Lecture 5: Gene to protein 1 Transcrition

Question 1

The information content of DNA is in the form:

Answer 1

of sequences of nucleotides (genes)

Question 2

proteins are the link between

Answer 2

genotype & phenotype

Question 3

Gene expression is the by which

Answer 3

DNA directs protein synthesis, includes two stages:

• transcription
• translation

Question 4

During transcription:

Answer 4

• During transcription, a DNA strand provides a template for the synthesis of a complementary RNA strand.
• Transcription produces messenger RNA (mRNA)

Question 5

Translation is..

Answer 5

..the synthesis of a polypeptide, using information in the mRNA.
- The sites of translation are the ribosomes, that facilitate the assembly of amino acids into polypeptide chains.

Question 6

Bridge between DNA ad protein synthesis is

Answer 6

the nucleic acid RNA

Question 7

RNA differs form DNA in that

Answer 7

its sugar is RIBOSE and swaps the nitrogenous base uracil for thymine

Question 8

RNA molecule usually consists of a

Answer 8

single strand

Question 9

transcription and translation in prokaryotic cells

Answer 9

• Bacteria lack nuclei, so their DNA is not segregated from ribosomes and other protein- synthesizing machinery.
• This allows the coupling of transcription and translation i.e. in prokaryotes, translation of mRNA can begin before transcription has finished.
• Ribosomes attach to the leading end of an mRNA molecule while transcription is still in progress.

Question 10

transcription and translation in eukaryotic cells

Answer 10

• In a eukaryotic cell, transcription occurs in the nucleus, and translation occurs at ribosomes in the cytoplasm.
• The transcription of a protein- coding eukaryotic gene results in pre-mRNA.
• Eukaryotic RNA transcripts are modified through RNA
  processing to yield the finished mRNA.

Question 11

Genes program protein synthesis via

Answer 11

genetic messages in the form of messenger RNA

Question 12

The central dogma

Answer 12

DNA –> RNA –> Protein

Francis Crick in 1956

Question 13

There are __ amino acids but there are only __ nucleotide bases in DNA

Answer 13

20

4

Question 14

Whats the smallest units of uniform length that can code of all the amino acids is

Answer 14

triplets of nucleotide bases

Question 15

a codon is a

Answer 15

triplet of nucleotide bases

Question 16

With a triplet code,three consecutive bases specify an

Answer 16

amino acid, creating 4^3 (64) possible codes

Question 17

During translation the mRNA codons are read in the

Answer 17

5’ to 3’ direction

Question 18

All __ codons were deciphered by the

Answer 18

mid-1960’s

Question 19

Of the 64 triplets, __ code for amino acids and __ triplets are ‘“stop” signals to end translation

Answer 19

61 for a.a.

3 =stop

Question 20

More than one codon may specify..

Answer 20

a particular amino acid but no codon specifies more than on amino acid

Question 21

The genetic code is

Answer 21

universal, shared by the simplest bacteria to the most complex animals

Question 22

Some genes can be _________ after being transfered from one species to another

Answer 22

transcribed and translated

Question 23

Type of RNA polymerases that synthesise RNA molecules in bacteria and eukaryotes

Answer 23

• Bacteria have a single type of RNA polymerase that synthesizes all RNA molecules.
• In contrast, eukaryotes have three RNA polymerases (I, II, and III) in their nuclei.

Question 24

RNA polymerase ll is used for

Answer 24

mRNA synthesis

Question 25

Transcription can be split into 3 stages

Answer 25

• Initiation
• Elongation
• termination of the RNA chain

Question 26

Initiation :

Answer 26

After RNA polymerase binds to the promoter, the DNA strands unwind and the polymerase initiates RNA synthesis at the start point on the template strand.

Question 27

Elongation:

Answer 27

The polymerase moves downstream unwinding the DNA and elongating the RNA transcript in the 5’ to 3’ direction. In the wake of transcription the DNA double helix reforms.

Question 28

termination:

Answer 28

Eventually the RNA transcript is released and the polymerase detaches from the DNA.

