A 4-nucleotidelong segment of
DNA. (arbitrary
choice
of bases)
Only variation
comes from
nitrogenous
base
side chains
The general structure of a nucleic acid in
terms of nucleotide subunits.
DNA varies from RNA in using
deoxyribose sugar not ribose,
and having thymine instead of uracil.
Otherwise it’s the same composition.
DNA GEOMETRY
Rosalind Franklin in early 1950s used
X-ray crystallography to study DNA structure.
Watson and Crick used her info
to develop a DNA double helix shape.
In 3-D, two DNA strands twist into
a double helix (spiral staircase).
DNA double helix like a spiral staircase
antiparallel strands:
each strand oriented 5‘3'
in opposite directions.
Strands are
complementary,
not identical.
Central bases pair
up to form “steps”
Base pairs composed
of one purine paired
with one pyrimidine.
Width of the helix stays roughly
equivalent with this arrangement
Phosphate–Sugar ester bonds are
covalent and very strong.
Base pairs are held together with
weak Hydrogen bonds.
This becomes important when we see how
DNA works.
Thymine to Adenine
Cytosine to Guanine
(a) Thymine to adenine base pairing involves
2 H bonds.
(b) Cytosine to guanine base pairing involves
3 H bonds.
DNA double helix
emphasizing the
hydrogen bonding
between bases on the
two chains.
Keep in mind the
weak H bonds
in the interior,
they are key!
For Heredity to Work:
Biological info must be accurately copied and
transmitted from each cell to all of its progeny.
Most cells divide frequently, so DNA has to be
easily copied (replicated) and separated
before a cell splits in two.
So how is DNA
replicated?
DNA replication:
2 strands of DNA double helix unwind (helicase
enzymes)
Separated strands = templates for new DNA
strands.
Free floating nucleotides pair with
complementary bases on separated strands.
Result is replication of DNA molecule.
The enzyme DNA polymerase catalyzes base
pairing and acts only in 5’-to-3’ direction.
One strand (top) grows continuously in the
direction of the unwinding, the other strand grows
in segments in the opposite direction.
The segments (Okazaki fragments) in this chain are
connected by a different enzyme, DNA ligase.
DNA replication usually occurs at
multiple sites within a molecule,
and replication is bidirectional from these
sites.
Summary
of DNA
Replication
Two
identical
daughter
strands
form.
Then
cell
divides.
Replication
animation
Review: can you…
•Compare and contrast the structure and
function of different types of nucleic acids
•Draw the basic structure of nucleosides
and nucleotides
•Explain the primary structure of nucleic
acids and compare it to protein structure
•Describe the structural properties of the
DNA double helix
•Draw the steps involved in DNA replication
•Compare & contrast RNA to DNA
Chromosomes are DNA
strands wrapped around
proteins.
DNA is the template for
protein production.
RNA is similar to DNA
but with these differences:
•The sugar in RNA is ribose not
deoxyribose
•The nitrogen base uracil replaces
thymine.
•RNA molecules are single, not
double stranded.
•RNA molecules are much smaller
than DNA molecules.
•RNA enters & leaves the nucleus
Protein Synthesis
• Proteins are synthesized in the cell
through a series of steps involving
– Transcription: DNA --> RNA
– Translation: RNA --> Protein
Genes are a segment on a
DNA strand that code for a
hnRNA/mRNA molecule.
The process of
copying a
complementary strand
of mRNA off of a DNA
gene is called
transcription
The transcription of DNA to form RNA involves
an unwinding of a portion of the DNA double
helix. Only one strand of the DNA is copied
during transcription.
RNA transcription
Initially heterogenous nuclear RNA (hnRNA) is formed
Non-coding pieces (introns) are spliced out using small
nuclear RNA and protein particles called “snurps”
(snRNP).