DNA and genes (L2-7) Flashcards
(52 cards)
What is the structure of DNA?
2 helical anti-parallel polynucleotide chains made of 4 bases
What are the purines
Adenine and guanine
What are the pyrimidines
Thymine and cytosine
What are the nucleotide base pairings?
A + T (2 H-bonds formed)
G+C (3 H-bonds formed)
What is DNA replication?
semi-conservative
This means each of the newly synthesised double helix is made of 1 parent strand and 1 newly synthesised strand
Parental strand acts as a template for the synthesis of the new strand
Summarise DNA synthesis
It’s the sequential polymerisation of nucleotide building blocks by DNA Polymerase
Incoming nucleotides can only be added too the free hydroxyl on the 3’ carbon terminal Deoxyribose
What is the overall polarity of DNA strands
5’ to 3’, orientated antiparallel to each other
Formation of base pairs require 2 DNA strands to adopt an antiparallel orientation
Summary of the DNA replication mechanism
Synthesis of new strands initiated by formation of replication fork where strands are pulled apart/separated
At the replication fork, 2 new strands are synthesised and have antiparallel orientation as DNA synthesis is only in 5’->3’ direction
What is the role of DNA helicase
Used to pull apart DNA strands in the first steps of DNA synthesis
DNA Helicase continuously unwinds parent DNA to give DNA Primase and Polymerase access to DNA template sequence
What is the role of DNA polymerase
synthesising new strands
Why can the new strands NOT be synthesised continuously?
Due to the antiparallel orientation of the parent stands and unidirectional orientation of the new DNA strand
What is the leading strand
It is continuous and proceeds the lagging strand
What is the lagging strand?
It is discontinuous and made of Okazaki fragments
What is the role of RNA primer?
Initiate DNA synthesis by extension of RNA primer
All Okazaki fragments and leading strands are intitated as short RNA molecules by a special RNA polymerase DNA Primase
What is the role of single-stranded Binding Proteins (SSBs)
they maintain the unwound parent DNA strands in a single stranded confirmation to ease replication fork progression
When does Okazaki fragment synthesis stop?
When DNA Polymerase reaches RNA Primer @ the 5’ end of another fragment
What happens to the RNA Primers by the end?
They are extended by DNA Polymerase then removed by Ribonuclease H and the gap is filled by DNA Polymerase
What does DNA ligase do by the end of Replication?
It covalently links the Okazaki fragments of the lagging strand to a continuous DNA strand
What is gene expression?
Gene expression is the process by which the heritable information in a gene, the sequence of DNA base pairs, is made into a functional gene product, such as protein or RNA. The basic idea is that DNA is transcribed into RNA, which is then translated into proteins
What are the 2 main steps of gene expression
- Transcription
mRNA synthesis - Translation
protein synthesis
What is the structural features of RNA molecules
- RNA backbone is ribose sugars and not deoxyribose sugars
2. RNA transcripts contain Uracil instead of Thymine, Uracil pairs with Adenine and still forms 2 hydrogen bonds
How is transcription initiated in prokaryotic genes?
By binding of RNA polymerase to DNA sequences in the promoter region which lies upstream of the transcription start site
This defines the position at which synthesis of RNA transcripts begin
What is transcript elongation and what does it do?
The stage where the RNA strand gets longer
Done by RNA Polymerase which yields and mRNA transcript (this is a copy of the DNA sequence)
Transcripts are single stranded
They are synthesised in a 5’-3’ direction
How is transcription in prokaryotes terminated?
When RNA Polymerase encounters the terminator region which promotes dissociation of RNA Polymerase from the DNA
