Lecture 4: Molecular basis of inheritance Flashcards
Your genetic endowment is your
DNA, contained in the 46 chromosomes you inherited from your parents.
Nucleic acids are
unique in their ability to direct their own replication.
The resemblance of offspring to their parents depends on
the precise replication of DNA and its transmission from one generation to the next.
It is this DNA program that directs the development of..
biochemical, anatomical, physiological, and (to some extent) behavioral traits.
Who used X-ray crystallography to study the molecular structure of DNA
Maurice Wilkins and Rosalind Franklin
Franklin’s X-ray crystallographic images of DNA enabled Watson to deduce that:
– DNA was helical,
– the width of the helix
– the spacing of the nitrogenous bases
– the DNA molecule was made up of two strands, forming a double helix
DNA: in April 1953
James Watson and Francis Crick proposed the double-helical model for the structure of DNA
DNA: in 1962
Crick, Watson and Wilkins were awarded the Nobel prize in Physiology/ Medicine.
DNA
deoxyribonucleic acid
DNA is a polymer of ____ each consisting of
NUCLEOTIDES
- A nitrogenous base, thymine, adenine, cytosine or guanine
- A sugar, deoxyribose
- A phosphate group
Nitrogenous bases of DNA
- Thymine
- Adenine
- Cytosine
- Guanine
Structure of DNA;
formation of sugar phosphate backbone
The P group of one nucleotide is attached to the sugar of the next to form the ‘backbone’ of the DNA molecule
Structure of DNA directionality:
from the 5’ end with the P group to the 3’ end with the OH group of the sugar.
DNA: 5’ refers to
the specific carbon of the sugar molecule to which the P group is attached
DNA: 3’ refers to
the carbon in the sugar molecule with the OH group.
The double helix of DNA is due to
the base pairing rules of the different nitrogenous bases.
Adenine and guanine are
purines with 2 organic rings.
Thymine (Uracil) and cytosine are
pyrimidines with a single ring
Adenine always pairs with thymine via
2 hydrogen bonds
Guanine pairs with cytosine via
3 hydrogen bonds
the two strands of DNA run
anti parallel to each other
5’ —–3’
3’——5’
The two strands are held together by
hydrogen bonds between the bases in the interior of the double helix
Watson and Crick noted that the specific base pairing suggested a
possible copying mechanism for genetic material
Each ‘daughter’ DNA molecule consists of
one parental strand and one new strand
DNA replication is a
Semi-conservative model of replication
Each nucleotide that is added to a growing DNA strand is a
Nucleoside triphosphate, adenine, thymine guanine or cytosine triphosphate. (dATP, dTTP, dGTP & dCTP)
DNA polymerases add nucleotides only to the free ___
3’ end of a growing strand
A new DNA strand can elongate only in the
5’ to 3’ direction
The copying of DNA is
remarkably fast and accurate
An E.coli cell has a ____ circular chromosome of about ______ nucleotide pairs which can be replicated to form 2 daughter cells in ~ __ hour.
An E.coli cell has a single circular chromosome of about 4.6 million nucleotide pairs which can be replicated to form 2 daughter cells in ~ 1 hour.
Humans have __ chromosomes that contain about _______ nucleotide pairs yet one cell take only ___ to replicate all the DNA
46
6 billion
a few hours
More than a dozen enzyme and other proteins
participate in DNA replication
DNA replication starts at sites called
Origins of replication
In circular chromosome of E. coil how many sites of origin of replications are there?
only one origin of replication
An origin of replication is a
short stretch of DNA with a particular sequence of bases
What happens at the origin of replication in E.coli
- Parental strands separate at the origin forming a replication bubble.
- The replication bubble has a fork at each end.
- Replication proceeds in both directions from each fork until the forks meet resulting in two daughter cells
In linear chromosomes of eukaryotes how many origins of replication are there?
MANY
in Linear chromosome what happens at the origins of replication
- Parental strands separate at the origins of replication forming replication bubbles.
- The replication bubbles have fork at either end.
- Replication bubbles expand as replication proceeds in both directions from each fork.
