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 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