2.1.2l Transcription & Translation Flashcards
see slide 2 for recap on base combinations for amino acids using chart
How does DNA code for polypeptides
- Genes are inside the cell nucleus but proteins are made in the cytoplasm, at ribosomes
- As the instructions inside the genes, on chromosomes, cannot pass out of the nucleus, a copy of each gene has to be TRANSCRIBED (copied) into a length of mRNA.
- In the form of mRNA, the sequence of bases (codons), can pass out the nucleus to the ribosome, ensuring that the coded instructions are TRANSLATED & the protein is assembled correctly from amino acids.
see slide 4 for diagram of making a protein from DNA
What is Transcription
The process of copying the code exactly to form a template of mRNA.
What is the DNA template in transcription also known as
The antisense strand
What is {a}-amanitin
A deadly toxin produced by some mushrooms. It works by inhibiting RNA polymerase
What is Translation
The process by which info in the code from the sequence of bases in mRNA controls the sequence of amino acids that a ribosome assembles.
What happens during Transcription (simple overview)
The 1st stage of protein synthesis.
During transcription, an mRNA copy of a gene is made in the nucleus
What happens during Translation (simple overview)
The 2nd stage of protein synthesis.
- Amino acids are joined tg by a ribosome in the cytoplasm to make a polypeptide chain (protein), following the sequence of codons carried by mRNA
EXAM Q: An error occurs during transcription that accidentally inserts a stop codon into the middle of an mRNA sequence.
What effect could this have on the protein that is eventually produced?
It could be shorter & so could be a different protein. Translation of the mRNA sequence only continues until a stop codon is reached. Any codons after the stop codon would not be translated into amino acids
Quick summary: where does transcription take place
Takes place in nucleus of cell. Involves RNA polymerase & mRNA
Quick summary: where does translation take pace
Takes place in the ribosomes in the cytoplasm of the cell. involves mRNA, tRNA, rRNA
What is RNA
- A single-stranded polynucleotide, which means it is made up of a no. of RNA nucleotide molecules joined to eachother w covalent bonds that form by condensation reactions
- RNA forms relatively short lengths of up to a few thousand nucleotides
- There are 3 types of RNA: mRNA, tRNA, rRNA
see slide 17 for dia of 3 types of RNA
3 points
What is mRNA
Messenger RNA
- Made in nucleus during transcription
- Carries the genetic code from the DNA in the nucleus to the cytoplasm, where it’s used to make a protein during translation. (It is small enough to leave the nuclear pores)
- In mRNA, three adjacent bases are called codons
(slide 18, pg38 for dia)
Key info about mRNA
mRNA contains Uracil (U) instead of Thymine (T).
Uracil always pairs with adenine during protein synthesis
3 points
What is tRNA
Transfer RNA
- Found in cytoplasm
- Has an amino acid binding site at one end & a sequence of three bases at the other end called an anticodon
- Folded into a clover shape. Hydrogen bonds between specific base pairs hold the molecule tg in this shape
Function of tRNA
- Carries the amino acids that are used to make proteins to the ribosomes during translation
What is rRNA
Ribosomal RNA
- Forms the 2 subunits in a ribosome (along with proteins)
- The ribosome moves along the mRNA strand during protein synthesis
- The rRNA in the ribosome helps to catalyse the formation of peptide bonds between the amino acids
Importance of rRNA
rRNA is important in maintaining the structural stability of the protein synthesis sequence
see images 07.12 for dia of 3 types of RNA
Why is mRNA important
- DNA molecules are found in the nucleus of the cell, but the ribosomes that make proteins are found in the cytoplasm
- DNA is too large to move out the nucleus, so a section is copied into mRNA. This process is called transcription
- The mRNA leaves the nucleus & joins w a ribosome in the cytoplasm, where it can be used to synthesis a protein. This process is called translation
(pg38)
Process of Transcription
1. RNA polymerase attaches to the DNA double-helix at the beginning of a gene
2. The hydrogen bonds between the 2 DNA strands in the gene break, separating the strands, & the DNA molecule uncoils
3. One of the strands is then used as a template to make an mRNA copy
4. The RNA polymerase lines up free RNA nucleotides alongside the template strand. Complementary base pairing means that the mRNA strand ends up being a complementary copy of the DNA template strand (except the base T is replaced by U in RNA)
5. Once the RNA nucleotides have paired up w their specific bases on the DNA strand theyre joined tg, forming an mRNA molecule
6. The RNA polymerase moves along the DNA, separating the strands & assembling the mRNA strand
7. The hydrogen bonds between the uncoiled strands of DNA re-form once the RNA polymerase has passed by & the strands coil back into a double-helix
8. When RNA polymerase reaches a stop codon it stops making mRNA & detaches from the DNA
9. The mRNA moves out of the nucleus through a nuclear pore & attaches to a ribosome in the cytoplasm, where the next stage of protein synthesis takes place (translation)
see pg40 for dia of transcription
Process of Translation
1. The mRNA attaches itself to a ribosome & tRNA molecules carry amino acids to the ribosome
2. A tRNA molecule, w an anticodon that’s complementary to the start codon on the mRNA, attaches itself to the mRNA by complementary base pairing
3. A 2nd tRNA molecule attaches itself to the next codon on the mRNA in the same way
4. rRNA in the ribosome catalyses the formation of a peptide bond between the 2 amino acids attached to the tRNA molecule. This joins the amino acids tg. The first tRNA molecule moves away, leaving its amino acids behind.
5. A 3rd tRNA molecule binds to the next codon on the mRNA. Its amino acid binds to the first two & second tRNA molecules move away
6. This process continues, producing a chain of linked amino acids (a polypeptide chain), until theres a stop codon on the mRNA molecule
7. The polypeptide chain moves away from the ribosome & translation is complete
see page 41 for dia of translation
