Eukaryotic Gene Organisation & Transcription Flashcards
Describe histones
Histones are proteins around which DNA is wound. Histones are the chief protein component of chromatins.
Describe chromatins
Consisting mainly of DNA and histones, chromatins are the substances in the nucleus which condense to form chromosomes during cell division
A piece of DNA around 146 bp long becomes wrapped around _____ histones to form a ____
A piece of DNA around 146 bp long becomes wrapped around 8 histones to form a structure known as a nucleosome.
When nucleosomes are packed tightly together . . .
When nucleosomes are packed loosely together . . .
When nucleosomes are packed tightly together RNA polymerase struggles to bind to the promoter region
When nucleosomes are packed loosely together RNA polymerase can bind to the promoter region more easily
When the nucleosomes are tightly compacted, this region of the chromosome is called ____
Heterochromatin
When the nucleosomes are spaced further apart, this region of the chromosome is called ____
Euchromatin
Describe the enhancer region
A region often 1000s of base pairs upstream of the gene. Transcriptional Activators bind here, & interact with General Transcription Factors & RNA polymerase at the Promoter which increases the assembly rate of RNA polymerase, and so speeds up the transcription rate.
Describe the promoter region
The region upstream of the strucutral genes where general transcription factors bind and help to assemble RNA polymerase
What is meant by a sense strand
In transcription only one DNA strand is copied. The sense strand does NOT act as a template and the mRNA strand formed by transcription will be identical to it (except from U instead of T) rather than complimentary to it
What is meant by an anti-sense strand
In transcription only one DNA strand is copied. The strand that is copied is known as the anti-sense strand. The mRNA strand produced by transcription will be complimentary to the anti-sense strand.
Name the 3 stages of transcription
- Initiation
- Elongation
- Termination
Describe the initiation stage of transcription
Transcription begins when one of the general transcription factors binds to the “promoter” located before the start of the gene.
(This can be assisted by Transcriptional Activators bound to Enhancer sequences located thousands of bp upstream of the promotor)
The transcription factor orientates at the promoter so that RNA polymerase will attach to the antisense strand of the DNA.
More General transcription factors then assemble, and recruit and assemble the RNA polymerase subunits until the active RNA polymerase holoenzyme is formed.
Transcription then begins upstream of the start codon
Describe the elongation stage of transcription
RNA polymerase unwinds and ‘unzips’ the DNA and builds a strand of mRNA that is complementary to the antisense strand using free ribonucleotides.
These ribonucleotides are floating free in the nucleus and align alongside complementary nucleotides in the DNA strand (A ribonucleotides align with T DNA nucleotides; C with G; G with C and U with A).
Hydrogen bonding holds the ribonucleotides in place.
Enzyme catalyses the forming of the phosphodiester bonds to create the RNA sugar-phosphate backbone in the 5’ to 3’ direction.
Describe the termination stage of transcription
Elongation stops at a termination sequence
Causes mRNA transcript to detach from the DNA antisense strand
RNA Polymerase also detaches and the DNA re-forms double helix
