Molecular Genetics Transcription

Question 1

What are the 3 main stages of Transcription and Translation?

Answer 1

3 main stages: initiation, elongation, termination

Question 2

Initiation

Answer 2

-RNA polymerase binds to a promoter region on the DNA

-Promoter region: a sequence of nucleotides in DNA that indicates where the RNA polymerase complex should bind to initiate transcription

-Key element of the promoter in eukaryotes is the TATA box (a portion of DNA with high percentage of Adenine and Thymine bases)

-Prokaryotes have a TATATT sequence instead for this -RNA polymerase binds to a promoter region on the DNA same purpose

Question 3

TATA box

Answer 3

-Adenine and thymine share only 2 hydrogen bonds

-Less energy is needed to break 2 bonds, RNA polymerase uses less energy opening up DNA helix in this region

Question 4

Elongation

Answer 4

-RNA polymerase complex works its way along DNA molecule

-RNA polymerase, unlike DNA polymerase, can begin making the complementary copy without needing a primer to be already in place

-Synthesizes mRNA strand that is complementary to template strand of DNA

-T is replaced with U

-RNA polymerases work in the 5ʼ → 3ʼ direction, using the 3ʼ→ 5ʼ DNA strand as a template strand.

Question 5

Defintion of Coding Strand, template strand, orientation of RNA strand being produced.

Answer 5

The opposite strand of DNA—the strand that is not being copied is known as the coding strand

-Coding strand: the DNA strand that is not being copied but contains the same sequence as the new RNA molecule

-The template strand contains the sequence that is complementary to the sequence that is going to be transcribed

-the beginning of the RNA strand is the 5ʼ end, and the other end is the 3ʼ end

Question 6

Elongation Pt 2 (Starts at as RNA polymerase moves along the DNA)

Answer 6

As RNA polymerase moves along the DNA, it unwinds the DNA at the forward end of the enzyme

-RNA strand grows as nucleotides are added, one by one forming a temporary RNA-DNA double helix with the template strand

-As the RNA polymerase passes, the DNA double helix reforms

-Once an RNA polymerase molecule has started transcription and progressed past the beginning of a gene, another molecule of RNA polymerase may start producing another RNA molecule if there is room at the promote

Question 7

Termination

Answer 7

The transcription is terminated when RNA polymerase recognizes a termination sequence

Question 8

Poly-A polymerase

Answer 8

-The pre-mRNA must undergo additional modifications before it can exit the nucleus and reach the ribosome

-One modification is the addition of a chain of 50 to 250 adenine nucleotides

-Does this one nucleotide at a time, to the 3’ end by an enzyme called poly-A polymerase

-The chain of nucleotides is called the poly(A)tail

Question 9

Poly(A)tail

Answer 9

-Poly(A) tail: a chain of adenine nucleotides added to the 3’ end of pre-mRNA molecule to protect it from enzymes in the cytosol

-Enables mRNA to be translated efficiently and protects from attack by RNA-digesting enzymes in the cytosol

Question 10

Addition of 5’ cap

Answer 10

-Involves covalent linkage of modified G nucleotides onto to 5’ end of pre-mRNA

-The cap is recognized by the protein synthesis machinery

-All eukaryotic mRNA undergo modifications on their ends

-These modifications convert precursors mRNA (pre-mRNA) to mature mRNA

Question 11

Splicing

Answer 11

Genes contain non-coding regions called introns interspersed among coding regions called exons

-The intron sequences are removed from pre-mRNA and exons are joined together to form mature mRNA

-This process is called splicing

• mRNA splicing occurs in a spliceosome: a complex formed between the pre-mRNA and a handful of small ribonucleoproteins called snRNPs

-snRNPs bind in a particular order to an intron in the pre-mRNA

-The first snRNPs recognize and form complementary base pairs with mRNA sequences at junctions of the intron and adjacent exons

-Other snRNPs recruited, causing intron to loop out and bring the two exon ends close together

-At this point, an active spliceosome has been formed, releasing the intron and joining together the two exons

Question 12

Alternative Splicing

Answer 12

• Exons may be joined in different combinations to produce different mRNAs from a single DNA gene sequence

-A mechanism called alternative splicing increases number and variety of proteins encoded by a single gene

-Allows more than one possible polypeptide to be made from a single gene

-Alternative splicing helps understand why humans with only 20 000 genes can produce approx. 100 000 proteins

-After final mRNA has been produced, ready to leave nucleus and be translated by a ribosome.