What is central dogma?From DNA to Protein

• All proteins consist of polypeptide chains– A linear

sequence of amino acids

• Each chain corresponds to the nucleotide base sequence of a gene

What is the first step?1. Transcription: Enzymes uses base

sequence of a gene as template to make strand of RNA

• Two DNA strands unwind in a specific region

• RNA polymerase assembles strand of RNA– Covalently bonds RNA nucleotides

(adenine, guanine, cytosine, uracil) according to nucleotide sequence of exposed gene

What is the second step?• 2. Translation

– Information in the RNA strand is decoded (translated) into a sequence of amino acids

Prokaryotes and eukaryotes

• In prokaryotic cells (no nucleus)– Transcription and

translation occur in cytoplasm

• In eukaryotic cells– Genes are

transcribed in the nucleus

– Resulting mRNA is translated in the cytoplasm

Three types of RNA

• Messenger RNA (mRNA)– Carries protein-building codes from

DNA to ribosomes• Ribosomal RNA (rRNA)

– Forms ribosomes (where polypeptide chains are assembled)

• Transfer RNA (tRNA)– Delivers amino acids to ribosomes

RNA and DNA compared• DNA

– exists as double-stranded molecules

– hereditary information– double helix– contains deoxoyribose sugar

• RNA– Disposable copies of

hereditary information and some are catalytic

– exists as a single stand.– contains ribose instead of

deoxyribose– contains uracil in place of

thymine

RNA Modification: Alternative Splicing

• Before mRNA leaves the nucleus:– Introns are

removed – Some exons are

removed along with introns; remaining exons are spliced together in different combinations

– Poly-A tail is added to 3’ end of new mRNA

What is the genetic code?

• Messenger RNA (mRNA) carries DNA’s protein-building information to ribosomes for translation

• mRNA’s genetic message is written in codons– Sets of three

nucleotides along mRNA strand

Codons

• Codons specify different amino acids– A few codon

signals stop translation

• Sixty-four codons constitute a highly conserved genetic code