Although DNA replication and cellular division are also important processes, this unit will explore protein synthesis. Students who can grasp the central dogma of molecular biology and the genetic code are those who are truly starting to make sense of this amazing molecule.
Let’s start with the nucleus. It is here where the DNA resides, and she never leaves her nucleus. So how does the information contained in DNA leave the nucleus? Her special messenger- RNA (m-RNA). Why does mRNA have to leave the nucleus? Because it needs the ribosomes to assist in protein synthesis. Also, the raw materials and other helpers needed to perform this process are found in the cytoplasm.
The base pairs in a DNA molecule are grouped into long chains called genes. Genes code for proteins therefore the hereditary information contained in DNA tells the cell how to make proteins. The language used by the nucleic acids is well understood by the cell and by molecular biologists and is known as the genetic code.
Imagine a gene about 300 base pairs long. Remember DNA structure- a double helix with nitrogenous base pairs hydrogen bonded to each other. Now think of the DNA molecule as a zipper, such as the one on a jacket. Open the zipper (molecule), and what you see are the two exposed metal sides (strands). The DNA molecule works just like that except where you use your hand to open the zipper, there is an enzyme that does the job for DNA. And just like you can zip your zipper back up, so can the DNA molecule! The RNA polymerase can move anywhere along the gene, opening and closing segments of DNA as needed.
The DNA strands are separated so that RNA can begin its job. RNA is the other type of nucleic acid, and has a few differences from DNA. First, it is single stranded. Second, its nucleotides do not contain the nitrogenous base thymine, but rather uracil. Third, its sugar is ribose; a sugar like DNA’s but with an oxygen bonded to the second carbon. Lastly, it leaves the nucleus. Similarities are that both contain nucleotides, and both follow same base pairing rules.
The nucleotides are grouped into threes, or triplets, called codons. Each codon codes for either a function (start, stop) or an amino acid. There are 20 different common amino acids which make all the proteins known to humans! Each of these amino acids has one or more codon. Therefore, the sequence of the nucleotides along the DNA specify the sequence of amino acids in the proteins.
Once DNA strands are separated, transcription can begin. Transcription is the transfer of information from DNA to mRNA. The way this occurs is as follows:
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1. RNA polymerase separates the strands and will then help the messenger RNA assemble itself.
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2. RNA polymerase knows when to begin once it finds the promoter and the start codon.
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3. Following the base pairing rules, a complementary strand of messenger RNA is formed.
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4. An mRNA strand continues to grow until it reaches a special terminator sequence on the DNA and the stop codon.
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5. Transcription ends when RNA polymerase releases the newly synthesized strand of mRNA and itself leaves the DNA allowing it to “zip” back up.
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6. Several copies of a gene can be transcribed simultaneously.
Once the messenger RNA transcript leaves the nucleus it heads for a ribosome. Now translation can begin. Briefly, translation is as follows:
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1. Messenger RNA transcript arrives at ribosome.
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2. Transfer RNA is responsible for bringing the amino acids into alignment to form the polypeptide.
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3. Attached to one end of the transfer RNA is the amino acid, the other end is an anticodon, which is a complementary codon to the messenger RNA transcript.
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4. Enzymes help couple each amino acid with its respective transfer RNA molecule and each transfer RNA molecule anticodon with messenger RNA codon.
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5. The messenger RNA transcript is secured within a ribosome and will temporarily couple with the appropriate transfer RNA while it is being translated.
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6. Once the transfer RNA is coupled with the messenger RNA, it transfers its attached amino acid to the chain which is growing as the as the messenger RNA strand is being read.
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7. Termination of translation occurs when there is a stop codon, and water rather than an amino acid is added to the protein (peptide chain) which cleaves it.
