Structure of DNA
• DNA is made up of a
long chain of
nucleotides
• Each nucleotide has
three parts
– A phosphate group
– A deoxyribose sugar
– A nitrogen-containing
base
Structure of DNA
• The nitrogen containing bases are the only difference
in the four nucleotides
Structure of DNA
• DNA is a double helix, in
which two strands are
wound around each
other
– Discovered by Watson
and Crick
• Nucleotides always pair
in the same way
• Base-pairing rules
– A pairs with T
– C pairs with G
Structure of DNA
• The backbone is
connected by covalent
bonds
• The bases are
connected by hydrogen
bonds
DNA Replication
• Replication- copies the
genetic information
• A single strand of DNA
serves as a template for a
new strand
• The rules of base pairing
direct replication
• When is DNA replicated
during cell cycle?
• Each body cell gets a
complete set of identical
DNA
DNA Replication
• How does replication work?
– Proteins carry out the process of replication
– DNA serves only as a template
– Enzymes and other proteins do the actual work of replication
• Enzymes unzip the double helix
• Free-floating nucleotides form hydrogen bonds with the template strand
DNA Replication
• How does replication happen?
– DNA polymerase enzymes bond the nucleotides together to
form the double helix
– Polymerase enzymes form covalent bonds between
nucleotides in the new strand
DNA Replication
• Two new molecules of DNA are formed, each with an
original strand and a newly formed strand
• Replication is fast and accurate
• DNA polymerase can find and correct errors
Transcription
• Transcription- converts a
gene into a single-stranded
RNA molecule
• RNA carries DNA’s
instructions
• Central Dogma- information
flows in one direction from
DNA to RNA to proteins
– Replication
– Transcription
– Translation
• RNA is a link between DNA
and proteins
Transcription
• How does DNA differ from RNA?
– RNA has ribose sugar instead of deoxyribose
– RNA has uracil instead of thymine
– RNA is single stranded instead of double stranded
Transcription
• Transcription copies DNA to make a strand of
RNA
• Makes three types of RNA
– Messenger RNA (mRNA) carries the message that
will be translated to form a protein
– Ribosomal RNA (rRNA) forms part of ribosomes
where proteins are made
– Transfer RNA (tRNA) brings amino acid from the
cytoplasm to a ribosome
Transcription
• How does transcription happen?
– RNA polymerase binds to DNA and separates the
DNA strands
– RNA polymerase then uses one strand of DNA as a
template from which nucleotides are assembled
into a strand of RNA
• How does RNA polymerase know where to
start and stop making a RNA copy of DNA?
– Start and stop signals- specific base sequences
Transcription
Translation
• Amino acids are coded
by mRNA base
sequences
– Translation converts
mRNA messages into
polypeptides or proteins
– Codon- sequence of
three nucleotides that
codes for an amino acid
• Genetic code- language of mRNA
instructions, matches each codon to
Translation its amino acid or function
– Start codon= AUG
– Stop codon= UGA, UAA, UAG
Translation
• Translation- production of proteins
– Ribosome
• What happens during translation?
– Messenger RNA is transcribed in the nucleus, and released into
cytoplasm
– Translation begins at AUG (start codon), as each codon of the
mRNA moves through the ribosome, amino acid is brought into
ribosome by tRNA, amino acid is transferred to growing
polypeptide chain
• tRNA has 3 unpaired bases (anticodon)- complimentary to one mRNA
codon
– Polypeptides join assembly line, continues until one of the three
stop codons is reached
– Protein synthesis
Translation