7.1

Hershey and Chase experiment→ Isotopic mediums used

Radioactive SULFUR→ Proteins

Radioactive PHOSPHORUS→ DNA

Pellet

The insoluable portion after centrifugation

Supernatant

The soluable portion after centrifugation

Result of the Hershey and Chase experiment

-Radioactive phosphorus in the PELLET

-Radioactive sulphur in the SUPERNATANT

Process of X-ray diffraction

-DNA fibres streched in a thin glass tube

-X-ray beam is targetted to the tube, it diffracts when it contacts an atom

-The structure is recorded on film

Oucomes of the X-ray diffraction

-DNA is a double-stranded molecule

-Phosphates form an outer backbone

-DNA twists at regular intervals o form a helix (every 34 Angstrom)

Helicase

-unwinds the double-stranded DNA by breaking the hydrogen bonds between base pairs

-creates the replication fork

Topoisomerase (DNA Gyrase)

-reduces the stain created by the undwinding of DNA

-Keeps the DNA from supercoiling

Single Stranded Binding Proteins

-Bind to the DNA strands after they have been separated and prevent the strands from re-annealing

-Prevent the single stranded DNA from being digested by nucleases

Primase

-Generates a short RNA primer(~10–15 nucleotides) on each of the template strands

-The RNA primer provides an initiation point for DNA polymerase III, which can extend a nucleotide chain but not start one

DNA Polymerase III

-Builds the new strand in a 5’ to 3’ direction

-It moves from 3’ to 5’ in the template strand

-It attaches the free nucleotides which are aligned opposite to their complementary bases

Replication Bubble

Open region of DNA where replication occurs

Okazaki Fragments

Synthesising of the DNA in pieces created Okazaki fragments in the lagging strand

DNA Replication is…

Semi-conservative

DNA Polymerase I

Removes the RNA primers from the lagging strand and replaces them with DNA nucleotides

DNA Ligase

Joins Okazaki fragments together to form a continuous strand by covalently joining the sugar-phosphate backbones together with a phosphodiester bond

Free nucleotides exist as…

-deoxynucleoside triphosphates (dNTPs)

-they have 3 phosphate groups

Energy required for DNA replication comes from…

-DNA polymerase splits the two additional phosphates

-uses the energy released to form a phosphodiester bond with the 3’ end of a nucleotide chain

Dideoxynucleotides- how are they different

Dideoxynucleotides (ddNTPs) lack the 3’-hydroxyl group necessary for forming a phosphodiester bond

Dideoxynucleotides

-ddNTPs prevent further elongation of a nucleotide chain and effectively terminate replication

Purines

-Adenine

-Guanine

(2 rings)

Pyrimidines

-Thymine

-Cytosine

(1 ring)

Coding DNA

EXONS

-Very small percentage (about 1.5%)

-codes for our characteristics

Non-coding DNA

-Large percentage

-Structural function

-“junk” DNA

-doesn’t code for proteins

-includes genes

Introns

-Non-coding sequences WITHIN genes

-are removed by RNA splicing prior to the formation of mRNA

Gene

A section of DNA (sequence of nucleotides) that codes for a protein (a sequence of amino acids)

Functions of Introns

-production of RNA

-gene expression (promotes or inhibits genes)

-telomeres

Telomeres

-located on the ends of eukaryote chromosomes

-protective function: DNA cannot replicate all the way to the ends so telomeres prevent loss of important genes

Short tandem repeats

-Structural component of heterochromatin and centromere

-Satellite DNA

-Non-coding

Uses of STRs

-Cut using restriction enzymes and then separated with gel electrophoresis

-individuals have different numbers of repeats at a given satellite DNA locus, they have unique DNA profiles

Structure of nucleosomes

-Eight histone proteins (an octamer) form a complex called a nucleosome

-Nucleosomes are linked by an additional histone protein (H1 histone) to form a string of chromatosomes

-These coil to form a solenoid structure which condense to form a fibre

-These fibres then form loops, which are compressed and folded around a protein scaffold to form chromatin