Title: Molecular Genetics: DNA Structure and the Genetic Code
1Molecular Genetics DNA Structure and the Genetic
Code
Textbook Readings Ch. 16-background only Ch.
17-most important Ch. 19 Sec. 19.1 also
important Ch. 18 and 20-not yet Ch. 15 (reminder)
Secs. 15.3-15.5
2Molecular Genetics DNA
- Four roles for DNA
- Information storage
- Replication (reproduction)
- Day to day instructions to the cell
- Mutation
3Molecular Genetics DNA
- Consists of two strands of nucleotides.
- Nucleotides bonded together in each chain with
phosphodiester bonds. - Hydrogen bonds hold the two chains to one another
- Adenine hydrogen bonds with Thymine.
- Cytosine hydrogen bonds with Guanine.
- Antiparallel
- Structure is compatible with four roles of
DNA-but there is one major problem-length of a
DNA molecule
4DNA from a bacterial nucleoid illustrates the
major problem with DNA
5Eukaryotes have a more difficult job to do
6Molecular Genetics Folded fiber model of
chromatin structure
Eukaryotic DNA must be highly folded to fit in
the nucleus.
7Molecular Genetics-information storage role for
DNA
- The sequence of bases carries a coded message
- Each base is like a letter in a book
- The letters are organized into words called
codons-3 bases long - The codons are arranged in sentences or messages
called genes - Each gene carries the information for a final
product that is either a protein or an RNA
molecule
8Molecular Genetics-information storage role for
DNA
- DNA does not carry out its own instructions.
- It relies on RNA or protein to accomplish tasks
for the cell. - DNA makes RNA (transcription)
- RNA makes protein (translation)
9Molecular Genetics overview of transcription and
translation
10Molecular Genetics-summary of transcription
- Catalyzed by enzymes (not shown)
- Produces different types of RNA (rRNA, mRNA, tRNA
are the 3 main types) - Some of the RNAs go on to produce protein
- Others have different jobs
11Molecular Genetics-codons
- mRNA contains a chain of successive codons
- It has start and stop signals in the chain
- Also has extra nucleotides at the end help to
regulate its activity - Single- not double-stranded
A cartoon of an mRNA showing codons but not
regulatory nucleotides
12Molecular Genetics-codon dictionary
Each codon corresponds to an amino acid But
there are more codons (64) than amino acids
(20)
13Molecular Genetics-codon dictionary properties
- Degenerate (more than 1 codon per AA)
- Unambiguous (each codon always corresponds to the
same amino acids) - Has start (MET) and stop (nonsense) codons
- Universal-almost
- The reading frame determines the sense of the
message - Most important
14Molecular Genetics-codon properties
- Because each codon has 3 bases
- There are 3 ways to read each sequence
- 3 reading frames
- Different messages
One sequence-3 reading frames with different
messages, 3 with premature UAG stop
15Molecular Genetics-codon definitions
- Open Reading Frame (ORF) start codon to stop
codon - Initiator codon AUG (methionine, MET)
- Termination codon UAA, UGA, UAG
16Molecular Genetics-information storage role for
DNA-summary
- The sequence of bases carries a coded message
- But the molecule that carries the messages has to
be very long-much longer than the cell.
Solution??? - The bases are organized into words called
codons-3 bases long and the messages contain
start and stop codons. - This leads to additional problems.