7.2 DNA Replication
7.2.1 State that DNA replication occurs in a 5' - 3' direction
DNA replication is semi-conservative, meaning that a new strand is synthesised from an original template strand
DNA replication occurs in a 5' - 3' direction, in that new nucleotides are added to the C3 hydroxyl group such that the strand grows from the 3' end
This means that the DNA polymerase enzyme responsible for adding new nucleotides moves along the original template strand in a 3' - 5' direction
Direction of DNA Replication
7.2.2 Explain the process of DNA replication in prokaryotes, including the role of enzymes (helicase, DNA polymerase, RNA primase and DNA ligase), Okazaki fragments and deoxynucleoside triphosphates
DNA replication is semi-conservative and occurs during the S phase of interphase
Helicase unwinds and separates the double stranded DNA by breaking the hydrogen bonds between base pairs
This occurs at specific regions (replication origins), creating a replication fork of two polynucleotide strands in antiparallel directions
RNA primase synthesises a short RNA primer on each template strand to provide an attachment and initiation point for DNA polymerase III
DNA polymerase III adds deoxynucleoside triphosphates (dNTPs) to the 3' end of the polynucleotide chain, synthesising in a 5' - 3' direction
The dNTPs pair up opposite their complementary base partner (adenine pairs with thymine ; guanine pairs with cytosine)
As the dNTPs join with the DNA chain, two phosphates are broken off, releasing the energy needed to form a phosphodiester bond
Synthesis is continuous on the strand moving towards the replication fork (leading strand)
Synthesis is discontinuous on the strand moving away from the replication fork (lagging strand) leading to the formation of Okazaki fragments
DNA polymerase I removes the RNA primers and replaces them with DNA
DNA ligase joins the Okazaki fragments together to create a continuous strand
Overview of DNA Replication
7.2.3 State that DNA replication is initiated at many points in eukaryotic chromosomes
Because eukaryotic genomes are (typically) much larger than prokaryotic genomes, DNA replication is initiated at many points simultaneously in order to limit the time required for DNA replication to occur
The specific sites at which DNA unwinding and initiation of replication occurs are called origins of replication and form replication bubbles
As replication bubbles expand in both directions, they eventually fuse together, two generate two separate semi-conservative double strands of DNA
7.3 Transcription
7.3.1 State that transcription is carried out in a 5' - 3' direction
Transcription is carried out in a 5' - 3' direction (of the new RNA strand)
7.3.2 Distinguish between the sense and antisense strands of DNA
DNA consists of two polynucleotide strands, only one of which is transcribed into RNA
The antisense strand is transcribed into RNA
Its sequence will be complementary to the RNA sequence and will be the "DNA version" of the tRNA anticodon sequence
The sense strand is not transcribed into RNA
Its sequence will be the "DNA version" of the RNA sequence (identical except for T instead of U)
7.3.3 Explain the process of transcription in prokaryotes, including the role of the promoter region, RNA polymerase, nucleoside triphosphates and the terminator
A gene is a sequence of DNA which is transcribed into RNA and contain three main parts:
Promoter: Responsible for the initiation of transcription (in prokaryotes, a number of genes may be regulated by a single promoter - this is an operon)
Coding Sequence: The sequence of DNA that is actually transcribed (may contain introns in eukaryotes)
Terminator: Sequence that serves to terminate transcription (mechanism of termination differs between prokaryotes and eukaryotes)
Transcription is the process by which a DNA sequence (gene) is copied into a complementary RNA sequence and involves a
