Transcription & Translation IB Biology Topic 2.6-2.7 Topic 7.1-7.3 Structure of DNA and RNA DNA Replication Transcription & Translation

Nucleotides 3 major nucleic acids: DNA & RNA DNA, RNA, ATP Made of nucleotides (monomer) Nucleotides: One phosphate One 5 carbon monosaccharide One nitrogenous base

Bases: Adenine Guanine Cytosine Thymine Purines and Pyrimidines: Uracil in place of thymine in RNA Purines and Pyrimidines:

Monomers  Polymers Backbone: Alternating pentose-phosphate Bases extend outward Order of bases determine genetic code for organism Covalent bonding between components

Single or Double Strand RNA- always SS A-U, G-C DNA always DS Bases are paired: complementary base-pairing A-T, G-C Hydrogen bonds between bases

Anti-Parallel Arrangement Deoxyribose + phosphate= phosphodiester bond (linkage) Forms between hydroxyl group of 3’ carbon of deoxyribose and phosphate group to 5’ carbon of deoxyribose New always added to 3’ carbon end Result: Double strand has 2 directions

DNA Packaging Paired with histone proteins for organization Nucleosome- 2 molecules of each of 4 different histones (8 histones total) DNA- (-) charge and histones (+) charge Between nucleosomes, often a 5th histone attached to linking string of DNA Additional histone supercoiling

DNA Packaging, cont. Supercoiling of DNA makes it inaccessible to transcription enzymes Form of regulation Only certain areas of DNA involved in protein synthesis

Types of DNA Sequences Highly Repetitive sequences: 5-300 base pairs 100,000 replicates of a certain type per genome Clustered in discreet areas- satellite DNA Dispersed throughout- Coding function not known

Types of DNA Sequences, cont. Protein Coding Genes Provide base sequences proteins via ribosomes Base sequences carried from the nucleus to the ribosomes via mRNA Genes not fixed sequence of bases, but coding info (exons) mixed with non-coding fragments (introns)

Types of DNA Sequences, cont. Structural DNA: Highly coiled- has no coding function Around centromere and near ends of chromosomes Pseudogenes: lost coding function due to mutation involved with a base change

Types of DNA Sequences, Cont. Short tandem repeats and DNA Profiling: Most of our DNA is identical to every other human Specific regions that show variation: polymorphisms Polymorphisms are used for DNA profiling Look at 13 specific loci: short tandem repeats (STRs) of 2-5 base pairs

Semi- Conservative Replication Watson and Crick Structure of DNA Single strand of DNA used as template Meselson and Stahl Confirmed findings Bacterial DNA replication process developed as result Bacterial DNA: Circular No histone proteins Single point of origin for replication of DNA Eukaryotic DNA: Linear Has histones Has thousands of points of origin

Semi-Conservative Replication, cont. Replication steps: 1. Begins at origin, appears as a “bubble” 2. Helicase unzips H bonds between bases. 3. End of bubble replication fork 4. Bubbles enlarge in both directions and eventually fuse to produce 2 identical daughter strands https://www.youtube.com/watch?v=dKubyIRiN84

DNA Replication Elongation of DNA strand 1. Primer produced by primase at replication fork Primer is 5-10 nucleotides of RNA Primase allows joining RNA nucleotides that match DNA sequence 2. DNA polymerase III allows the addition of nucleotides in a 5’ to 3’ direction 3. DNA polymerase I removes primer from 5’ end and replaces it with DNA nucleotides 4. Topoisomerase (DNA gyrase) stabilizes DNA above helicase

DNA Replication, cont. Anti-Parallel strands One 5’ 3’, one 3’ 5’ DNA only assembled in 5’  3’ direction due to DNA pol III. 3’ 5’ template strand made quickly and continuously- leading strand 5’ and 3’ template strand made discontinuously and in pieces- lagging strand Pieces are called Okazaki fragments Fragments are joined by DNA ligase

DNA Replication, cont. 1. Leading strand assembled continuously 2. Lagging strand assembled in Okazaki fragments 3. Primer, primase, and DNA pol III required to begin formation of each Okazaki fragment 4. Primer and primase needed only once for leading strand 5. DNA ligase attaches sugar-phosphate backbone together

Replication Proteins Replication in prokaryotes and eukaryotes is almost identical

Speed and Accuracy of Replication 4000 nucleotides per second Bacteria- Speed essential, divide every 20 minutes Remarkably accurate- Few errors occur Cells have repair enzymes that detect errors Repair enzymes also used when cells are damaged by chemicals or high energy waves

Replication, DNA Sequencing, Human Genome Project HGP required actual representation of nucleotide sequence in humans. Sequencing DNA- 1970s Sanger developed sequencing procedure Fragments of DNA copied through PCR (polymerase chain reaction) Read and outline section on page 332-333 bring questions to class.