16-17 ch (Many proteins work together in DNA replication and repair (Each…
Many proteins work together in DNA replication and repair
Each strand of DNA can be used to copy itself both strands are complementary
Replication begans at a particular site called origins of replication that are short stretches of DNA that have a specific sequence of nucleotides
Proteins that initiate DNA replication recognize this sequence and attach to the DNA separating the two strands and opening up a replication bubble
Replication then proceeds in both direction until the entire molecule is copied. At each end of the bubble is a replication fork a y shaped region where the parental strands of DNA are being unwound.
Helicases are enzymes that untwist the double helix at the replication forks, separating the two parental strands and making them available as template strands
After the parental strands separate single strand binding proteins bind to the unpaired DNA strands keeping them from repairing causes double helix to twist tighter ahead of the fork
Topoisomerase is and enzyme that helps relieve this strain by breaking swiveling and rejoining DNA strands
The enzymes can not initiate the synthesis of a ply nucletide they can only add DNA nucleoties so DNA synthesis is actually a short stretch of RNA called a primer and is synthesized by the enzyme primase. The new DNA strand will start from the 3 end of the RNA primer
DNA polymerases are enzymes that catalyze the synthesis of new DNA by adding nucleotides to the 3 end of a prexisting chain.Major DNA polymerases are DNA polymerase 3 and DNA polymerase 1. There are 11 in eukaryotic cells
The rate is 500 nucleotides per second in bacteria and 50 per second in human cells.
Each nucleotide added to the strand consists of a sugar attached to a base and to three phosphate groups. Each monomer is added using a dehydration reaction
Strand can only be replicated from 5->3 direction.DNA pol 3 remains in the replication fork on that template strand and continuously adds nucleotides to the new complementary strand as the fork progresses. The DNA strand made by this mechanismis called the leading strand
DNA pol 3 must work along the other template strand in the direction away from the replication fork. The DNA strand elongating in this direction is called the lagging strand.
Okazaki fragments- is a series of segments that are synthesized in the lagging strand
DNA pol 1 replaces the RNA nucleotides of the adjacent primer with DNA nucleotides one at a time but can not join the final nucletoide of this replacement DNA segment the to the firs DNA nucletide of the adjacent Okazaki fragment. DNA ligase accomplishes this task joining the sugar phosphate backbones of all the Okazaki fragments into a continuous DNA strand
In eukaryotic cells multiple copies of the complex perhaps grouped into factories may be anchored to the nuclear matrix a framework of fiber extending through the interior of the nucleus. As a theory not sure of this
Initial error os base pairing are 10^5 nucleotides. However errors is the completed DNA molecule amount to only one in 10^10 nucleotides (10 billion) error rate is 100,000 times lower. In mismatch repair- other enzymes remove and replace incorrectly paired nucleotides that have resulted from replication errors.
Nuclease-cutting enzyme for DNA
Nucleotide excision repair- DNA repair system
For linear DNA the usual replication machinery cannot complete the 5 ends of daughter DNA strands resulting in each replication have less and less DNA (eukaryotic not in prokaryotic)
Eukaryotic chromosomal DNA have a defensive from this called telomeres which do not contain genes instead the DNA typically consists of multiple repetitions of one short nucleotide sequence. In human DNA TTAGGG is repeated between 100 and 1,000 tims
Telomeres have two protective function-First specific proteins associated with telomeric DNA prevent the staggered ends of the daughter molecule from activation the cell's system for monitoring DNA damage. Second telomeric DNA acts as a kind of buffer zone that provides some protection against the organism's genes shortening somewhat like how the plastic wrapped ends of a shoelace slow down its unraveling.
telomerase- catalyzes the lengthening of telomeres in eukaryotic germ cells
DNA is the genetic material
transformation- defined as a change in genotype and phenotype due to the assimilation of external DNA by a cell
Evidence that DNA was the genetic material came from studies of bacteriophages.
DNA a polymer of nucleotides each having three components : a nitrogenous base a penotse called deoxyribose and phosphate group.
Bases inclue adenine thymine guanine and cytosine
Roughly same percents of A bases of the T bases and the G bases for the C bases
DNA has a helix shape specifically a double helix
The two sugar phosphate backbones are anti parallel their sub units run in opposite direction.