- Eventually the bubbles fuse resulting in the synthesis of two daughter molecules.
Separation of DNA strands at the Replication fork; what happens at the forks?
DNA strands are being unwound
Separation of DNA strands at the Replication fork; & PROTEINS
Several proteins participate in the unwinding of parental DNA strand including topoisomerases, helicases and single strand binding protein
Topoisomerase
Topoisomerase corrects “overwinding” ahead of replica6on forks by breaking, swiveling, and rejoining DNA strands
Helicases:
Helicases are enzymes that untwist the double helix at the replica6on forks
Single-stranded binding proteins
Single-strand binding proteins bind to and stabilize single-stranded DNA
The RNA primer is
Short and the 3’ end serves as the starting point for the new DNA strand
What are the enzymes called that
catalyse the synthesis of new DNA at a replication fork.
In E.coli there are several different DNA polymerases but ________ are important.
DNA polymerase III and I (DNA pol III and DNA pol I)
In eukaryotes there are at least ___ different DNA polymerases discovered so far but the general principles are the same
11
Most DNA polymerases require a
Primer and a DNA template strand
In E.coli DNA pol ||| adds a
DNA nucleotide to the 3’ end of the primer and then continues to add DNA nucleotides
DNA polymerases cannot initiate _____ they can only _____
synthesis of a polynucleotide; they can only add nucleotides to the 3ʹ end.
The initial nucleotide strand is a
short RNA primer
Primase:
is an enzyme that starts an RNA chain from scratch adding RNA nucleotides one at a tie using the parental DNA as a template
Synthesis of leading strand:
Along one template strand DNA pol III can synthesize
a complimentary strand continuously by elongating the DNA in the mandatory 5’ to 3’ direction.
Synthesis of leading strand:
DNA pol lll remains
in the replication fork on that template strand continuously adding nucleotides to the new complimentary strand as the fork progresses.
Synthesis of lagging strand:
to elongate the new strand
called the lagging strand, DNA polymerase must work in the direction away from the replication for
The lagging strands synthesised as a series of segments called
Okazaki fragments
Synthesis of lagging strand:
Primate synthesises short
RNA primer
Synthesis of lagging strand: DNA pol lll
starts DNA synthesis at the 3’ end of the primer and con6nues in the 5’ to 3’ direc6on un6l it reaches the fragment ahead
Synthesis of lagging strand:
DNA pol l
then replaces the RNA primer with DNA nucleo6des.
Okazaki fragments are joined together by
DNA ligase
DNA replication machine:
The proteins that participate in DNA replication form a large complex
Recent studies support a model in which DNA polymerase molecules
“reel in” parental DNA and “extrude” newly made daughter DNA molecules
the DNA replication machine may be
stationary during the replication process
A chromosome consists of
a DNA molecule packed together with proteins
The bacterial chromosome is a
double-stranded, circular DNA molecule associated with a small amount of protein.
In bacterium DNA is
“supercoiled” and found in a region of the cell called the NUCLEOID
Eukaryotic chromosomes have:
Linear DNA molecules associated with a large amount of protein. The complex of DNA and protein is called chromatin, and chromosomes are found in the nucleus of eukaryotic cells.
Chromosomes fit into the nucleus through an
elaborate, multi-level system of packing
Histones:
Proteins which are responsible of the first level of picking in chromatin
The nucleosome consist of
DNA wound twice around a protein core composed of 2
molecules of each of the 4 main histone types.
Next level of packing in eukaryotic chromosome results from
interactions between the histone tails of one nucleosome and the linker DNA and nucleosomes on either side. A fifthh histone (H1) is involved.
-The interactions cause the fibre to coil forming a chromatin fibre 30 nm thick
the 30 nm fibre forms loops called
looped domains attached to a chromosome scaffold made of proteins thus making up a 300 nm fibre.
In mitotic chromosome the looped domains themselves:
coil in a way that is not fully understood compacting the chromatin to produce a characteristic metaphase chromosome (700 nm in width)
Histones :
H2A, H2B, H3, H4
+ H1 at later stage
