Chapter 11 - DNA replication Flashcards
what macromolecule did scientists in the early 1900s believe to carry genetic material
proteins because they had the most diversity (20 amino acids vs 4 nucleotides)
Four criteria necessary for genetic material
- information
- replication
- transmission
- variation
Describe Griffith’s experiment
- Rough and heat-killed smooth is non-toxic
- Smooth S. pneumoniae toxic because of its capsule
- Rough with heat-killed smooth is toxic
Conclusion: the genetic material had been transferred from the heat-killed S-type to the R-type - gave R the capsule-secreting trait that was passed on to their offspring
Avery, MacLeod, and McCarty
- used DNase, DNase, and proteases
- only DNase resulted in to transformation
- concluded that DNA is the genetic material
genome
complete complement of genetic material in an organism
How are the two strands of DNA aligned?
antiparallel in a double helix
three components of DNA
- phosphate group
- pentose sugar
- deoxyribose - nitrogenous base (a,g,c,t)
three components of RNA
- phosphate group
- pentose sugar
- ribose
- OH at 2’ - nitrogenous base (a,g,c,u)
numbering of nucleotides
- sugar carbons numbered 1’-5’
- base attached to 1’
- phosphate attached to 5’
- exposed 3’ of one sugar attaches
backbone of DNA
- made of phosphates and sugars
- phosphodiester bonds
- written: 5’-TACG-3’
how was the structure of DNA solved
- Watson and Crick proposed the structure of the double helix using the ball and stick model
- Relied on Rosalin Franklin’s X-ray diffraction findings suggesting a double helix with
Erwin Chargoff
- analyzed base composition of DNA
- found that the amount of adenine was always the same as thyme and cytosine the same as gaunine
- supports idea of a uniform diameter
features of DNA structure
- right-handed double helix
- antiparallel, complementary strands
- sugar-phosphate backbone
- base pairing on the inside via H-bonding
- consistent distance of about 2nm
- one helical turn is about 10 nucleotides
major vs minor grooves
- major grooves are larger and allow proteins to bind to help with gene expression
semiconservative model
- DNA replication produces DNA molecules with 1 parent and 1 daughter strand
- discovered by Meselson and Stahl using isotopes
- not conservative or dispersive
Meselson and Stahl
- used isotopes of nitrogen to discover the semiconservative model
Origin of replication
- sequence that signals the opening of the replication fork
- allows the beginning of DNA replication
- bacteria have one
- eukaryotes have multiple
DNA helicase
binds to DNA and travels 5’ to 3’ to separate the strands
topoisomerase
- removes knots caused by the helicase
single strand binding proteins
hold parent strands open to be replicated
DNA polymerase 3
- covalently links nucleotides
- requires primer because it needs something to build off of
- builds 5’ to 3’
- reads 3’ to 5’
deoxynucleoside triphosphate (DNTP)
- free nucleotides with three phosphate groups
- two terminal phosphates are broken off for energy in to bind nucleotides together in DNA synthesis
how does primer work?
- primate adds and RNA primer must so polymerase can add nucleotides
- DNA polymerase 1 removes the primer after replication and makes it DNA (for okazakis - can’t do this for leading)
Leading vs. lagging strand
Leading:
- synthesized 5’-3’ as one long molecule
Lagging:
- synthesized 5’-3’ as Okazaki fragments
- fragments covalently bonded by ligase
both have RNA primer that is later replaced by DNA
3 mechanisms for DNA replication accuracy
- H-bonding between A and T and between G and C is more stable than mismatched combinations
- DNA polymerase is unlikely to form bonds if the pairs are mismatched
- DNA polymerase can proofread to removed mismatched pairs
- other DNA repair enzymes also exist
DNA polymerases
- there are many different types of DNA polymerases that perform different functions
- bacteria and human cells have different types of polymerases with different specialized functions
telomeres
- series of nonsensical nucleotide sequences at the 3’ end of chromosomes
- cannot be replicated because there is nowhere for upstream primer to be made
- also called the 3’ overhang
- makes sure that all important DNA can be replicated
- progressively shorten
how does telomerase work
- makes telomeres
- attaches many copies of DNA repeat sequence to the ends of chromosomes
what is a chromosome
- discrete unit of genetic material
- composed of chromatin
- about 1 meter in length
three levels of DNA compaction
- DNA wrapping
- 30 nanometer fiber
- formation of loop domains
DNA wrapping
- DNA wraps around proteins called histones to form a nucleosome
- shortens DNA 7x
30nm fibers
- asymmetric, 3D zig zag of nucleosomes
- shortens length another 7x
radial loop domains
- interactions between 30nm fibers and nuclear matrix
- each chromosome located in discrete territory
Chromosomes during cell division
- chromosomes compact more during cell division
- Euchromatin - not as compact
- heterochromatin - more compact
how can the charge of histone affect the compactness of chromatin
- DNA is negatively charged
- Positive histones favor compactness/heterochromatin
- negative histones will favor euchromatin
