Lecture 5: Gene to protein 1 Transcrition Flashcards
The information content of DNA is in the form:
of sequences of nucleotides (genes)
proteins are the link between
genotype & phenotype
Gene expression is the by which
DNA directs protein synthesis, includes two stages:
- transcription
- translation
During transcription:
- During transcription, a DNA strand provides a template for the synthesis of a complementary RNA strand.
- Transcription produces messenger RNA (mRNA)
Translation is..
..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.
Bridge between DNA ad protein synthesis is
the nucleic acid RNA
RNA differs form DNA in that
its sugar is RIBOSE and swaps the nitrogenous base uracil for thymine
RNA molecule usually consists of a
single strand
transcription and translation in prokaryotic cells
- 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.
transcription and translation in eukaryotic cells
- 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.
Genes program protein synthesis via
genetic messages in the form of messenger RNA
The central dogma
DNA –> RNA –> Protein
Francis Crick in 1956
There are __ amino acids but there are only __ nucleotide bases in DNA
20
4
Whats the smallest units of uniform length that can code of all the amino acids is
triplets of nucleotide bases
a codon is a
triplet of nucleotide bases
With a triplet code,three consecutive bases specify an
amino acid, creating 4^3 (64) possible codes
During translation the mRNA codons are read in the
5’ to 3’ direction
All __ codons were deciphered by the
mid-1960’s
Of the 64 triplets, __ code for amino acids and __ triplets are ‘“stop” signals to end translation
61 for a.a.
3 =stop
More than one codon may specify..
a particular amino acid but no codon specifies more than on amino acid
The genetic code is
universal, shared by the simplest bacteria to the most complex animals
Some genes can be _________ after being transfered from one species to another
transcribed and translated
Type of RNA polymerases that synthesise RNA molecules in bacteria and eukaryotes
- 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.
RNA polymerase ll is used for
mRNA synthesis
Transcription can be split into 3 stages
- Initiation
- Elongation
- termination of the RNA chain
Initiation :
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.
Elongation:
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.
termination:
Eventually the RNA transcript is released and the polymerase detaches from the DNA.
RNA Polymerase Binding and Initiation of Transcription at a Prokaryotic promoter
- 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
Initiation of Transcription at a Eukaryotic Promoter:
- 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.
Elongation of the RNA strand:As RNA polymerase moves along the DNA
it untwists the double helix, 10 to 20 bases at a time
Elongation of the RNA strand: Transcription progresses at a rate of
40 nucleotides per second in eukaryotes
Elongation of the RNA strand: nucleotides are added to the
3” end of the growing RNA molecule
Elongation of the RNA strand:
a gene can be transcribed simultaneously by several
RNA polymerases which increases the amount of mRNA transcribed from it which means the encoded protein can be made in large amounts
Termination mechanism in bacteria
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.
Termination mechanism in eukaryotes
- 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.
Pre-mRNA undergoes processing:
- bothendsoftheprimarytranscript are usually altered
* Someinteriorpartsofthe molecule are cut out, and the other parts spliced together.
Processing of pre-mRNA in eukaryotic cells: Modification of the ends of pre-mRNA
• 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).
Processing of pre-mRNA in eukaryotic cells: RNA splicing
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.
RNA splicing: in some cases it is carried out by
spliceosomes
a spliceosome consists of
small nuclear ribonucleoproteins (snRNPs) and other proteins
within the spliceosome
snRNA base pairs with nucleotides at specific sites along the intron.
How does the spliceosome work:
- 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.
Do introns have any function?
- 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
Functional significance of alternative splicing
-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
