1.2 - Structure & Replication Of DNA Flashcards
What does a molecule of DNA consist of?
2 strands each composed of repeating units called nucleotides.
What does a DNA nucleotide consist of?
- a molecule of deoxyribose sugar
- a phosphate group
- an organic base
How many carbons does the deoxyribose sugar contain?
Deoxyribose is a 5 carbon sugar
What part of the DNA nucleotide is attached to carbon 5 of the deoxyribose sugar?
The phosphate group is attached to carbon 5 of the deoxyribose sugar.
What is attached to carbon 1 of the deoxyribose sugar?
The organic base is attached to carbon 1 of the deoxyribose sugar.
What join together to form the DNA molecules sugar phosphate backbone?
Nucleotides join together to form the DNA molecules sugar-phosphate backbone.
Sugar-phosphate backbone - bonding
A strong chemical bond forms between the phosphate group of one nucleotide and the carbon 3 of the deoxyribose sugar on another nucleotide.
Complementary base pairing
DNA is a double helix which means there are 2 strands of nucleotides joined together.
The 2 strands of nucleotides are joined together by weak hydrogen bonds between their complementary bases.
- Adenine-Thymine, Cytosine-Guanine.
are known as the complementary base pairs.
Antiparallel strands
The deoxyribose sugar is at the 3’ end and the phosphate at the 5’ end of each strand.
The chain is only able to grow by adding nucleotides to its 3’ end.
This leads to the two DNA strands with their Sugar-phosphate backbones running in opposite directions. This is described as antiparallel.
Double helix
In order for the complementary base pairs to align with each other, the 2 strands in a DNA molecule form a twisted coil called a double helix.
The Sugar-phosphate backbones are on the outside and the complementary base pairs are on the inside (of the DNA molecule).
What does the base sequence of DNA form?
The genetic code
Where does DNA Replication take place?
DNA replication is performed at the beginning of cell division.
DNA replication takes place in the nucleus of the cell and is the process by which a cell makes an identical copy of its DNA.
Whar does DNA replication ensure?
DNA replication ensures that an exact copy of a cells genetic information is passed on to each new daughter cell making it genetically identical to the original cell.
DNA replication therefore maintains the chromosome complement in the new daughter cell.
Requirements for DNA replication
For DNA replication to occur, the nucleus must contain:
- DNA to act as a template
- primers
- a supply of the 4 types of nucleotides.
- enzymes - DNA polymerase, ligase
- supply of ATP for energy
Beginning DNA replication
- The shape of DNA is a double helix
- Before DNA can be replicated, it must first be unwound.
- The weak hydrogen bonds between the bases are then broken to form two template strands.
What enzyme replicates DNA
DNA polymerase
Why are primers needed
DNA is replicated by the enzyme DNA polymerase.
DNA polymerase can only add nucleotides to a pre-existing chain of nucleotides.
Therefore for DNA polymerase to function, a primer must be present.
A primer is a short strand of nucleotides which binds to the 3’ end of the template DNA strand allowing DNA polymerase to add DNA nucleotides.
What is a primer
A primer is a short strand of nucleotides which binds to the 3’ end of the template DNA strand allowing DNA polymerase to add DNA nucleotides.
Adding nucleotides
DNA polymerase adds DNA nucleotides,.using complementary base pairing, to the deoxyribose 3’ end of the template DNA strand.
DNA polymerase can only add DNA nucleotides is one direction.
This results is the formation of a leading strand and lagging strand.
Leading strand
On the leading strand, nucleotides are added from the 3’ end of the parental DNA molecule continuously.
The primer is then replaced by DNA.
Lagging strand
The lagging strand is antiparallel to the leading strand but its replication cannot begin at the 5’ end.
The lagging strand is instead replicated in fragments.
Replication of the lagging strands starts with primer at the 3’ end and adds nucleotides discontinuously as more primer is added.
The fragments are joined together by the enzyme ligase.
The primer is the replaced by DNA.
What technique is polymerase chain reaction (PCR)
Polymerase chain reaction (PCR) is a technique used to create many copies of a piece of DNA. This is known as DNA amplification.
How does PCR amplify DNA?
PCR amplifies DNA using complementary primers for specific target sequences.
PCR - primers
In PCR, primers are short strands of nucleotides which are complementary to specific target sequences at the two ends of the region of DNA to be amplified.
Polymerase chain reaction - stages
- DNA is heated between 92°C and 98°C to break the hydrogen bonds between the bases and separate the two strands.
- The DNA is then cooled to between 50°C and 65°C to allow primers to bind to the target sequences.
- DNA is then heated to between 70°C and 80°C for heat-tolerant DNA polymerase to replicate the region of DNA.
- Repeated cycles of heating and cooling, amplify this region of DNA.
PCR cycles
Each PCR cycle takes approximately 5 minutes.
PCR causes exponential growth in the number of DNA molecules present.
Practical applications of PCR
Throught the use of PCR, a tiny amount of DNA can be amplified to provide enough material to:
- help solve crimes
- settle paternity suits
- diagnose genetic disorders
DNA shape and what holds it together
DNA is a double helix which means there are 2 strands of nucleotides joined together.
The 2 strands of nucleotides are joined together by weak hydrogen bonds between their complementary bases.
Nucleotides on complementary strands difference
The nucleotides on complementary strands of DNA are exactly the same but are antiparallel (opposite directions)
What holds the 2 strands on nucleotides together
The 2 strands of nucleotides are joined together by weak hydrogen bonds between their complementary bases.
What determines an organisms genotype?
An organisms genotype is determined by the sequence of bases.
