Protein Synthesis Flashcards
How do genes express themselves?
Via protein synthesis (i.e. the information contained in a gene codes for the production of a single protein)
What is the central dogma of how genes work?
DNA —> mRNA —> protein
Which two processes are involved in the expression of genes?
TRANSCRIPTION and TRANSLATION
Describe genetic transcription
DNA is transcribed into a form of RNA known as messenger RNA (mRNA)
Describe genetic translation
The mRNA created during transcription travels out of the nucleus and into the cytoplasm where it binds to the RNA in a ribosome (rRNA). The ribosome “reads” (translates) the mRNA code and matches it with the appropriate transfer RNA molecule (tRNA). Attached to each specific tRNA is a corresponding amino acid. By assembling the tRNA molecules coded for by the mRNA, an amino acid (protein) chain is created
In what ways is RNA different from DNA?
It uses RIBOSE SUGAR in place of deoxyribose sugar
It replaces the base thymine with URACIL
Describe mRNA
Messenger RNA: A single stranded molecule complimentary to the DNA template.
It varies in length depending on the gene that has been copied. It acts as the intermediary between DNA and the ribosomes. It is translated into protein by ribosomes. It is the RNA version of the gene encoded by DNA
Ex. If DNA = G T A C G A T
Then mRNA = C A U G C U A
Describe rRNA
Ribosomal RNA: Found as part of the ribosome. Ribosomes are composed of two subunits (50s and 30s) which in turn are composed of RNA associated with some 50 proteins. The two subunits are synthesized in the nucleolus and fit together to form a functional ribosome.
It functions as the delivery system of amino acids to ribosomes as they synthesize proteins. It is very short, only 70 to 90 base pairs long
Describe tRNA
Transfer RNA: They float freely in the cytoplasm. The cell contains more than 60 different kinds of tRNA molecules, each one carrying 1 of 20 possible amino acids. The amino acid associated with the tRNA is specific to the anticodon.
It binds with the ribosomes to form proteins. It varies in length
Genetic transcription is divided into which three sequential processes?
Initiation, elongation, and termination
Describe transcription initation
RNA POLYMERASE (enzyme) binds to a particular sequence of DNA bases upstream from the edge of a gene. This sequence of bases is called the promoter and is rich in A-T base pairs (easier to break than C-G base pairs)
Describe transcription elongation
RNA polymerase moves along the DNA TEMPLATE assembling an mRNA STRAND with a nucleotide sequence COMPLEMENTARY to the DNA template strand (the other strand of DNA is called the coding strand). RNA polymerase builds the mRNA in a 5’ —> 3’ direction and does not require a primer
Describe transcription termination
When RNA polymerase reaches a STOP SIGNAL at the end of the gene, it disengages from the DNA and releases the mRNA TRANSCRIPT
What post-transcription modifications of the mRNA occur once the primary transcript has been synthesized and prior to it leaving the nucleus?
Capping and tailing; introns and exons
Describe post-transcriptional capping of the mRNA
The addition of a 5’ cap: 7-methyl guanosine is added to the 5’ end of the primary transcript.
The cap protects the mRNA transcript from being digested by nucleuses and phosphatases as it enters the cytoplasm. The cap also plays a role in the initiation of translation of the transcript - joining the mRNA to the ribosome
Describe post-transcriptional tailing of the mRNA
The addition of a poly-A tail: The enzyme poly-A polymerase adds about 200 adenine ribonucleotides to the 3’ end of the mRNA transcript. The poly-A tail appears the protect the mRNA from degradation in the cytoplasm, allowing the mRNA to last longer and thus be translated more than once
What are introns and exons
The primary transcript of mRNA contains CODING REGIONS called EXONS with NON-CODING REGIONS called INTRONS interspersed between them. The introns are cut out of the primary transcript by spliceosomes (particles made of RNA and proteins). The spliceosomes then join the exons together to form the finished mRNA transcript
What does a primary mRNA transcript look like?
Exon 1: Intron 1: Exon 2: Intron 2: Exon 3
What does the final mRNA transcript look like?
5’ cap: Exon 1: Exon 2: Exon 3: -AAAAA
Describe genetic translation
The transcribed mRNA travels to the cytoplasm. A RNA molecule within the ribosome binds to the leading end of the mRNA. The ribosome moves along the mRNA, reading groups of 3 nucleotides at a time (codons). At each codon, a tRNA with a complimentary anticodon sequence binds to it, bringing with it an amino acid specific to that codon/anticodon pairing. Peptide bonds form between adjacent amino acids as the ribosome moves along, forming the protein chain. The tRNA is released from the ribosome once its amino aid has been incorporated into the growing polypeptide. The ribosome continues along the mRNA one codon at a time, until it reaches a stop signal at the end of the mRNA. The ribosome then disengages from the mRNA and releases its polypeptide
Draw and label a diagram of the tRNA structure
Amino acid attachment point, D loop, TC loop, anticodon loop, anticodon, codon, mRNA
Review “The Genetic Code: DNA to Proteins — Answer Sheet”
Define “transcription”
Mechanism by which the information encoded in DNA is transcribed into a complementary RNA copy
Which type of nucleic acid is able to exit the nucleus and enter the cytosol?
RNA (during transcription)
Where does transcription occur?
In the nucleus of eukaryotic cells
Define “translation”
Mechanism by which the information coded in the nucleic acids of RNA is copied into the amino acids of proteins
Where does translation occur?
On the ribosomes in the cytosol
Draw and label a diagram of DNA to Transcription to Translation
DNA template strand: 3’ C A A C G G T T T G G A 5’
Transcription
mRNA: 5’ G U U G C C A A A C C U
Translation
Protein: Valine - alanine - lysine - proline
Review “Genetic Code Codons”
