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 proteinbuilding 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