The Structure and Function of DNA
Early geneticists didn’t know what the genetic material actually was
Some thought it was protein
This alphabet has 20 “letters”
Others thought it was DNA
This alphabet has 4 “letters”
Previous information
In the 1950’s, the race was on
Scientists knew a fair amount about DNA but nobody had figured out how everything
was arranged
But Watson and Crick didn’t do it all alone
Rosalind Franklin used x-ray crystallography on the DNA
DNA structure explains its function
Contains information
Information is contained in order of the 4 bases
Easy to copy
Each strand serves as a template for making a new strand
Accounts for diversity
Different alleles have difference sequences
DNA replication
DNA must be replicated before the cell divides
This ensures that each new daughter cell receives an identical complement of
chromosomes
DNA replication =
Chromosome duplication
The double helix also provided a model for how DNA could be replicated
Base-pairing is the key!
Each parental strand will serve as the template for the synthesis of each new strand
The main enzyme involved is DNA polymerase
Gene expression
In order for a cell to carry on its normal business (including division), it must have
the proper proteins in the proper amounts
Information to make proteins is stored in DNA
Genes carry the genetic information to produce a polypeptide
Transcription and translation
Genes are transcribed to give mRNA copies in the nucleus
mRNA is translated to give the polypeptide on ribosomes in the cytoplasm
Transcription
Complementary base pairing occurs between the template DNA strand and the new
RNA molecule
RNA polymerase is the enzyme
The RNA polymerase recognizes signals present in the DNA sequence (promoter)
This is the site of unwinding of the DNA helix and the initiation of transcription
Since the promoter sequences are near each gene, this allows transcription to
produce gene-length RNA molecules
In all eukaryotes, the initial primary transcript is not the finished mRNA
It must undergo RNA processing
One major step in processing is RNA splicing - removal of non-coding introns putting the coding exons together
Translation
Translation takes place on ribosomes and involves three different types of RNA
The mRNA carries the code for a particular amino acid sequence
rRNAs join with ribosomal proteins to give ribosomes
tRNAs are unique adaptor molecules acting as bridges by binding amino acids
and mRNAs
During translation, every 3 mRNA nucleotides specifies 1 amino acid
Each 3 nucleotides is a codon
Each tRNA with a particular anticodon will have the same amino acid attached
Ribosomes are composed of ribosomal proteins and rRNA molecules
Each ribosome is composed of a large and a small subunit
The two subunits come together to initiate translation on a mRNA
The ribosome has two important sites - P and A
The mRNA moves through the different sites
When the A site is unoccupied, a tRNA with the appropriate anticodon brings
the next amino acid to add to the chain
Translation involves three steps - initiation, elongation, and termination of translation
Initiation of translation first involves the small ribosomal subunit, the mRNA, and an
initiator tRNA with methionine attached
Later, the large ribosomal subunit will attach giving an intact ribosome
Elongation of translation brings additional amino acids to the ribosome to add to the
growing polypeptide chain
The sequence of the amino acids added is dependent on the sequence of codons
on the mRNA
Termination
Occurs when a stop codon is present in the A site
The polypeptide is released
The ribosome splits into its subunits
Multiple ribosomes can bind to and translate a single mRNA at the same time
This can increase the rate of translation
Mutations
Mistakes in replication are rare
DNA polymerase has proofreading ability
Repair enzymes spot and repair mismatched bases
Nonetheless, sometimes mistakes do wind up in the DNA
These occur only rarely but can have profound effects
Any change in the original DNA sequence is a mutation
Can be due to errors in DNA replication
Effects of mutagens/carcinogens
Mutations can have profound effects
The effects are rarely positive (advantageous)
Some have a negative effect (disadvantageous)
Most are neutral
Mutations can range from single base pair changes to large chromosomal
disturbances
Types of Mutations
Mutations within a gene can be divided into two general categories:
1. nucleotide substitution
2. nucleotide deletions or insertions
Because the reading frame for translation is set when the first codon is
“read”, these can have very different consequences
Missense mutations involve a single nucleotide and change a single amino acid
Nonsense mutations change an amino acid codon into a stop codon
Mutations involving the deletion or insertion of one or more nucleotides in a gene,
called frameshift mutations, often have disastrous effects
Adding or subtracting nucleotides may alter the triplet grouping of the genetic
message
Frameshift mutations usually produce non-functioning polypeptides
So some mutations occur spontaneously due to errors in DNA replication or
recombination; others can be induced by mutagens
Life depends on the incredible ability of genetic systems to store, use, and pass on
information