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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