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

Translation• Genetic code- language of mRNA

instructions, matches each codon to 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