Protein Synthesis (5) Flashcards
Proteins, which include enzymes are the workhorses of all living systems; as many as a ____ ____ of them may be busy in any cell at any moment.
hundred million
There are different types of RNA made in the nucleus, each of which has a function in the synthesis of proteins. They are: (3)
- messenger RNA (mRNA)
- transfer RNA (tRNA)
- ribosomal RNA (rRNA).
What happens in the nucleus?
mRNA is formed in the nucleus in the same way as DNA is replicated. The process is called transcription as the coded message in DNA is carried across (transcribed) into the new mRNA molecule, which carries it to the ribosome.
Explain the process of transcribing DNA. (8)
- The process starts when a small piece of DNA, a gene, unwinds.
- The process is catalyzed by the enzyme RNA polymerase which causes the two strands of DNA to separate by breaking the hydrogen bonds between complementary DNA nucleotides.
- The RNA polymerase attaches to and moves along the DNA strands, causing new one pair with their nucleotides to complementary nucleotides. This DNA strand is called the template (pattern) as it carries the code.
- The nucleotides join up, a sugar-phosphate backbone is added and a new strand of mRNA is formed.
- The sequence of nucleotides is therefore determined by the sequence of the template DNA nucleotides. In other words, the DNA transcribes its genetic code to the mRNA. -The genetic code, therefore, is simply the sequence of some nucleotides in a DNA strand.
- Note that a uracil base (not a thymine base) will pair with an adenine base.
- A completed strand of mRNA breaks away from the DNA. The DNA then re-zips.
- The relatively small mRNA moves through the pores of the nuclear membrane and carries the genetic code to the ribosomes which are the sites of protein synthesis.
What determines which protein is made?
A protein is a long chain (polymer) of small units (monomers) called amino acids. There are twenty different amino acids that are involved in protein synthesis. These may combine in various numbers in various sequences to form thousands of different proteins, - the shortest having 50 amino acids.
Think of how many different words can be formed from the 26 letters of the alphabet. The order in which the amino acids are linked determines what kind of protein is made, e.g. the protein keratin has a different sequence of amino acids from the protein haemoglobin.
The sequence of amino acids is determined by the instructions from the genetic code in the DNA molecules which is passed on to ____.
mRNA
What is the role of mRNA? (5)
- The genetic code is carried as a sequence of ‘codewords’ which are transcribed to the mRNA. Each ‘codeword’ is made up of three bases and is called a cytosine - adenine - guanine.
- There are 64 different codons and all except three, code for one of the twenty amino acids used to form proteins. Some amino acids are coded for by more than one codon. any 8ə ‘uopoɔ - The three codons that do not code for an amino acid are called stop codons. (UGA, UAA and UAG)
- A codon is written, using the first letter of the different bases. For example, the sequence CCG (cytosine, cytosine, guanine) is the codon for the amino acid glycine and CAG (cytosine, adenine, guanine) is the codon for valine.
- The triplet code of bases is the basis of the genetic code as a gene is made up of a group of codons that code for the synthesis of one protein.
- The order of codons in mRNA will therefore determine the sequence of the amino acids which will determine which protein is made.
What happens at the ribosomes?
The mRNA binds to the ribosome at the start codon (first codon). The codons of the mRNA act as a template (pattern) that determines the order in which the amino acids are linked.
What is the role of tRNA? (3)
- There are at least 64 different tRNA molecules, made from nucleotides found in the nucleoplasm of cells.
- Each tRNA has three bases at one end called an anticodon which picks up a specific amino acid found in the cytoplasm and transfers it to a ribosome.
- The most important feature of tRNA is that it can bind to an amino acid at one end and to mRNA at the other, depositing its amino acid in the correct position to form a specific protein.
Translation of RNA into proteins. (5)
- One of the codons, the ‘start signal’, begins the process of making a protein from amino acids. Stop signals’ that Three of these codons act as to indicate that the message is over and the protein chain is complete. All the other codons code for specific amino acids.
- The anticodon bases link up to their complementary bases of the codon. This process is called translation, as the code on the mRNA is translated into a sequence of amino acids. For example, if the codon on mRNA is GGA, the anticodon of the tRNA will be CCU. This enables amino acids to link up in the correct sequence.
- The tRNA molecule is released to carry more of its specific amino acid to the ribosome.
- Catalysed by enzymes, the amino acids link together with peptide bonds to form a polypeptide chain.
- The polypeptide chains link together to form the final functional protein.
Summarise translation.
Translation is the process by which a specific protein is formed from a chain of amino acids due to the sequence of codons in the mRNA, which, in turn, was coded by the DNA.
What is the role of rRNA?
rRNA is the most common form of RNA in the cell and it, together with proteins, makes up the ribosomes. The rRNA moves from codon to codon along the mRNA, reading the code. rRNA, therefore, plays an important role in controlling the process of protein synthesis.
What are pharmacological drugs and what do they do and how do they do this?
Pharmacological drugs such as antibiotics are produced to counteract bacterial infections. They do this by interacting with the bacterial ribosomes and inhibiting their function of protein synthesis. The ribosomes of bacteria (prokaryotes) and eukaryotes are different enough that antibiotics can specifically target those of the bacteria and not those of the host. Proteins are essential for the production and growth of new cells so, if bacteria are prohibited from making proteins, they will not be able to make new cells to spread the infection.
What are the antibiotics that inhibit protein synthesis by interacting with the bacterial ribosomes in a different way?
- the tetracyclines prevent the attachment of tRNAs carrying amino acids
- chloramphenicol, prevents the formation of peptide bonds.
By targeting different stages of the mRNA translation, antibiotics can be changed if resistance develops.
