Prok & Euk Part 1: Organisation Flashcards
What are introns?
Non-coding sequences found within a gene
What is the function of introns?
- Introns have no involvement in the translation of an mRNA and hence are excised. During splicing, introns are excised and exons are subsequently joined together to form a mature RNA
- Alternative RNA splicing of a single pre-mRNA can produce different mature RNA depending on which combination of exons are spliced together, meaning there can be more than one type of polypeptide.
What are promoters?
Non-coding DNA sequences located just upstream of the transcription start site of a gene
What are the functions of promoters?
It serves as a recognition site for the binding of general transcription factors and RNA polymerase to start transcription. Sequences within promoter determines the strength of promoter and in turn efficiency of RNA polymerase and frequency of transcription.
What specific sequences can be found in a promoter?
TATA box, CAAT and GC box
What is the function of the TATA box?
It functions as the recognition and binding site for general transcription factors which recruits RNA polymerase to form transcription initiation complex, thus it determines precise location of transcription start site.
What are enhancers and silencers?
Short non-coding DNA sequences located far away from the promoter.
What is the function of enhancers?
It helps to increase transcription frequency. It binds to activators, specific transcription factors, promoting assembly of a transcription initiation complex.
What is the function of silencers?
It helps to decrease transcription frequency. It binds to repressors, specific transcription factors, inhibiting the assembly of a transcription initiation complex
What are telomeres?
Telomeres are non-coding regions of DNA made up of a series of tandem repeat sequences found at both ends of eukaryotic chromosomes.
What are the functions of telomeres?
- Telomeres ensure genes are not lost/eroded with each round of DNA replication due to the end replication problem, preventing loss of viral genetic information.
- Telomeres protect and stabilise the terminal ends of chromosome
- Telomeres alow their own extension, by providing an attachment point for the correct positioning of the enzyme telomerase.
What are centromeres?
Constricted regions on chromosomes, with repetitive non-coding DNA, where spindle fibres attach to during nuclear division. It can be located anywhere along length of chromosome.
What are the functions of centromeres?
- Centromeres ensure proper nuclear division by allowing sister chromatids to adhere to each other.
- Centromeres ensure proper nuclear division by allowing proteins called kinetochore proteins and subsequently spindle fibres to attach so that sister chromatids can be separated to opposite poles.
What is the function of the CAAT and GC boxes?
Although they are not always present but it improves efficiency of promoter by helping to recruit general transcription factors and RNA polymerase to the promoter
How does telomerase work?
- A short 3-nucleotide segment of RNA within telomerase binds to part of a DNA repeat in the 3’ overhang by complementary base pairing.
- The adjacent part in telomerase used as template.
- Telomerase synthesizes phosphodiester bonds btween DNA overhang and incoming DNA
- After repeat is made, telomerase translocates 6 nucleotides to the right in the 5’ to 3’ direction of overhang and begins to make another repeat
- Primase makes an RNA primer near the end of the telomere. DNA polymerase adds nucleotides to the end of the primer, synthesizing a complementary strand. The nick is sealed by ligase and primer eventually removed.
