Introductory Biology

10.1 From Genes to Proteins

Proteins are made by decoding the information in DNA since the specific instructions in DNA determine the traits of proteins. There are three main things RNA and DNA differ in. DNA is double stranded, deoxyribose (does not contain oxygen) and has thymine. RNA is single stranded, ribose (contains oxygen) and has uracil. In RNA, the Thymine is replaced by Uracil so when “u” is spotted, its RNA. There are two steps in gene expression: transcription and translation. Transcription is the process in copying DNA to RNA and translation is building proteins from RNA.

**RNA polymerase is the active enzyme that reads the DNA and makes mRN. It unwinds/unzips DNA, adds new nucleotides to the growing RNA polymer using the complimentary rules and re-zips the DNA as it goes across adding nucleotides.

Transcription is the process in which the information from DNA is transferred into RNA. During the transcription stage, the DNA nucleotide sequence is used as a template for producing RNA and the RNA nucleotides are linked together with strong covalent bonds by the RNA polymerase. Transcription in prokaryotic cells takes place in the cytoplasm because they lack nuclei, but in eukaryotic cells it occurs in the nucleus. Up to 100 RNA polymerase molecules can be transcribing the same gene at the same time. These are the four steps during transcription:

  1. The RNA polymerase attaches tot he promoter (the start signal of the sequence of DNA)
  2. The RNA polymerase unwinds/unzips the DNA double helix
  3. At the initiation site, the RNA polymerase reads the template strand of DNA and builds a complementary RNA strand (elongation stage)
  4. When the RNA polymerase reaches the termination site on the DNA, it releases the RNA and the RNA polymerase from the DNA

**Messenger RNA (mRNA) delivers the instructions for making a protein to a ribosome for translation. The order of mRNA nucleotides determines the sequence of amino acids in a protein. Transfer RNA (tRNA) carries specific amino acids from the cytoplasm to ribosomes and makes sure that the proper amino acid is placed in the proper sequence in a protein. Each of the tRNA has a particular anticodon on one end (which corresponds to a specific codon in mRNA). Ribosomal RNA (rRNA) works with proteins to create the structure of ribosomes, the sites of translation, and the process of building proteins. The ribosomes are made up of: a large subunit and a small subunit. The ribosomes have 2 attachment sites for tRNA: the P-site and the A-site. Each ribosome holds an mRNA and 2 tRNAs which match codons with anticodons to produce proteins with a specific amino acid sequence.

** Condons are three-nucleotide sequences (each corresponding to a specific amino acid). Anticodons are the site of the 2-base sequence that recognizes and matches up with the codon on the mRNA molecule. For example, if the codon was UGG then the anticodon will be ACC.

The genetic code matches each of the condon to a specific amino acid. It has a job of translating the DNA language to the protein language. The DNA has 4 letters/nucleotides and the protein has 20 amino acids. The genetic code is universal and the DNA codes for assembly of amino acids. The code is read in a sequence of three bases called: triplets on DNA, codons on mRNA, anticodons on tRNA. The start codon is AUG. The three stop codons are: UAA, UAG, and UGA. The genetic code is the same for ALL organisms (which implies that all life has descended from a common ancestor).

  • 2 letter code = (4)2 combinations
  • = 16 “words”
  • 3 letter code = (4)3 combinations
  • = 64 “words”

Translation is when the RNA translates to proteins. It can be seen as the nucleotides are being translated into a totally different language. The rate of protein synthesis may speeds up if the mRNA moves across the ribosome while another ribosome reads the same mRNA. These are the seven steps in translation:

  1. The mRNA (the ribosomal subunits) and the tRNA (carrying methionine) binds together
  2. The tRNA with the amino acid specified by the codon in the A-site arrives and its anticodon matches with the mRNA codon
  3. A peptide bond forms between the adjacent amino acids in the P and A-site.
  4. The P-site detaches and exits while it leaves its amnio acid behind
  5. The tRNA in the A-site shifts to the P-site. The tRNA carrying the amino acid specified by the mRNA codon in the A-site comes.
  6. A peptide bond is formed and attaches the A-site amino acid to the P-site polypeptide and the tRNA in the P-site exits.
  7. This process is repeated until the ribosome reaches a stop codon where the newly made protein is released, as well as the ribosome.

This process of translation can be concluded as: enter, bond, move, and exit. First, the small ribosome subunit binds to the AUG start codon. Then, the tRNA (with the amino acids) ENTERs in the A site. They make a covalent peptide BOND with the P-site. The P-site MOVES to the E-site and the A-site to the P-site. FInally, the P-site that has been shifted to the E-site EXITs and goes away when the release factor comes along. This process makes a growing peptide chain and the protein wraps up to its small shape.

**Details on the sites:

  • A site: carrying its amino acid
  • P site: the growing chain of amino acids growing polypeptides (amino acid)
  • E site: exiting through the E site

This whole process gives the amount of oxygen for the brain, muscles, and etc in our body.


One thought on “10.1 From Genes to Proteins

  1. Yurika,

    Excellent entry for 10.1 on transcription and translation. You’ve done a great job of covering the main ideas of how genetic information is transferred to proteins by numbering the steps.

    Mr. F.

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