DNA --> RNA --> Protein
Step 1: Transcription
So you start off with DNA floating inside the nucleus in the middle of the cell. An enzyme called RNA polymerase comes to the double stranded DNA, unwinds it and breaks the weak hydrogen bonds between the bases of each strand. It basically unzips it. This process is similar to DNA duplication except a different enzyme is involved there - DNA polymerase - and it bind complementary DNA free-floating nucleotides (A/T/C/G) to bind to the sense strand (master template). The master template is the top half of the unzipped double strand and it goes in the 5' - 3', left to right direction. By this process it doubles the DNA before cell replication so that the new cell gets a copy of the entire genome.
RNA polymerase partially unwinds and unzips the DNA to bind complimentary RNA (A/U/C/G) Nucleotides to the sense strand. The enzyme binds to the start codon in the DNA, travels along the strand and dettaches when it gets to a stop codon A single RNA strand is forged as the enzyme moves along the strand and when the enzyme dettaches the free RNA strand, now called messenger RNA (mRNA) , begins to float outside of the nucleus. Outside the Nucleus it eventually finds its way to a protein complex called a Ribosome where step 2, translation, occurs.
Step 2: Translation
So the mRNA strand which is floating in the plasma of the cell reaches a Ribosome. The ribosome is shaped a bit like a grasped hand and the RNA strand goes through it three bases at a time. Free floating anticodon triplets in a special molecule called Aminoacetylated Transfere RNA (tRNA) molecules in the cytoplasm come to the ribosome. These tRNA molecules have a triplet of bases at one end and an amino-acid attached at the other end. So the anti-codon triplet on the tRNA binds to the RNA strand in a complementary fashion; in fact three tRNA molecules are docked into the ribosome at a time and the amino acids on the other end of the tRNA join up together and this process continues for the length of the RNA strand and the amino acids continue to join up and form a long strand - a polypeptide (protein) strand.
Triplet codons
Each triplet of bases (called a codon) of the DNA/RNA strand represents an amino acid. For example, AUG codes for the amino acid Methionine. AUG (ATG in DNA) is the start codon. There are 64 possible combinations that can be made from a triplet of bases since there are 4 different bases (4 * 4 * 4 = 64). But yet there are 20 possible types of amino acids so this means that some triplets code for the same amino acid like CUS, CCS, CAS and CGS will all code for the amino acid Serine.
Promoter Sequences
Consider the start codon ATG in DNA. There is a 1/64 probability by chance alone that we find it while going along the DNA strand. It can't be that every time the ATG codon is found by RNA polymerase it starts transcription. No, in order to identify which occurance of the ATG codon signifies the beginning of a gene a special sign needs to be present - this is the "Promoter Sequence". Only when the promoter sequence is found a few bases before the start tag will the transcription of that length of DNA occur. In prokaryotes the promoter lies about -10 or -35 bases upstream of the start codon while in eukaryotes it is far more complex but in approxiamatly 20% of the cases a TATA code is found in it. There are various promoters and there can be very many bases upstream of a gene before a promoter is found.
So you start off with DNA floating inside the nucleus in the middle of the cell. An enzyme called RNA polymerase comes to the double stranded DNA, unwinds it and breaks the weak hydrogen bonds between the bases of each strand. It basically unzips it. This process is similar to DNA duplication except a different enzyme is involved there - DNA polymerase - and it bind complementary DNA free-floating nucleotides (A/T/C/G) to bind to the sense strand (master template). The master template is the top half of the unzipped double strand and it goes in the 5' - 3', left to right direction. By this process it doubles the DNA before cell replication so that the new cell gets a copy of the entire genome.
RNA polymerase partially unwinds and unzips the DNA to bind complimentary RNA (A/U/C/G) Nucleotides to the sense strand. The enzyme binds to the start codon in the DNA, travels along the strand and dettaches when it gets to a stop codon A single RNA strand is forged as the enzyme moves along the strand and when the enzyme dettaches the free RNA strand, now called messenger RNA (mRNA) , begins to float outside of the nucleus. Outside the Nucleus it eventually finds its way to a protein complex called a Ribosome where step 2, translation, occurs.
Step 2: Translation
So the mRNA strand which is floating in the plasma of the cell reaches a Ribosome. The ribosome is shaped a bit like a grasped hand and the RNA strand goes through it three bases at a time. Free floating anticodon triplets in a special molecule called Aminoacetylated Transfere RNA (tRNA) molecules in the cytoplasm come to the ribosome. These tRNA molecules have a triplet of bases at one end and an amino-acid attached at the other end. So the anti-codon triplet on the tRNA binds to the RNA strand in a complementary fashion; in fact three tRNA molecules are docked into the ribosome at a time and the amino acids on the other end of the tRNA join up together and this process continues for the length of the RNA strand and the amino acids continue to join up and form a long strand - a polypeptide (protein) strand.
Triplet codons
Each triplet of bases (called a codon) of the DNA/RNA strand represents an amino acid. For example, AUG codes for the amino acid Methionine. AUG (ATG in DNA) is the start codon. There are 64 possible combinations that can be made from a triplet of bases since there are 4 different bases (4 * 4 * 4 = 64). But yet there are 20 possible types of amino acids so this means that some triplets code for the same amino acid like CUS, CCS, CAS and CGS will all code for the amino acid Serine.
Promoter Sequences
Consider the start codon ATG in DNA. There is a 1/64 probability by chance alone that we find it while going along the DNA strand. It can't be that every time the ATG codon is found by RNA polymerase it starts transcription. No, in order to identify which occurance of the ATG codon signifies the beginning of a gene a special sign needs to be present - this is the "Promoter Sequence". Only when the promoter sequence is found a few bases before the start tag will the transcription of that length of DNA occur. In prokaryotes the promoter lies about -10 or -35 bases upstream of the start codon while in eukaryotes it is far more complex but in approxiamatly 20% of the cases a TATA code is found in it. There are various promoters and there can be very many bases upstream of a gene before a promoter is found.
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