From DNA to Protein

Chapter 8

Terminology

• Genetics• Genome• Chromosome

• Gene • Locus• Alleles

• Genotype/Phenotype

• Heredity

• Living cells must accomplish two general tasks to multiply and survive – DNA replication– Gene expression

• Expression involves two process– Transcription– Translation

– Flow of information from DNA to RNA to protein

• Polymer of nucleotides

• Hydrogen bonds between complementary bases

– AT and CG

• Antiparallel

• New nucleotides can only be added to the “free” 3’ end

DNA

DNA synthesis involves anabolic polymerization

Monomers (Triphosphate deoxyribonucleotides) provide required energy for DNA synthesis

One DNAdouble helix.

Semi-conservative DNA replication

Two identical DNAdouble helixes, eachwith one parentalstrand (blue) and onenew strand (pink).

Replication produces two DNA double helixesEach contains one original strand and one new strand

• DNA replication in bacteria is bi-directional – due to closed circular chromosome– replication forks eventually meet and two

complete loops are separated

• Bacterial DNA is attached at several points to the cell membrane– Enzymes need for replication are membrane

proteins

• Topoisomerase (DNA gyrase)

• Helicase

• Primase

• DNA polymerases

• DNA Ligase

• Bacteria replication involves methylation of daughter stands– Methylase

– Adds methyl group (-CH3) to nitrogenous bases (typically adenine)

• Methylation functions:– Initiation of DNA repliction– Control of genetic expression – Protection from viral infection– Repair of DNA

DNA Replication

• As DNA unwinds, it creates a replication fork

• As nucleotides are added, the replication fork moves down the parental strand

– Leading strand• Is synthesized CONTINUOUSLY as the DNA

polymerase moves towards the replication fork

– Lagging strand • Is synthesized DISCONTINUOUSLY in pieces

as DNA polymerase moves away from the replication fork

• Okazaki fragments

Single Strand Binding Proteins

• DNA contains the instructions for protein synthesis – Genes

• RNA carries out the instructions

• Genetic information flows from DNA to RNA to protein

• Central Dogma of Molecular Biology– DNA Transcribed RNA – RNA Translated Protein

Gene Expression

• Transcription– RNA polymerase synthesizes complementary mRNA from

DNA template– Cytoplasm of prokaryotes and the nucleus of eukaryotes

Concurrent RNA transcription

Multiple copies of RNA can be transcribed simultaneously

• Eukaryotic DNA is more complex– Requires post-

transcriptional modifications

– Spliceosome– Cap and tail

• Translation– The language of mRNA is in the form of codons

• Three nucleotides situated next to each other on DNA

– Sequence of codons determines sequence of amino acids in the protein

– 64 codons make up the “alphabet”• 61 are sense codons• 3 “stop codons”

•The site of translation is the ribosome

• tRNA brings appropriate amino acid to site of translation• Each tRNA has an anticodon

– complementary sequence to the mRNA codon

In a prokaryotes, many molecules of mRNA can by transcribed simultaneously

Why can translation begin before transcription is completed in a prokaryote but not in a eukaryote?

template DNA strand

complementaryDNA strand

DNA

gene

codons

anticodons

amino acids

mRNA

tRNA

methionine glycine valineprotein