Download Central dogma

•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
••
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
•
•
RNA Modification: Alternative Splicing
•
•
•
•
•
•
•
–
–
–
–
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
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