DNA is a huge information database that carries the complete set of instructions for making all the proteins a cell will ever need! Although there are only four different bases in DNA (A, C, G and T), the order in which the bases occur determines the information to make a protein, just like the 26 letters of the alphabet combine to form words and sentences: Compare: RAT – TAR – ART – same 3 letters; completely different meanings. And with DNA: GAC – AGC – CGA – same 3 ‘letters’; completely different meanings to the cell (specifies the amino acids Aspartic Acid, Serine, and Arginine)
Q: Review: What are genes?
A: The DNA in each chromosome that DOES provide the instructions for a protein is called a gene. * In the 1940s, scientists proposed, fairly correctly, that each gene “codes for” (contains the instructions for) one protein. This is referred to as the “one-gene, one-protein” hypothesis. * One gene will code for perhaps two or more related proteins. * Scientists realized that we had only about 30,000 genes, coding for 100,000 different proteins – rather than the 100,000 genes that had been estimated for the human genome * The basic hypothesis is still the same, but we know a lot more details. Q: If DNA is in the nucleus and proteins are synthesized in the cytoplasm, on ribosomes and in the RER, how to they “get together”? A: The answer: use a “messenger” to carry the instructions from DNA out into the cytoplasm. A nucleic acid very similar to DNA, called mRNA or messenger RNA, is a copy of a gene, and serves this function the “bridge” between DNA and protein: The Central Dogma:DNA encodes the information to make RNA and RNA molecules function together to make protein|
II. What is RNA and how is it different from DNA?
Two big differences between DNA and RNA:
* 1. The sugar in DNA is deoxyribose; in RNA it is ribose * 2. The nitrogenous base uracil (U) is used in RNA in place of T (they are very similar bases; in RNA U= A just like T = A.)
III. Transcription: Re-writing DNA into RNA
DNA has a sense strand and an antisense strand. The antisense strand is complementary to the sense strand. It is also known as the template strand, as this is the strand, which is used for transcription – making mRNA. DNA is “transcribed” or re-written into RNA in a very complicated process called transcription. Simply stated, during transcription, one gene (DNA) is ‘re-written’ into RNA in the nucleus: * A team of enzymes and proteins binds to the promoter, or starting region, of a gene. * These enzymes and proteins unzip the DNA double helix just at the region of the gene. * The enzyme RNA polymerase uses one of the DNA strands to make an RNA copy of that one gene. * This copy, which contains the instructions to make 1 protein, is called an mRNA or messenger RNA. * After the mRNA is made, it is trimmed down to a final size, and shipped out of the nucleus. * When the mRNA gets into the cytoplasm, it is made into protein * Complete transcription of an RNA molecule.
1. The resulting RNA nucleotides are added on the 3’ end of the growing of mRNA strand. 2. RNA polymerase detaches itself and mRNA is released.
3. Eukaryote pre- mRNA contains exons and introns:
* Introns are non-coding regions that need to be removed before translation
The structure of tRNA matches its functions :
Funtion : to bring amino acids from the cytoplasm to the growing polypeptide and to attach them in the current location. tRNA is activated by a tRNA activating enzyme. tRNA delivers amino acids to the growing polypeptide chain in translation.
It picks up new amino acids when activated by a specific tRNA activating enzyme.
IV. Connection between mRNA and protein:
The order of the bases in the DNA specifies the order of bases in the mRNA, and
The order of bases in the mRNA specifies the order of amino acids in a protein.| The genetic code is a triplet code (handout)
1. Nucleotides on mRNA are read “three at a time” by the ribosome. * Every three nucleotides in an mRNA (a ‘codon’) specify the addition of one amino acid in a protein. * For example, a 600 nucleotide mRNA will code for a 200 amino acid protein. 2. The amino acids corresponding to all 64 codons have been determined. All proteins start with the initiation codon AUG (Met) * All proteins end with stop codons -either UAA, UGA, or UAG * Some codons that differ in the third nucleotide can still code for the same amino acid
The genetic code chart represents the sequence on the mRNA codon. V. Translation = De-coding RNA into protein
During translation, the mRNA transported to the cytoplasm is “de-coded” or “translated” to produce the correct order of amino acids in a protein. Translation requires numerous enzymes. rRNA = ribosomal RNA; these RNA molecules associate with other proteins to form the ribosomes. Each ribosome can accept two tRNAs at a time (carrying amino acids) and one mRNA. tRNA = transfer RNA; small RNA molecules that carry a specific amino acid at one end and an anticodon region that recognizes and binds mRNA at the other end. The tRNA that binds to that mRNA codon determines what amino acid is added to a protein chain. The Three RNAs (mRNA, tRNA, and rRNA) all work together to turn the information in DNA into a 3-dimestional protein.
The steps of translation:
1. Initiation: mRNA enters the cytoplasm and becomes associated with ribosomes (rRNA + proteins). tRNAs, each carrying a specific amino acid, pair up with the mRNA polys inside the ribosomes. Base pairing (A-U, G-C) between mRNAcodons and tRNA anticodons determines the order of amino acids in a protein. 2. Elongation: addition of amino acids one-by-one:
As the ribosome moves along the mRNA, the tRNA transfers its amino acid to the growing protein chain, producing the protein – codon by codon! 3. Termination: when the ribosomes hit a stop codon – UAA, UGA, or UAG – the ribosome falls apart. The same mRNA may be used hundreds of times during translation by many ribosomes before it is broken down by the cell.