LGS A-Level OCR Biology - Unit 2 - Nucleic Acids and DNA Replication

Deoxyribonucleic acid

ribonucleic acid

Nucleotides

phosphate group

pentose (5 carbon sugar) either deoxyribose or ribose

nitrogenous base

thymine

guanine

adenine

cytosine

uracil

thymine

adenine

guanine

cytosine

uracil

adenine

guanine

cytosine

Uracil

Thymine

condensation reactions occur

between the phosphate on carbon 5 of one nucleotide and the hydroxyl group on carbon 3 of another nucleotide

water is released

phosphodiester bond formed

Phosphodiester

From the 5’ end to 3’ end

Purines and pyrimidines

Double ringed

Single ringed

Adenine and guanine

Cytosine, thymine, and uracil

-DNA to be copied and transcribed

Hydrogen bonds

2

3

It must be rotated through 180 degrees

From 5’ to 3’ and the other from 3’ to 5’

The antiparallel nature of DNA

sample ground up using pestle and mortar

sample mixed with detergent

salt is added

protease is added

ice cold ethanol is added down the side of the test tube

it breaks down the cell walls

breaks down the cell membrane, releasing the cell contents into solution

neutralises charges on phosphates in sugar phosphate backbone

breaks the hydrogen bonds between DNA and water

makes DNA less soluble

breaks down the proteins associated with DNA

Causes the DNA to form a white precipitate between the layer of the sample and the ethanol

Reduces rate of enzyme controlled reactions that break down DNA

Helps to make DNA more insoluble

S phase

ensures the continuation of species

| - ensures cells and structures in a species are maintained

Each one of the two daughter molecules contains one original strand of DNA and one newly synthesised strand

the original strand acts as a template for a new strand

They are genetically identical

they are identical

DNA helicase

| DNA polymerase

unwinds DNA from histones

| - unzips DNA by breaking Hydrogen bonds betwem complementary base pairs

-catalyses the reaction to form phosphodiester bonds between the free nucleotides to form a sugar phosphate backbones

DNA helicase unwinds DNA from histones and unzips DNA by breaking Hydrogen bonds between complementary base pairs

free nucleotides are attracted to the exposed nucleotides on the strand of DNA by base complementarity

DNA polymerase forms phosphodiester bonds between nucleotides

two genetically identical molecules of DNA are formed

They are exposed

Free nucleotides are attracted to the exposed nucleotides by base complementarity and form hydrogen bonds

Exogenous come from our diet.

| We can also synthesis nucleotides.

From the cytoplasm through the nuclear pores

A short piece of RNA or DNA with a complementary sequence to part of the DNA template

5' to 3'

The leading strand is built continuously whereas the lagging strand is built discontinuously

The antiparallel nature of DNA

DNA polymerase requires them in order to bond to the 3' OH group

Okazaki fragments

The Meselsohn and Stahl experiment

Mass labelling and density centrifugation

N- 15

It could be incorporated into the nitrogenous bases and therefore track the movement of them

The nitrogenous bases

A CsCl gradient is produced by centrifugation

more dense molecules move to the bottom and lighter molecules to the top

When DNA is placed into this solution it migrates to the position in the gradient with the same buoyant density

CsCl (caesium chloride)

E.coli placed in growth factor containing N-15 (heavy nitrogen) which the E.coli incorporated into their DNA, DNA extracted and centrifuged with CsCl

E.coli placed in growth factor containing N-14 (normal nitrogen) which the E.coli incorporated into their DNA, DNA extracted and centrifuged with CsCl

N-15, the DNA settled further down the density gradient whilst the N-14 DNA was higher up the density gradient

the environment in which it grows can be carefully controlled

it would incorporate the different isotopes of Nitrogen into its DNA

it divides every 20 minutes

They allowed the position of where each N isotope sits after centrifugation depending on its density to be identified

if there was a mixing of the two isotopes a band would be seen inbetween

E.coli was grown in a growth media containing N-15 for several divisions

it was then transferred to a growth media containing N-14 and only allowed to divide once

the DNA was then extracted and centrifuged with CsCl

A band of DNA was formed inbetween the N-15 and N-14 bands

the initial molecule of DNA contained only N-15 nucleotides

as DNA replication is semi-conservative the DNA which was newly synthesised in the N-14 growth media would contain one strand with N-15 nucleotides and one strand with N-14 nucleotides

so the band would be in the middle

A band of DNA at the N-14 and N-15 positions

A band at N-14 and in the middle in the ratio 1:1

A band at N-14 and in the middle in the ratio 3:1

reduces occurrences of mutation

allows for formation of hydrogen bonds

DNA is replicated without error

the tubes were spun for the same amount of time

the tubes were spun at the same speed

the solution the DNA is put in is the same concentration

A change in the sequence of DNA bases within a gene

A base changing

evolution

| - genetic disorders

-DNA has a double stranded structure whilst RNA has a

single stranded structure

-In DNA A bonds to T and G bonds to C, in RNA A bonds to U and G bonds to C

-DNA is too big to leave the nucleus, RNA is small enough to leave the nucleus