2.7 + 7.2 DNA transcription and gene expression

Question 1

Define DNA transcription

Answer 1

DNA transcription: mRNA synthesis from a DNA gene

• in interphase (S) - only in this phase chromosomes despiralise - can be read

Question 2

Sequence of transcription + enzymes

Answer 2

1. INITIATION: RNA polymerase binds to promoter sequence - unwinds DNA - NTPs (E) used for mRNA synthesis from template strand (antisense)
2. ELONGATION: synthesis of mRNA in 5’ to 3’ end
3. TERMINATION: gene synthesised until terminator sequence - RNA polymerase and mRNA detach from template strand (antisense)

RNA trasncript is identical to sense strand (5’-3’) and complementary to antisense strand (3’-5’)

Question 3

Sections of a gene

Answer 3

• 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

Question 4

Sense strand vs antisense strand

Answer 4

SENSE strand: coding strand, not transcribed, identical to mRNA (T->U)

ANTISENSE strand: template strand, transcribed, complementary to mRNA

Question 5

Post-transcriptional modifications

Answer 5

1. Capping: addition of methyl group to 5’ end, protection against exonucleases, can be recognised by cell’s translation machinery
2. Polyadenylation: addition of poly-A tail to 3’ end, improves stability, facilitates export out of nucleus
3. Splicing: introns are removed - mature mRNA
4. Alternative splicing: removal of some exons - different proteins from same gene

Question 6

Exons vs introns

Answer 6

INTRONS - intruding sequence

EXONS - expressing sequence

Question 7

What regulates gene expression?

Answer 7

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)

Question 8

Types of control elements in transcription

Answer 8

Distal control elements: distant from promoter, regulatory proteins bind

Proximal control elements: close to promoter, transcription factors bind

Question 9

Examples of environmental influences on gene expression

Answer 9

1. Hydrangeas: pH of soil determines - pink/blue flowers
2. Himalayan rabit produces different fur colour depending on the temperature
3. Humans produce different amounts of melanin depending on light exposure
4. Certain fish/reptiles change sex depending on social cues

Question 10

Histone tail modifications and their influence on gene expression

Answer 10

ACTIVATION - DEACTIVATION of genes by nucleosome packing

1. Acetylation: neutralises the + charge of histone tails - DNA less tightly coiled - increases transcription of those genes - EUCHROMATIN
2. Methylation: maintains + charge of histone tails - DNA more coiled - transcription of those genes reduced - HETEROCHROMATIN
3. Phosphate group addition

Question 11

DNA methylation

Answer 11

DNA methylation decreases gene expression - prevents transcription fcator binding

Question 12

Define epigenetics

Answer 12

EPIGENETICS: study of changes in phenotype due to variation in gene expression levels

Shows DNA methylation patterns - change over life

MEethylation infleunced by:

• heritability but not genetically pre-determined (identical twins different methylation patterns)
• environmental effects (diet, exercise, smoking)

Question 13

Reverse transcription

Answer 13

In retroviruses - by reverse transcriptase - cDNA (complemenatry) 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)