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CST Review PowerPoint
The Chemistry of DNA and
Molecular Genetics
Introducing…DNA
-DNA is a nucleic acid that
contains the genetic instructions
used in the development and
functioning of all known living
organisms and some viruses.
-The DNA segments that carry this
genetic information are called
genes, but other DNA sequences
have structural purposes, or are
involved in regulating the use of
this genetic information.
Important!
The DNA molecule has two important
properties:
• It can make copies of itself. If you pull
the two strands apart, each can be
used to make the other one (and a new
DNA molecule).
• It can carry information. The order of
the bases along a strand is a code - a
code for making proteins.
Structure of DNA
-Chemically, DNA consists of
two long polymers of
simple units called
nucleotides, with
backbones made of sugars
and phosphate groups
joined by bonds.
-These two strands run in
opposite directions to each
other and are therefore
anti-parallel. Attached to
each sugar is one of four
types of molecules called
bases. It is the sequence of
these four bases along the
backbone that encodes
information.
Strands go 5’ to 3’ one way and 3’
to 5’ the other way…
DNA vs. RNA
DNA
RNA
Number of
Strands
2
1
Sugar present
Deoxyribose
Ribose
Bases
A, G, C, T
A, G, C, U
Nucleus
Nucleus and
Cytoplasm
Location
DNA and Chromosomes
-DNA molecules are tightly
packed around proteins
called histones to make
structures called
chromosomes.
Q: Do you remember how
many chromosomes are
present in a human
diploid cell (somatic
cells)? Gametes?
DNA Replication
-Every time a cell divides, DNA must
be able to make a copy of itself.
-To make a copy of itself, the
twisted, compacted double helix
of DNA has to unwind and
separate its two strands. Each
strand becomes a pattern, or
template, for making a new strand,
so the two new DNA molecules
have one new strand and one old
strand.
-A host of enzymes, DNA
polymerase, helicase, ligases, play
a part in the complicated steps
involved in DNA synthesis.
Salient points…
-Complementary Base Pairing- A bonds with T & C bonds with G.
-DNA polymerase can only act in the 5’ to 3’ direction (so
nucleotide synthesis proceeds at two different direction at the
replication fork).
-A replication fork forms at the site where DNA synthesis is
occurring at the exposed parent strands.
Continuous and
Discontinuous Synthesis
• The direction of
replication is 5’  3’.
• Replication is
continuous when at
the 3’-5’ parent
strand and
discontinuous at the
5’-3’ parent strand.
• Okazaki fragments
form where
synthesis is
discontinuous.
Central Dogma
• The central dogma of molecular biology
explains how information flows from DNA to
RNA to proteins.
• Protein synthesis involve two stages:
Transcription and Translation.
Transcription
-is RNA synthesis.
-mRNA is created inside the
nucleus using a parent DNA
template.
-RNA polymerase is involved.
-Before the mRNA transcript
leaves the nucleus, it is
processed.
-Question: Quiz: Transcribe RNA
from a strand of DNA “A G C T
G T A C A.” [Remember: there
is no Thymine in RNA].
Translation
-is the actual
manufacture of
proteins using mRNA.
-each ‘trio’ of bases
on RNA serves as a
code (codon) for an
amino acid. This code
is universal.
-ribosomes are involved and the site of synthesis is
in the cytoplasm.
-tRNA’s carry specific amino acids to the ribosomes.
Key Players
tRNA
The anticodon site of tRNA binds
to the codons of mRNA. The
anticodon is complementary to
the mRNA codons.
Ribosomes come in two sizes: a
small sub-unit and a large subunit.
Animated movie:
Watch the steps at:
http://www.aboutkidshealth.ca/HowTh
eBodyWorks/Steps-ofTranslation.aspx?articleID=10190&cat
egoryID=XG-nh4-05a
Ready for the
practice questions?