DNA
The Genetic Material
QUESTION
Is the structure and function of DNA
the same in almost all organisms?
Fredrick Griffith and
Transformation
Hershey and Chase Show That
Genes Are Made of DNA
Structure of DNA
Nucleotides Are the Building Blocks.
• There are three components
to a nucleotide
– Sugar (5 carbon)
– Nitrogenous base
(adenine, quanine, thymine,
cytosine)
– Phosphate group
Semi-conservative Model of DNA
Replication
• Prokayotes
– One circular
chromosome
– 5 million base
pairs
– Replicates in less
than one hour
• Eukaryotes
– Humans have 46
chromosomes
– 6 billion base pairs
– Replicated in a
few hours
– DNA is separated
from most of the
replication
machinery by the
nuclear envelop.
Both carry out replication very quickly and with
tremendous accuracy – 1 error /billion nucleotides.
The Bases Are Either Purines or
Pyrimidines.
A always pairs with T and C always pairs with G. This
is because of the number of bonds formed between the
bases. Two hydrogen bonds form between A and T
and three between C and G.
Write the compliment for GGCTATTGGCA.
DNA Replication
• Steps in replication
– DNA helix is unwound by helicase.
– The point where the DNA strands separate is
called the replication fork (Y)
– At the replication fork, the enzyme DNA
polymerase adds bases according to the basepairing rules.
– Two new DNA helixes are formed.
Replication Begins at the Origin of Replication
• Prokaryotes have only one (1) origin of
replication.
• Eukaryotes have 100’s or 1000’s of origins
of replications.
• Replication proceeds in both directions,
forming a replication bubble.
• At the ends of the replication bubble is the
replication fork – a” Y shaped” region.
• DNA polymerase has the
ability to proofread and
make corrections if an
error has been made.
After proofreading the
rate of errors is one per
billion nucleotides.
• There are multiple DNA
replication forks in a
strand of linear DNA
creating replicating
“bubbles.” (Without
multiple replication forks,
it would take 16 days to
copy one DNA molecule.)
What Is the Energy Source for
Replication?
• The energy source is a
nucleoside triphosphate.
• This molecule is similar to
ATP except the sugar is
deoxyribose and the sugar
in ATP is ribose.
• The energy comes from
the hydrolysis of the
phosphate tail.
DNA Is Anti-parallel
Structure of A Gene
• Genes are DNA –encoded information that
specifies particular proteins; each gene is
made of a specific sequence of nucleotides.
• Genes are composed of coding and
noncoding sequences.
– Coding sequences are exons (code for amino
acids)
– Noncoding sequences are introns (intervening)
Telomeres
• Telomeres are special
nucleotide sequences
located at the end of
chromosomes.
• They do not contain
genes but mutliple
repetitions of short
nucleotide sequences
(100-1000)
• Telomerase
When the introns are removed, the resulting
fragments are stitched together to make a
protein.