22. Transcription Flashcards
Define DNA transcription
DNA transcription: mRNA synthesis from a DNA gene
- in interphase (S) - only in this phase chromosomes despiralise - can be read
Sequence of transcription + enzymes
- INITIATION: RNA polymerase binds to promoter sequence - unwinds DNA - NTPs (E) used for mRNA synthesis from template strand (antisense)
- ELONGATION: synthesis of mRNA in 5’ to 3’ end
- TERMINATION: gene synthesised until terminator sequence - RNA polymerase and mRNA detach from template strand (antisense)
RNA transcript is identical to sense strand (5’-3’) and complementary to antisense strand (3’-5’)
Sections of a gene
- Promoter: non-coding, initiates trasncription, binding site for RNA polymerase (controlled by transcription factors in euk.)
- Coding sequence: transcribed by RNA polymerase into mRNA
- Terminator: non-coding, stops transcription, RNA polymerase detaches
Sense strand vs antisense strand
SENSE strand: coding strand, not transcribed, identical to mRNA (T→U)
ANTISENSE strand: template strand, transcribed, complementary to mRNA
Post-transcriptional modifications
- Capping: addition of methyl group to 5’ end, protection against exonucleases, can be recognised by cell’s translation machinery
- Polyadenylation: addition of poly-A tail to 3’ end, improves stability, facilitates export out of nucleus
- Splicing: introns are removed - mature mRNA
- Alternative splicing: removal of some exons - different proteins from same gene
Exons vs introns
INTRONS - intruding sequence
EXONS - expressing sequence
What regulates gene expression?
TRANSCRIPTION FACTORS: from a complex with RNA polymerase - transcription cannot be started without - levels regulate expression
REGULATORY PROTEINS: bind to DNA sequences (control elements) and interact with transcription factors, enhancer (activator, increases rate of transcription) - silencer (repressor, decreases rate of transcription)
Types of control elements in transcription
Distal control elements: distant from promoter, regulatory proteins bind
Proximal control elements: close to promoter, transcription factors bind
Examples of environmental influences on gene expression
- Hydrangeas: pH of soil determines - pink/blue flowers
- Himalayan rabit produces different fur colour depending on the temperature
- Humans produce different amounts of melanin depending on light exposure
- Certain fish/reptiles change sex depending on social cues
Histone tail modifications and their influence on gene expression
ACTIVATION - DEACTIVATION of genes by nucleosome packing
- Acetylation: neutralises the + charge of histone tails - DNA less tightly coiled - increases transcription of those genes - EUCHROMATIN
- Methylation: maintains + charge of histone tails - DNA more coiled - transcription of those genes reduced - HETEROCHROMATIN
- Phosphorylation: phosphate group addition
DNA methylation
DNA methylation decreases gene expression - prevents transcription factor binding
Define epigenetics
EPIGENETICS: study of changes in phenotype due to variation in gene expression levels
Shows DNA methylation patterns - change over life
Methylation influenced by:
- heritability but not genetically pre-determined (identical twins different methylation patterns)
- environmental effects (diet, exercise, smoking)
Reverse transcription
In retroviruses - by reverse transcriptase - cDNA (complementary) transcribed from mRNA - cDNA represent sequences which are actively transcribed
- used to determined gene expression in healthy - diseased
- cDNA used in gene transfer (don’t possess introns)
Explain codons how many of them
- codons code for specific amino acids, three bases translated into one amino acid (reading frame)
- 64 codons in total: 61 code for the 20 possible amino acids, 3 STOP codons
- Several codons can code for the same amino acid, but one codon corresponds only to one amino acid
- AUG (Met) start codon
What are the three RNA polymerases and what to they synthesise?