7.1 DNA Structure and Replication Flashcards
How did Franklin and Wilkins investigate DNA structure?
they used X-ray diffraction
they took images of crystallised DNA which proved the double helix model
How did Hershey and Chase prove that DNA is the genetic carrier of material?
they took bacteriophages where the P in the DNA was radio labelled
they then took some other pages and had the S in their proteins radio labelled
these phages were incubated separately with bacteria
radioactivity was seen in the bacteria infected with radioactive DNA but not in the bacteria infected with radioactive protein
hence proving that DNA was the carrier of genetic information
How does DNA supercoil in Eukaryotes?
DNA is first wound around proteins called histones which are grouped in bead like structures of eight molecules
this grouping is called a nucleosome
when a cell divides the chromosomes have to be wound up so they do not tangle
to do this, the beaded coils form tight chromatin coils called chromatin fibres
these then form loops called looped domains
these then coil and form chromosomes
Exons
exons are coding regions of DNA
only make up around 10% of DNA
Satellite DNA
tandemly repeating sequences of DNA that make up a part of structural heterochromatin and centromeres
repeat sequences of 2-4 base pairs showing a variation in the number of repeats
are called short tandem repeats or STRs (used in forensics)
Telomeres
regions of repetitive DNA at the end of a chromosome that protect chromosomes from being damaged during DNA replication
Introns
non-coding regions within genes that are removed before the formation of mRNA
Non-coding RNA genes
codes for RNA that is not translated into protein but result in the formation of other molecules such as tRNA
How does DNA Replication work? HL
DNA polymerase can only add nucleotides in the 5’ to 3’ end (i.e. only to the 3’ end of a primer)
meaning that one strand is built continuously (leading strand) by polymerase III enzyme
and the other needs to be built in sections (lagging strand)
the lagging strand must be created using prefabricated sections called Okasaki fragments
RNA polymerase (primase) makes RNA Primers that attach the Okasaki fragments
DNA polymerase III extends the primer by joining Okasaki fragments
DNA polymerase I extends small sections of the DNA strand and has an important role in repair if there are any errors in the base pairing
the fragments are then joined by DNA ligase
How do dideoxynucleotides (ddNTPs) work?
they stop DNA replication in the preparation of samples for base sequencing
they don’t have the 3’- hydroxyl group used for bonding to phosphate groups
if they are present during DNA sequencing the nucleotide chain cannot be elongated so replication is stopped