Question 29

RNA Polymerase Binding and Initiation of Transcription at a Prokaryotic promoter

Answer 29

• Promoters signal the transcriptional start point and usually extend several dozen nucleotide pairs upstream of the start point.
• RNA polymerase binds in a precise location and orientation on the promoter, determining where transcription starts and which of the two strands of the DNA helix is the template
• In bacteria RNA polymerase itself recognizes and binds to the promoter

Question 30

Initiation of Transcription at a Eukaryotic Promoter:

Answer 30

• In eukaryotes transcription factors mediate the binding of RNA polymerase and the initiation of transcription.
• A promoter sequence called a TATA box is crucial in forming the initiation complex in eukaryotes. A transcription factor recognising the TATA box must bind to the DNA before RNA polymerase can do so.
• The transcription factors and RNA polymerase II bound to a promoter is called a transcription initiation complex. The DNA double helix unwinds and RNA synthesis begins at the start point on the template strand.

Question 31

Elongation of the RNA strand:As RNA polymerase moves along the DNA

Answer 31

it untwists the double helix, 10 to 20 bases at a time

Question 32

Elongation of the RNA strand: Transcription progresses at a rate of

Answer 32

40 nucleotides per second in eukaryotes

Question 33

Elongation of the RNA strand: nucleotides are added to the

Answer 33

3” end of the growing RNA molecule

Question 34

Elongation of the RNA strand:

a gene can be transcribed simultaneously by several

Answer 34

RNA polymerases which increases the amount of mRNA transcribed from it which means the encoded protein can be made in large amounts

Question 35

Termination mechanism in bacteria

Answer 35

the polymerase stops transcription at the end of a specific RNA sequence known as the terminator and the mRNA can be translated without further modification.

Question 36

Termination mechanism in eukaryotes

Answer 36

• In eukaryotes, RNA polymerase II transcribes the polyadenylation signal sequence (AAUAAA) in the pre-mRNA.
• At a point about 10 to 35 nucleotides past this sequence, the RNA transcript is cut from the polymerase.
• This releases the pre-mRNA, which then undergoes processing.

Question 37

Pre-mRNA undergoes processing:

Answer 37

• bothendsoftheprimarytranscript are usually altered

* Someinteriorpartsofthe molecule are cut out, and the other parts spliced together.

Question 38

Processing of pre-mRNA in eukaryotic cells: Modification of the ends of pre-mRNA

Answer 38

• Each end of a pre-mRNA molecule is modified in a particular way – The 5ʹ end receives a modified nucleotide 5ʹ cap
– The 3ʹ end gets a poly-A tail
• These modifications share several functions
– They facilitate the export of mRNA to the cytoplasm – They protect mRNA from hydrolytic enzymes
– They help ribosomes attach to the 5ʹ end
• The parts of the mRNA that will not be translated into protein are referred to as UTRs (untranslated regions).

Question 39

Processing of pre-mRNA in eukaryotic cells: RNA splicing

Answer 39

Most eukaryotic genes and their RNA transcripts have long noncoding stretches of nucleotides that lie between coding regions.
• These noncoding regions are called introns.
• The other regions are called exons because they are eventually expressed
and usually translated into amino acid sequences.
• RNA splicing removes introns and joins exons, creating an mRNA molecule with a continuous coding sequence.

Question 40

RNA splicing: in some cases it is carried out by

Answer 40

spliceosomes

Question 41

a spliceosome consists of

Answer 41

small nuclear ribonucleoproteins (snRNPs) and other proteins

Question 42

within the spliceosome

Answer 42

snRNA base pairs with nucleotides at specific sites along the intron.

Question 43

How does the spliceosome work:

Answer 43

• The spliceosome cuts the pre-mRNA, releasing the intron for degradation and splices the exons together.
• The spliceosome comes apart releasing mRNA which now contains only exons.

Question 44

Do introns have any function?

Answer 44

• Some introns contain sequences that may regulate gene expression
• Some genes can encode more than one kind of polypeptide, depending on which segments are treated as exons during splicing
• This is called alternative RNA splicing
• Consequently, the number of different proteins an organism can produce is much greater than its number of genes

Question 45

Functional significance of alternative splicing

Answer 45

-Sexual differentiation in Drosophila (fruit fly) is regulated by a protein called sex-lethal (sxl) protein.
• The female embryo expresses functional sxl proteins whereas the male embryo expresses non-functional sxl proteins.
• Their difference is a result of alternative splicing