Unit 6
DNA
Griffith Experiment
DNA Structure
• DNA is a polymer made of monomers called
nucleotides
• Each nucleotide is made of:
– A phosphate group
– Deoxyribose (sugar)
– Nitrogen containing base
DNA Structure
• DNA is in the shape of a double helix
• Each nucleotide is paired
Types of Nucleotides
• C – Cytosine
Pyrimidines
• T – Thymine
• A – Adenine
Purines
• G – Guanine
Base-Pairing Rules
• Nucleotides always pair in the same way
– Thymine (T) always pairs with Adenine (A)
– Cytosine (C) always pairs with Guanine (G)
– Ex. TTACGTAG
AATGCATC
DNA Replication
• Replication is a process by which DNA is
copied
• Occurs during the S stage of the cell cycle
DNA Replication
• Enzymes are responsible for splitting and
copying DNA
• DNA polymerase- enzyme that binds DNA
strands together
DNA Replication
_________
• Steps of Replication:
1. DNA strand is unzipped
2. Free nucleotides bind to each template
strand
3. Two identical DNA strands are formed
DNA Transcription
• Transcription is the process of copying a
sequence of DNA into a message called RNA
• RNA is a temporary copy of DNA
• Three types of RNA:
1. mRNA- message
2. rRNA- forms part of ribosomes
3. tRNA- transfers amino acids to help make
proteins
DNA Transcription
Transcription Animation
• Steps of transcription:
1. Gene is recognized, DNA unwinds
2. RNA strand binds to template strand
- GC, TA, AU (not T)
3. Completed RNA strand separates
DNA Translation
• Translation- process converts, mRNA message
into a polypeptide
– Polypeptides make up proteins
• Codon- 3 nucleotide sequence that codes for
an amino acid
DNA Translation
Steps of
Translation
1. The start mRNA codon
attracts a
complimentary tRNA
anticodon
2. Each amino acid is
added to the next tRNA
3. Process continues until
a stop codon is
reached.
4. Protein is then released
Gene Expression
• Genes can make a lot of
protein, little, or none
• Transcription is controlled
by promoters and
operators
• Promoter- DNA segment
that allows a gene to be
transcribed
• Operator- DNA segment
turns a gene “on” or “off”
Gene Expression
• Operon– region of DNA that
includes a promoter,
operator and genes that
code for certain proteins
mRNA Processing
• Sometimes pieces of
mRNA are not needed
1. Introns are removed
2. A cap and tail are
added
3. Molecule joins
together
Mutations
• mutation- change in an organism’s DNA
• Single gene mutations usually occur in
replication
• Mutations affecting a group of genes happen
in meiosis
Mutations
• Types of mutations:
1. Point mutation- one nucleotide is substituted for
another
2. Frameshift mutation
1. Insertion- addition of a nucleotide
2. Deletion- removal of a nucleotide
3. Gene duplication
4. Gene translocation
Point mutation
Gene duplication
Frameshift
Translocation