Structure and replication of DNA (Base pairing (The ends of the DNA…
Structure and replication of DNA
structure of DNA
holds instructions for every living thing, double helix
held by complementary base pairs
basic units of DNA are nucleotide s,consist of deoxyribose sugar, phosphate and base.
nucleotide are identical expect for base can be an adenine, thymine, guanine or cytosine.
there are chemical cross links between two strands of DNA
formed by pairs of bases held by hydrogen bonds
always pair up in specific duo C-G A-T
these basic units form strong bonds between deoxyribose sugar and phosphate these form sugar phosphate back bone
The ends of the DNA strand are called the 5' at the phosphate end and 3' at the deoxyribose
the two strands are anti parallel one runs 5' to 3' and 3' 5'
this creates twisting of DNA structure
all cells store information in base sequence of DNA
Organization of DNA
two groups in how DNA is organized , eukaryotes and prokaryotes.
bacteria are prokaryotes
they do not have a membrane bound nucleus
and DNA is free in cytoplasm
bacteria have one circular chromosome in the center of the cell
holds all the genes needed for the bacteria
also have extra circles of DNA called plasmids
these plasmids contain additional genes such as antibiotic resistance
animals plants and fungi are eukaryotes
they have a membrane bound nucleus
and their chromosomes are linear rather than circular
the DNA found in the linear chromosome is tightly coiled
packaged around special proteins called histones
found in nucleus
circular chromosomes are also found in mitochondria and chloroplast
they use DNA to make proteins needed for their function
DNA replication is the process by which a cell makes an identical copy of its DNA
performed at the beginning of every cell division
so that when the cell divides, each daughter cell will inherit an identical copy of the DNA.
Requirements for DNA replication
Original DNA template
Each strand can be used as a template to create a new DNA molecule.
Free DNA nucleotides
needed to form the new strands.
an enzyme that adds new nucleotides to a growing strand of DNA.
A primer is a short strand of nucleotides
Binds to 3'
allowing DNA polymerase to add free DNA nucleotides.
DNA Replication stages
DNA is unwound and unzipped
helix structure is unwound. Special molecules break the weak hydrogen bonds
process occurs at several locations on a DNA molecule.
DNA polymerase will add the free DNA nucleotides to the 3' this will allow the new DNA strand to form.
Adenine pairs with thymine, thymine with adenine, cytosine with guanine
and guanine with cytosine. A primer is needed to start replication.
Leading strand is synthesised continuously. DNA polymerase adds nucleotides to the deoxyribose (3’) ended strand in a 5’ to 3’ direction.
laging strand is synthesised in fragments. Nucleotides cannot be added to the phosphate (5’) end
because DNA polymerase can only add DNA nucleotides in a 5’ to 3’ direction. The lagging strand is therefore synthesised in fragments.
The fragments are then sealed together by an enzyme called ligase.
The two new strands twist to form a double helix. Each is identical to the original strand.
DNA Replication (PCR)
The Polymerase Chain Reaction (PCR) is a technique for the amplification of DNA in vitro
PCR amplifies DNA using complementary primers for specific target DNA sequences.
allows scientists to easily and cheaply turn a specific target sequence of DNA into millions of copies
The practical applications of PCR are to amplify target sequences of DNA to help solve crimes, settle paternity suits and diagnose genetic disorders.
Requirements for PCR
DNA the original strand of DNA which needs amplified.
Complementary primers primers are short complementary sequences of nucleotides needed to start DNA synthesis.
Thermal cycler equipment that varies the temperature of the reaction.
Heat-tolerant polymerasean enzyme which will add nucleotides to the growing strand and which is not denatured by the high temperatures used in the reaction.
Supply of nucleotides to synthesise the new strands of DNA.
The PCR process
DNA heated to between 92 and 98°C- to denature the DNA and separate the two strands.
DNA cooled to between 50 and 65°C - to allow primers to bind to target DNA sequences.
Complementary primers added - which are complementary to the target sequences at the two ends of the region to be amplified.
Heated to betweeen 70 and 80°C -tolerant DNA polymerase added - which replicates the region of DNA to be amplified. Two strands are formed.
Repeated cycles of heating and cooling amplify the target region of DNA.