– Self-complementary regions form helical stems– Three-dimensional structure allows functionality of RNA

• Active research area: non-coding RNAs…– Once upon a time, before DNA and protein, RNA did all

Central Dogma

DNA

RNA

Protein

Condon• The genetic code defines a mapping between tri-nucletide sequences

called codons and amino acids.• Condon is defined by the initial nucleotide from which translation starts.

– For example, the string GGGAAACCC, if read from the first position, contains the codons GGG, AAA and CCC; and if read from the second position, it contains the codons GGA and AAC; if read starting from the third position, GAA and ACC.

– Every sequence can thus be read in three reading frames. With double-stranded DNA there are six possible reading frames. three in the forward orientation on one strand and three reverse (on the opposite strand).

– If the DNA is eukaryotic, the reading frame may contain introns.• Start/stop codons

Translation starts with a chain start codon. The most common start codon is AUG, which codes for methionine, so most amino acid chains start with methionine. Nearby sequences and initiation factors are also required to start translation. Stop condons: UAG-amber, UGA-umber, and UAA-ochre.

Degeneracy of the genetic code

• The genetic code has redundancy but no ambiguity. – Both Codons GAA and GAG -> glutamic acid (redundancy),

neither of them specifies any other amino acid (no ambiguity).

• The codons encoding one amino acid may differ in any of their three positions. – the amino acid glutamic acid is specified by GAA and GAG

codons (difference in the third position), – the amino acid leucine is specified by UUA, UUG, CUU,

CUC, CUA, CUG codons (difference in the first or third position)

– the amino acid serine is specified by UCA, UCG, UCC, UCU, AGU, AGC (difference in the first, second or third position).

Proteins carry out the cell’s chemistry

• More complex polymer– Nucleic Acids have 4 building blocks– Proteins have 20. Greater versatility– Each amino acid has specific properties

• Sequence -> Structure -> Function– The amino acid sequence determines thethree-dimensional fold of protein– The protein’s function largely depends onthe features of the 3D structure

• Proteins play diverse roles– Catalysis, binding, cell structure, signaling,transport, metabolism

Protein structures• Primary structure - the amino acid sequence of the peptide

chains.• Secondary structure - highly regular sub-structures (alpha

helix and strands of beta sheet) which are locally defined, meaning that there can be many different secondary motifs present in one single protein molecule.

• Tertiary structure - Three-dimensional structure of a single protein molecule; a spatial arrangement of the secondary structures.

• Quaternary structure - complex of several protein molecules or polypeptide chains, usually called protein subunits in this context, which function as part of the larger assembly or protein complex.

Summary