Th helix makes one full turn every 3.4nm along its length. Bases are stacked just 0.34 nm apart. Ten layers of base pairs or rungs of the ladder in each full turn of the helix
Base pairs combination of Adenine with thymine and guanine with cytosine
Eukaryotic cells modify RNA after transcription
RNA processing- enzymes in the eukaryotic nucleus modify pre-mRNA in specific ways before the genetic message is dispatched to the cytoplasm
Poly-A-tail-At the 3 end an enzyme then adds 50-250 more adenine nucleotides
RNA splicing- where large portions of the RNA molecules are removed and the remaining portions are reconnected
Introns-the noncoding segments of nucleic acid that lie between coding regions
Exons- they are eventually expressed usually by being translated into amino acid sequences
Spliceosome- large complex made of proteins and small RNAs
Ribozymes-RNA molecules that function as enzymes
Alternative RNA splicing- Many genes are known to give arise to two or more different polypeptides depending on which segments are treated as exons during RNA processing
Domains- proteins often have a modular architecture consisting of discrete structural and functional region
Mutations of one or a few nucleotides can affect protein structure and function
mutations-changes to the genetic information of a cell
point mutations- changes in a single nucleotide pair of a gene
Nucleotide pair substitution-is the replacement of one nucleotide and its partner with another pair of nucleotides
silent mutation- has no observable effect on the phenotype
Missense mutations- substitutions that change one amino acid to another one
Insertions and deletions- are additions or losses of nucleotide pairs in a gene
Frameshift mutation- whenever the number of nucleotides inserted or deleted is not a multiple of three
Mutagens- physical and chemical agents that interact with DNA in ways that cause mutations
Transcription is the DNA directed synthesis of RNA: a closer look
RNA polymerase- enzyme that pries the two strands of DNA apart and joins together RNA nucleotides complementary to the DNA template strand
promoter-Where RNa polymerase attaches and initiates transcription
Terminator- sequence that signals the end of transcription
Transcription unit- the stretch of DNA downstream from the promoter that is transcribed into an RNA molecule
transcription factors- mediate the binding of RNA polymerase and the initiation of transcription
Transcription initiation complex-The whole complex of transcription factors and RNA polymerase 2 bound to the promoter
TATA box- crucial promoter DNA sequence in forming the initiation complex at a eukaryotic promoter.
Translation is the RNA directed synthesis of a polypeptide: a closer look
Transfer RNA- to transfer an amino acid from the cytoplasmic pool of amino acids to a growing polypeptide in a ribosome
anticodon- the particular nucleotide triplet that base pairs to a specific mRNA codon
Aminoacyl-tRNA synthetases-correct matching up of tRNA and amino acids
Wobble-flexible base pairing at a codon
signal peptide-targets the protein to the ER
Signal recognition particle-Recognizes the signal peptide a sequence of about 20 amino acids at or near the leading end as it emerges from the ribosome
Polyribosomes-strings of ribosomes
A chromosome consists of a DNA with proteins
DNA consists of about 4.6 million nucleotide pairs representing about 4,400 genes
Chromatin- eukaryotic DNA precisely combined with a large amount of protein fits into the nucleus through an elaborate multilevel system of packing.
Heterochromatin-highly condensed state chromatin visible as irregular clumps with a light microscope
Gene expression- the process by which DNA directs the synthesis of proteins
Genes specify proteins via transcription and translation
The one gene one enzyme hypothesis- states that the function of a gene is to dictate the production of a specific enzyme.
Genes provide the instruction for making specific proteins
The bridge between DNA and protein synthesis is the nucleic acid RNA.
RNA consists of one strand and the bases A G C or U
Getting form DNA to protein requires two major stages transcription and translation
Transcription- is the synthesis of RNA using information in the DNA.
Messenger RNA- because it carries a genetic message from the DNA to the protein synthesizing machinery of the cell
Translation- is the synthesis of a polypeptide using the information in the mRNA. During this stage there is a change in language: The cell must translate the nucleotide sequence of an mRNA
Ribosomes-molecular complexes that facilitate the orderly linking of amino acids into polypeptide chains
The initial RNA transcript from any gene including specifying RNA is not translated into protein is called a primary transcript
Triplet code- the flow of information from gene to protein on this code
Template strand- the DNA strand that is transcribed
Codons- the mRNA nucleotide triplets written in the 5->3 direction
Coding strand- non template DNA strand by convention the sequence of the coding strand is used when a gene's sequence is reported
Reading frame- our ability to extract the intended message on reading the symbols in the correct groupings