Chapter 8
From DNA to Proteins
Replication
The Components and Structure of DNA
DNA is a long molecule made up of
nucleotides. Each nucleotide has
three parts.
• 5-carbon sugar called
deoxyribose
• a phosphate group
• one of four nitrogenous bases
–Two of the nitrogenous bases are
adenine and guanine and they
belong to the purine group. They
have 2 rings in their structure.
–The other two nitrogenous bases
are thymine and cytosine. They
belong to the pyrimidine group
and have only one ring in its
structure.
–The backbone of the DNA chain is
formed by sugar and phosphates of
each nucleotide.
–The nitrogenous bases stick out
sideways but facing inward and any
sequence of bases is possible.
Chargaff’s Rules – An American
biochemist, Erwin Chargaff,
discovered the percentages of
guanine and cytosine to be almost
equal and the same was true for
adenine and thymine.
He concluded that A=T, and C=G. From
that he was given credit for Chargaff’s
Rules.
Adenine pairs with thymine and guanine
pairs with cytosine.
The double helix is made up of a chain of
nucleotides. The two strands are held
together by hydrogen bonds between
adenine and thymine and between
guanine and cytosine. Scientists realized
that this principal is base pairing
explained by Chargaff’s rules.
What happens during DNA
replication?
Prokaryotic (ex. bacteria) cells lack a
nucleus (and many other organelles)
• Their DNA molecules are located in the
cytoplasm (not in the nucleus-like
eukaryotic cells)
• Most have single, circular DNA (located in
ONE chromosome)
Eukaryotic cells have DNA that is
more complicated
• More than 1000X more DNA than
prokaryotes
• DNA Is found in nucleus (not in
cytoplasm-like prokaryotic cells)
–Number of chromosomes differs from one
species to the next
–# of chromosomes Humans—46, Fruit
flies—8, Giant sequoia tree—22
DNA Replication
–DNA molecules contain everything
they need (within themselves) to
replicate
–The parent (original) strand is used to
make the new (replicated) strand
• Strands are made by base pairing (AT, CG)
DNA Replication
Base pairing:
»Ex:
A T C C A G G T G
»DNA copy:
Duplicating DNA strands
–Replication - the
duplication/copying of DNA
–During DNA replication, the DNA
molecule separates into two
strands
»It then produces two new
complementary strands (following the
rules of base pairing)
»Each strand of DNA serves as a template
(model) for the new strand
DNA replication results in two DNA molecules
Each DNA molecule has:
• One new strand
• One original strand
What do you know???
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Name the parts of a nucleotide
List the bases in DNA
What does DNA stand for?
When does replication take place?
What enzymes are involved in replication?
What direction are the strands copied?
What is the result of replication?