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a change in genotype andphenotype due to the assimilation of external DNA by a cell
- a nitrogenous (nitrogen-containing)base, a pentose sugar called deoxyribose, and a phosphate group
- adenine (A) 2(no O), thymine(T)1(CH3), guanine (G)2(O), or cytosine (C) 1(N-H-H)
- sugar-phosphate backbones are antiparallel
- A, G = purines
- C,T = pyrimidine
- Pairing = purine + pyrimidine, A+T, C+G
- Bond with hydrogen bonds wiht nitrogenous base
- A+T : 2 Hydrogen bonds
- G+C : 3 Hydrogen bonds
Have unique a unique order or base sequence
- (1) thebase composition varies between species,
- (2) within aspecies, the number of A and T bases are equal and the numberof G and C bases are equal.
Three alternative models of DNAreplication.
Conservative model - The two parental strands reassociate afteracting as templates for new strands,thus restoring the parental double helix.
Semiconservative model.The two strands of the parental moleculeseparate, and each function sas a template for synthesis of a new, complementary strand
Dispersive model. Each strand of both daughter molecules contains a mixture of old and newly synthesized DNA.
Meselson and Stahl Experiment
- comfirmed semi-conservative model
- cultured E.coli for several generations in N15 and N14
- result : band of hybrid DNA
- result 2: both light and hybrid DNA
origins of replication
- site where replication of a DNA molecule begins
- short stretches of DNA having a specific sequence of nucleotides
- Ex. E.coli and bacteria
- only one origin of replication is present
- The parental strands separate at the origin, forming a replication bubble with two forks. Replication proceeds in both directions until the forks meet on the other side, resulting in two daughter DNA molecules
replication in a eukaryotic cell
- DNA replication begins when replication bubbles form at many sites along the giant DNA molecule
- The bubbles expand as replication proceeds in both directions.
- the bubbles fuse and synthesis of the daughter strands is complete
Y-shaped region where the parental strands of DNA are being unwound
enzymes that untwist the double helix at the replication forks
single strand binding proteins
bind to the unpaired DNAstrands, keeping them from re-pairing.
helps relieve this strain by breaking, swiveling, and rejoining DNA strands.
- added to nucleotides
- The initial nucleotide chain that is produced
- is synthesized by the enzyme primase
starts a complementary RNA chain from a single RNA nucleotide
- Enzymes that catalyze the synthesis of new DNA by adding nucleotides to a pre existing chain(primer)
- require a primer and a DNA template strand, along which complementary DNA nucleotides line up.
- bacteria: 500 nucleotides/sec
- human: 50 nucleotides/sec
DNA polymerases can add nucleotides only to the free 3' end of a primer or growing DNA strand, never to the 5' end
dATP, nucleoside triphosphate
- supplies an adenine nucleotide to DNA,is the sugar component, which is deoxyribose in the building block of DNA
- used for DNA synthesis are chemicall y reactive, partly because their triphosphate tails have an unstable cluster of negative charge
- 3' End
- Only one primer is required for DNA pol III to synthesize the leading strand
- DNA pol III remains in the replication fork on that template strand and continuously adds nucleotides to the new complementary strand as the fork progresses ( same direciton as the fork) right
- elongates continuously
- 5' end
- DNA pol III work in direction away from the replication ,right
- synthesized discontinuously, as a series of segments (Okazaki fragments)
- 1) Primase joins RNA nucleotides into a primer
- 2)DNA pol III adds DNA nucleotides to the primer, forming Okazaki fragment 1
- 3)After reaching the next RNA primer to the right, DNA pol III detach
- 4) DNA pol I replace RNA with DNA andding 3' to frag 2.
- 5)DNA ligase - connect the fragments
- other enzymes remove and replace incorrectly paired nucleotides that have resulted from replication errors
- changes inDNA are usually corrected before they become permanent changes—mutations—perpetuated through successive replications.
- DNA repair enzymes: 100 in E.coli, 130 in humans
Nucleotide excision repair
- 1. enzymes detect and repair damaged DNA
- 2.A nuclease enzyme cuts the damaged
- 3.DNA polymerase fills in the missing nucleotides
- 4. DNA ligase seals the free end
Ex. thymine dimers ( cuase by ultraviolet)
- acts as a kind of buffer zone that protects the organism’s genes.
- proteins associated with telomeric DNA prevent the staggered ends of the daughter molecule
shortening telomeric can lead to self destruction of tumor cell
catalyzes the lengthening of telomeres in eukaryotic germ cells, thus restoring their original length and compensating for the shortening that occursduring DNA replication
circular DNA molecule that is associated with a small amount of protein
- qukaryotic chromosome: one linear DNA molecule associated with a large amount of protein
- E.coli has 4.6 million nucleotide pair/4400 gene
proteins responsible for the first level of DNA packing in chromatin
- basic unit of DNA packing; the “string”between beads is called linker DNA
- contains 4 histones
dense region of DNA in a bacterium, not bounded by membrane
complex of DNA and protein,fits into the nucleus through an elaborate, multilevel system of packing
DNA -> Histones(5) -> nucleosome (1st 4 histones) -> 30-nmfiber (5th hsitone H1) -> Looped domains -> Metaphase
- exist in a highly condensed state similar to that seen in a metaphase chromosome
- largely inaccessible to the machinery in the cell responsible for transcribing the genetic information coded in the DNA,
distinguish it from the lesscompacted, more dispersed