BIO1022 Chapter Notes - Chapter Prescribed : Telomerase, Reverse Transcriptase, Microorganism
30 May 2018
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BIO1022 – Readings – Week 2 – Molecular Genetics
16.1 and 16.2
- DNA is the genetic material
- transformation - a change in genotype and phenotype due to the assimilation of
external DNA by a cell.
- viral DNA can program cells
• viruses - bacteriophages - phages
• a virus is little more than DNA or sometimes RNA enclosed by a protective
coat - often simply protein.
• to produce more viruses - a virus must infect a cell and take over the cell’s
metabolic machinery
- structure of DNA - arrangement of covalent bonds in a nucleic acid polymer
• two strands - double helix
• two sugar phosphate back bones are antiparallel - their subunits run in
opposite directions
• Adenine and guanine are purines, nitrogenous bases with two organic
rings, while cytosine and thymine are nitrogenous bases called
pyrimidines, which have a single ring
- many proteins work together in DNA replication and repair
- DNA replication - semi-conservtative model
• when a double helix replicates, each of the two daughter molecules will
have one old strand from the parental molecule and one newly made
strand.
- DNA replication process - e. coli
• the replication of a chromosome begins at particular sites called origins of
replication - short stretches of DNA having a specific sequence of
nucleotides
• proteins that initiate DNA replication recognise this sequence and attach
to the DNA - separating the two strands and opening up a replication.
• replication proceeds until the entire molecule is copied
• eukaryotes have thousands of replication origins
• at the end of a replication bubble is a replication fork - a Y shaped region
where the parental strands of DNA are being unwound
•
o several kinds of proteins participate
• helices 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
• topoisomerase - helps relieve this strain by breaking, swivelling and
rejoining DNA strands.
• the enzymes that synthesise DNA cannot initiate the synthesis of a
polynucleotide - they can only add DNA molecules to the end of an
already existing chain that is base-paired with the template stand.
• the initial nucleotide chain that is produced during DNA synthesis is a
short stretch of RNA not DNA
•
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o this RNA chain is called a primer and is synthesised by the enzyme
primase
o primase starts a complementary RNA chain from a single
nucleotide adding more RNA nucleotides at a time - using the
parental DNA as a template stand
o completed primer is base paired to the template strand
- synthesising a new DNA strand
• enzymes called DNA polymerases catalyse the synthesis of new DNA by
adding nucleotides to a preexisting chain.
•
o most common DNA polymerase III and DNA polymerase I
o
▪ bacteria
o eukaryotes have at least 11 different DNA polymerases
o most DNA polymerase require a primer and a DNA template
strand
o in e.coli - DNA polymerase II adds a DNA nucleotide to the RNA
primer and then continues adding DNA nucleotides
complimentary to the parental DNA strand to the growing end of
the new DNA strand
• each nucleotide to be added to a growing DNA strand consists of a sugar
attached to a base and to three phosphate groups.
• as each monomer joins the growing end of a DNA strand - two phosphate
groups are lost as a molecule of pyrophosphate
- antiparallel elongation
• the antiparallel arrangement of the double helix affects replication
because of their structure -n DNA polymerase can add nucleotides only to
the 3’ end of a primer or growing DNA strand never to the 5’ end
•
o thus a new DNA strand can elongate only in the 5’ - 3’ direction.
• leading strand - only one primer is required for DNA pol III to synthesise
the entire leading strand.
• to elongate the other new strand of DNA in the mandatory 5’ - 3’ direction
- DNA pol III must work along the other template strand in the direction
away from the replication fork - the lagging strand.
•
o synthesised discontinuously - as a series of segments
o
▪ these segments are called Okazaki segments
▪ each Okazaki fragment on the lagging strand must be
primed separately
▪ after DNA pol III forms an Okazaki fragment another DNA
polymerase - DNA pol 1 replaces the RNA nucleotides of the
adjacent primer with DNA nucleotides
▪ DNA pol I cannot join the final nucleotide of this
replacement DNA segment to the first DNA nucleotide of
the adjacent Okazaki fragment
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▪ another enzyme DNA ligase accomplishes this - joining the
sugar-phosphate back bones of all the Okazaki fragments
into a continuos DNA strand
- DNA replication complex
• include various proteins that participate in DNA replication
- proof reading and repairing DNA
• errors in DNA molecules - very rare - because during DNA replication
DNA polymerases proofread each nucleotide against its template as soon
as it is covalently bonded to the growing strand.
• upon finding the incorrectly paired nucleotide - DNA pol removes the
nucleotide and then resumes synthesis.
• mismatched nucleotides sometimes evade proofreading by a DNA pol
•
o in mismatch repair - other enzymes remove and replace
incorrectly paired nucleotides that have resulted in replication
errors.
• incorrectly paired or altered nucleotides can also arise after replication.
•
o DNA molecules are constantly subjected to harmful chemicals and
physical agents - such as X-rays
o such changes are usually corrected - before they become
mutations.
• most cellular systems for repairing incorrectly paired nucleotides use a
mechanism that takes advantage of the base-paired structure of DNA.
•
o in many cases - a segment of the strand containing the damage is
cut out by a DNA cutting enzyme - a nuclease - and the resulting
gap is then filled with nucleotides using the undamaged strand as a
template.
o the enzymes involved in filling the gap are DNA pol and DNA
ligase.
o
▪ called nucleotide excision repair.
- replicating the ends of DNA molecules
• for linear DNA - such as the DNA of eukaryotic chromosomes - the usual
replication machinery cannot complete the 5’ ends of the daughter DNA
strands.
•
o this is a consequence of the fact that a DNA polymerase can only
add nucleotides to the 3’ end of a preexisting polynucleotide.
o results in shorter strands - each replication.
o eukaryotic chromosomal DNA molecules have a special nucleotide
sequence called telomeres at their ends
o
▪ do not contain genes
▪ instead they consist of multiple repetitions of one short
nucleotide sequence.
▪ have two protective functions
▪
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find more resources at oneclass.com
Document Summary
Bio1022 readings week 2 molecular genetics. Transformation - a change in genotype and phenotype due to the assimilation of external dna by a cell. Many proteins work together in dna replication and repair. Dna replication - semi-conservtative model: when a double helix replicates, each of the two daughter molecules will have one old strand from the parental molecule and one newly made strand. Dna replication complex include various proteins that participate in dna replication. Proof reading and repairing dna: errors in dna molecules - very rare - because during dna replication. Nucleic acid hybridisation - the base pairing of one strand of a nucleic acid to the complementary sequence on a strand from another nucleic acid molecule. Genetic engineering - the direct manipulation of genes for practical purposes. To separate and visualise dna fragments of different lengths - researches carry out a technique called gel electrophoresis: uses a gel to separate a mixture of nucleotides.
