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Biology

Chapter 10: DNA Transcription and Translation

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Lecture

Expressing Genes

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Gene Expression

Genes are expressed or turned from genetic information into cell components through a series of steps that copy, filter, and move the information from the nucleus to cell organelles for use.

Transcription: DNA to RNA

In transcription, the information in the DNA is copied to an RNA form called messenger RNA (mRNA).

  1. First the DNA strands separate.
  2. Then the enzyme RNA polymerase joins the DNA strand at a promoter sequence (initiation) and "walks" the strand, adding nucleotides to the RNA chain which are dictated by the DNA template (elongation).
    • If the current DNA nucleotide to which the polymerase is attached has an adenine base, the polymerase adds uracil to the mRNA strand.
    • If the DNA has a guanine base, the polymerase adds cytosine.
  3. The polymerase stops adding nucleotides to the mRNA when it reaches a terminator codon, one of the three-nucleotide sequences which mean "STOP" (termination).
  4. When the single strand of messenger RNA is done, it peals away from the DNA.
  5. The mRNA travels through the nuclear membrane to the cell's cytoplasm.
  6. In the cytoplasm, the mRNA forms a complex with a ribosome embedded in the endoplasmic reticulum, and the complex produces a protein whose amino acid sequence is dictated by the nucleotide sequence of the mRNA.
transcription translation

Translation: RNA to protein

The mRNA now contains the instructions for a protein complex. In order to actually manufacture the protein strand, the mRNA must use translation RNA, or tRNA, molecules. Each tRNA molecule is about 80 nucleotides long, and folded over on itself to form double-stranded regions held together by hydrogen bonds, just like double-helix DNA. Several single stranded loops remain, one of which contains an anticodon, or triplet of nucleotides which is complementary and can attach to a specific mRNA codon. Another site on the tRNA can attach to a specific amino acid with the aid of an appropriate enzyme.

Manufacture of proteins occurs in the ribosomes, which are complexes of ribosomal RNA [we won't go into this form of RNA here]. One mRNA binds to a ribosomal subunit at a start codon (initiation); if more ribosomal units are required for the translation process, the cell adds them. When the ribosomal complex is complete, it holds the mRNA in a specific position which it can manipulate. A tRNA molecule with the anticodon matching the mRNA codon is bound to it at the newly-formed ribosomal complex "P site". The peptide chain assembled so far (one amino acid long!) is attached to this tRNA molecule. The adjacent mRNA codon (at the "A site")binds with another tRNA (carrying its amino acid) at the A site. An enzyme facilitates the transfer of the peptide on the tRNA at the P site to the amino acid on the tRNA amino acid (peptide formation) at the A site. The tRNA at the P site is released, and the tRNA at the A site moves the P site, while the ribosome locks the next tRNA required by the mRNA into the A site (translocation). This elongation process until the the A site gets to a stop codon in the mRNA strand.

Note that since the ribosome is really only manipulating a few codons at a time. If the mRNA is long enough, multiple ribosomes can attach to it, each one manufacturing a protein. Such a complex is called a polyribosome.

Special consideration for Eukaryotes

Prokaryotic cells (bacteria cells) have a relatively simple protein synthesis process. Since prokaryotes are single-celled organisms with no nuclear membrane and one circular DNA gene, once transcription has begun, ribosomes can attach to the mRNA and begin translation. The mRNA in eukaryotic cells must undergo some further processing between transcription and translation. First, a special nucleotide is added during transcription to a position near the beginning of the mRNA strand. This nucleotide or cap; appears to help a ribosome attach to the mRNA, and also keeps the mRNA from disintegrating easily. Also, a tail of adenine nucleotides are added the end of the mRNA strand, but the reason for this polyadenylated tail is not known. Finally, the genes in eukaryotic cells are complex and redundant. A single gene may have segments (introns) that are not used embedded within segments of DNA (exons) that are used. The whole gene is transcribed, but before it can be used, the intron sections must be excised (cut out), and the used portions spliced together.

All cells, whether eukaryotic or prokaryotic, however, use the same genetic code. The codon CCU codes for proline in Escheria coli (a bacteria) and in you, despite all the differences between single-celled prokaryotes and multi-celled eukaryotes. This universality of genetic information is one of the reasons many biologists think all life forms must be essentially related, descended from some common ancestor in which the code originated.

But nothing's perfect: Mutations

InsertionMutation

A mutation is a change in the genetic code. It can have a number of causes, and take several forms. The simplest mutation involves a change in a single nucleotide base pair in the DNA, which results in the mRNA translating a different amino acid (missense mutation) or a stop codon (nonsense mutation) instead of the correct amino acid. Both mutations can be disastrous, although it is possible that a missense mutation will have little effect if the new amino acid is closely related to the old, or in a functionally unimportant location on the resulting protein.

Frameshift mutations occur when a codon loses one or two of its nucleotides or a nucleotide is interjected into a codon. The whole sequence is shifted, so that every subsequent codon is "reassessed" as a different amino acid or stop sequence:

Frameshift mutations almost always destroy the ability of the resulting strand to produce a functional protein or enzyme.

Viruses and DNA exchange

As we discussed earlier, Griffith's experiments show that bacteria could exchange DNA, and Hershey and Chase found that viruses inject their DNA into their host cells. Once in the host, the new DNA strands begin to use cell resources to replicate their own proteins and DNA, causing the cell to function differently, and eventually to make lots of new virus cells. Viruses which cause their hosts to die are called pathogens or phages.

Two kinds of bacteriophages are known. A lytic bacteriophage destroys the host cell. First the virus attaches to the wall of the cell, then it injects its own DNA into the cell. The phage DNA is replicated when the host cell replicates its own DNA, and the phage proteins are synthesized along with the host proteins. Of course, this weakens the host cell, since many of its chemical resources and energy go into making viral DNA and proteins instead of its own. Eventually, the viral DNA and proteins are assembled into new virus bodies. When enough viruses have been made that nothing much is left of the host cell, it lyses, or falls apart, and the new virus bodies escape to infect more bacteria.

The second type of virus follows the same patter of attaching and injecting DNA, but instead of destroying the host, the viral DNA become integrated with the bacterial host DNA. The infected bacteria is called a lysogenic cell, and it exhibits new properties, such as manufacturing new chemicals (often toxic ones), but it doesn't die. Two examples are the bacteria that cause diphtheria and botulism. Both have a harmless form and a lethal form; the only difference is that the lethal form contains viral DNA which causes the bacteria to manufacture the toxin.

Viruses do not limit their field of activity to bacteria: many infect animal or plant cells. The virus must have the ability to attach to the cell and inject its DNA or to attract to the cell and be picked up by endocytosis. Since this ability can vary from organism to organism and even from one tissue type to another within a single organism, some viruses target specific tissues of a single host type. Once inside, they act as lytic or lysogenic bacteria, either destroying the cell as they multiply, or using the cell as host for their own activities. Some viruses (called retroviruses) go even further and use an enzyme called DNA polymerase to create new viral DNA from their own RNA; the new DNA makes more viral RNA and the virus reproduces or creates proteins which destroy the host's ability to function normally. Viruses which attack humans include chickenpox, mumps, rubella, warts, influenza, hepatitis, AIDS, and Ebola.