Lecture 8 - DNA replication, transcription and translation Flashcards
Where does DNA replication begin along the DNA molecule?
At origins of replication.
Describe this image.
The double helix is broken apart and two polynucleotide strands open forming replication forks. New strands grow at forks- they read along from the origin of replication.
DNA is copied at many points along the DNA molecule. As DNA strands open along the DNA molecule, replication bubbles form. The bubbles eventually get bigger, join and we have identical copies of both DNA strands.
Role of enzyme helicase in DNA replication.
Enzyme helicase unwinds and separates the two strands by breaking the hydrogen bonds between base pairs
Role of single strand binding proteins in DNA replication
Single-strand binding proteins attach to each strand and keep the two DNA strands separated and untwisted.
Role of topoisomerase
Topoisomerase binds further along the DNA molecule. It attaches to the two forks of the bubble and its role is to relieve stress on the DNA molecule caused from the unwinding/separating of the double helix.
Role of primase enzyme and primase.
RNA primers are synthesised by primase enzyme- the primers kick start the addition of new nucleotides
Describe the work of DNA polymerase in DNA replication. Explain the direction in which the new strand is built and bonds that form in the process.
DNA polymerase moves along the strand and adds nucleotides to the 3’end of the DNA so the new strand is built in a 5’ to 3’ direction. Phosphodiester bonds form between the sugar and phosphates of the new nucleotides and hydrogen bonds from between complementary base pairs. Same process on repeat to form the polynucleotide strand.
As we know, DNA polymerase only adds new nucleotides to the …… end of the template strand so the new strand is made in the …….-……… direction.
3’ end
5’-3’ direction
The DNA polymerase can only add the new nucleotides to the 3’ end of the template strand so makes the new strand in the 5’ to 3’ direction, as we know.
But, we also know that the two polynucleotide strands in a DNA molecule are antiparallel. So, what is the effect?
In DNA replication we have a leading strand and a lagging strand.
What is the leading strand?
The leading strand is synthesized as a single strand from the origin towards the opening of the replication fork.
Describe the lagging strand.
The lagging strand is synthesized discontinuously- in small fragments (Okazaki fragments) against the overall direction of replication- from the replication fork towards the origin.
What enzyme joins Okazaki fragments together to make one continuous strand?
DNA ligase.
DNA must be proofread. Explain.
- DNA polymerase will make about 1 in 10,000 base paring errors.
- DNA polymerase can proofread and correct the mistakes. It can tell if something isn’t base paired correctly and will chop it off.
What does semi conservative replication mean?
The two strands of the parental molecule separate and each strand acts as a template for a new complementary strand. New DNA consists of 1 parental/original and 1 new strand of DNA.
DNA is not just used to store genetic info but also to synthesise proteins, therefore determining the cell structure and function. What is the definition of gene expression?
Gene expression is defined as the transformation of DNA information into functional molecules.
Two stages of protein synthesis.
transcription and translation.
What is transcription?
The production of a copy of a section of DNA carrying information related to one or more genes as a messenger mRNA molecule.
How does transcription differ from RNA replication?
The enzyme is RNA polymerase
RNA polymerase does not require a primer
RNA polymerase does not need to proof read the synthesis
the sugars in the nucleic acid backbone are ribose not deoxyribose
the four bases used are adenine, guanine, cytosine and uracil
mRNA synthesis starts at many different small sections on the DNA sequence and stops at specific places
a single stranded mRNA molecule is produced
In transcription of prokayrotes, there is a position along the DNA molecule at which transcription starts. What is the section of DNA called?
promoter region.
Every gene- every section of DNA which codes for a protein- has a promoter region. What is it’s role? If one or more genes share a promoter what is it called?
The promoter directs the RNA polymerase to move along the strand in only one direction. If one or more genes share a promoter, this group of genes is described as an operon.
Give the stages of transcription.
- RNA polymerase binds to promoter sequence in duplex DNA- closed DNA complex
- The RNA polymerase breaks apart the duplex DNA near transcription start site forming a transcription bubble- open complex
- The RNA polymerase catalyses phosphodiester linkages of two initial ribonucleotides.
- RNA polymerase moves along the template strand in the 3’-5’ direction, breaking apart the duplex DNA molecule and joining one ribonucleotide at a time to the growing RNA polynucleotide chain. We name this step elongation.
- Termination: at the transcription stop site, polymerase releases and the complete RNA dissociates from DNA.
What is translation?
The sequence of information on the mRNA molecule is used to synthesise a polypeptide strand- a sequence of amino acids
Where does translation take place?
ribosomes
During translation, three bases along the mRNA are read at a time. What do we call these three bases?
triplet/codon
