Lectures 10/11, Module 1 - DNA, Protein and the Central Dogma: DNA o o

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1 Lectures 10/11, Mdule 1 - DNA, Prtein and the Central Dgma: DNA Genetic material Plymer cnsisting f nucletides Nucletides cnsist f a nitrgenus base, a pentse sugar (dexyribse) and a phsphate grup Bases can be Adenine (A) Thymine (T) Guanine (G) Cytsine (C) Arranged in a duble helix Sugar phsphate backbnes cvalently bnded t base pairs A-T (jined by 2 hydrgen bnds) and C-G (jined by 3 hydrgen bnds) A and G are purines (tw rganic rings) and T and C are Pyrimidines (ne rganic ring) Backbnes run antiparallel (in ppsite directins) DNA Replicatin Semi-cnservative Parental mlecules have tw cmplementary strands f DNA, each base pair is jined by hydrgen bnding The strands separate and each parental strand serves as a template fr a new strand Cmplementary nucletides line up and are cnnected t frm tw new duble helixes (each cnsisting f an ld parental strand and a new strand) New strand=limited methylatin, ld strand=fully methylated A clser lk at replicatin Replicatin begins at particular sites with a specific sequence f nucletides called rigins f replicatin Prteins attach t this sequence and separate the tw strands frming a replicatin bubble Replicatin prceeds in bth directins Multiple bubbles frm and eventually fuse At the ends f each bubble replicatin frks frm Y-shaped regins where the parental DNA is being unwund Helicases are enzymes that untwist the duble helix at the replicatin frks After the strands are separated, single stranded binding prteins bind t the DNA strands and keep them frm repairing (keeps them stable) Untwisting f the duble helix causes tighter twisting and strain ahead f the replicatin frk Tpismerase relieves this strain by breaking, swivelling and re-jining the strands The enzymes that synthesise DNA cannt initiate the synthesis f a plynucletide - they can nly add base pairs t an already existing chain that is base-paired with the parent strand The enzyme primase synthesises RNA primers using the DNA strand as a template RNA nucletides are added ne at a time The cmpleted primer is generally abut 5-10 nucletides lng The new DNA strand will start frm the 3' end f the RNA primer

2 New nucletides are added by nucleside triphsphates New strand can elngate nly in 5' t 3' directin and nucletides can nly add t that 3' end f the RNA primer DNA has "ne directin" Leading strand DNA pl III remains at the replicatin frk n the new cmplementary strand and cntinuusly adds nucletides as the frk prgresses Only ne RNA primer is required Lagging strand On the ther strand, DNA pl III must wrk away frm the replicatin frk in the mandatry 5' - 3' directin

3 This strand in synthesised in a series f segments (Okazaki fragments) Each fragment requires a separate primer Ends f DNA Mlecules The ends f eukarytic chrmsmal DNA cnsist f telmeres (multiple repetitins f ne shrt nucletide sequence) that d nt cntain genes Buffer that prtects the rganism s genes by pstpning the ersin f genes lcated near the ends f DNA mlecules Repairing DNA Incrrectly paired, r altered nucletides are crrected by enzymes (apprx. 130 in humans) befre they becme mutatins Nuclease enzymes cut damaged DNA strands at tw pints s the segment can be remved Repair synthesis by DNA plymerase fills in the missing nucletides DNA ligase seal the new DNA t the ld DNA A summary: Must be able t redraw this - knw what all the enzymes d!!!!! Must understand DNA replicates in ne directin (leading and lagging)

4 Chrmsmes Histnes Respnsible fr the first level f DNA packaging int chrmatin H2A, H2B, H3, and H4 are mst cmmn in chrmatin Nuclesmes (10nm fibre) Cnsists f DNA wund twice arund a prtein cre cmpsed f tw mlecules each f the fur main histne types The amin end (N-terminus, r tail) f each histne extends utward frm the nuclesme 30nm fibre Ciling due t interactins between the histne tails f ne nuclesme and the linked DNA nuclesmes n either side Lped dmains (300nm fibre) 30nm fibre frms lps attached t a chrmsme scaffld made f prteins Metaphase chrmsme Lped dmains cil and fld t further cmpact the chrmatin t prduce a characteristic metaphase chrmsme The Central Dgma DNA -> RNA -> Prteins Transcriptin: Initiatin The DNA sequence where RNA plymerase attaches and initiates transcriptin is knwn as the prmter The prmter includes the transcriptin start pint (a single nucletide) In bacteria, the RNA plymerase recgnises the start pint In eukarytes, transcriptin factrs mediate the binding f RNA plymerase and the initiatin prcess The whle cmplex f transcriptin factrs and RNA plymerase II bund t the prmter is called a transcriptin initiatin cmplex.

5 Once the RNA plymerase is attached in the crrect rientatin, it unwinds the DNA strands and starts t transcribe the template Elngatin RNA plymerase mves alng the DNA and cntinues t unwind it and add nucletides at the 3' end f the grwing RNA strand 5' - 3' directin The RNA strand peels away and the DNA duble helix refrms A single gene can be transcribed simultaneusly by several mlecules f RNA plymerase fllwing each ther Terminatin Bacteria Transcriptin prceeds thrugh a terminatr DNA sequence The transcribed RNA f the terminatr sequence functins as the terminatin signal The plymerase detaches frm the DNA and releases the transcript which can be immediately translated Eukarytes In eukarytes, RNA plymerase II transcribes a sequence n the DNA called the plyadenylatin signal sequence, which cdes fr a plyadenylatin signal (AAUAAA) in the pre-mrna. Then, at a pint abut nucletides dwnstream frm the AAUAAA signal, prteins assciated with the grwing RNA transcript cut it free frm the plymerase, releasing the pre-mrna. RNA Prcessing (Eukarytes) Alteratin f pre-mrna ends The 5 end is synthesized first; it receives a 5 cap (mdified frm f G nucletide) added nt the 5 end after transcriptin f the first nucletides. An enzyme adds mre adenine (A) nucletides, frming a ply-a tail at the 3' end. The 5 cap and ply-a tail share several imprtant functins. Facilitate the exprt f the mature mrna frm the nucleus Help prtect the mrna frm degradatin by hydrlytic enzymes. Help ribsmes attach t the 5 end f the mrna nce the mrna reaches the cytplasm. Split Genes and RNA Splicing Eukarytic Genes and their RNA transcripts have lng nn-cding stretches that are spliced ut Intrns Nn-cding sequences (f DNA and RNA) that lie between cding sequences Exns The cding areas (f RNA and DNA) Sequences f RNA that exit the nucleus Intrns are cut ut frm the mlecule and the exns are jined tgether t frm the mrna invlved in translatin