Specialized Cell Structure and Function
The making of the various types of protein is one of the most important events for a cell because protein not only forms structural components of the cell, it also composes the enzymes that catalyze the production of the remaining organic biomolecules necessary for life. In general, the genotype coded for in the DNA is expressed as a phenotype by the protein and other enzyme-catalyzed products.
The DNA housed in the nucleus is too large to move through the nuclear membrane, so it must be copied by the smaller, single-stranded RNA (transcription), which moves out of the nucleus to ribosomes located in the cytoplasm and rough endoplasmic reticulum to direct the assembly of protein (translation). The genes do not actually make the protein, but they provide the blueprint in the form of RNA, which directs the protein synthesis.
Transcription occurs in the cell nucleus and represents the transfer of the genetic code from DNA to a complementary RNA. The enzyme RNA polymerase …
- Attaches to and unzips the DNA molecule to become two separate strands.
- Binds to promoter segments of DNA that indicate the beginning of the single strand of DNA to be copied.
- Moves along the DNA and matches the DNA nucleotides with a complementary RNA nucleotide to create a new RNA molecule that is patterned after the DNA.
The copying of the DNA continues until the RNA polymerase reaches a termination signal, which is a specific set of nucleotides that mark the end of the gene to be copied and also signals the disconnecting of the DNA with the newly minted RNA.
The three types of RNA are …
- mRNA (messenger RNA) is transcribed from DNA and carries the genetic information from the DNA to be translated into amino acids.
- tRNA (transfer RNA) “interprets” the three-letter codons of the nucleic acids to the one-letter amino acid word
- rRNA (ribosomal RNA) is the most abundant type of RNA, and along with associated proteins compose the ribosomes.
When the RNA polymerase is finished copying a particular segment of DNA, the DNA reconfigures into the original double-helix structure. The newly created mRNA moves out of the nucleus and into the cytoplasm.
Translation is the conversion of information contained in a sequence of mRNA nucleotides into a sequence of amino acids that bond together to create a protein. The mRNA moves to the ribosomes and is “read” by tRNA, which analyzes sections of three adjoining nucleotide sequences, called codons, on the mRNA and brings the corresponding amino acid for assembly into the growing polypeptide chain. The three nucleotides in a codon are specific for a particular amino acid. Therefore, each codon signals for the inclusion of a specific amino acid, which combines in the correct sequence to create the specific protein that the DNA coded for.
The assembly of the polypeptide begins when a ribosome attaches to a start codon located on the mRNA. Then tRNA carries the amino acid to the ribosomes, which are made of rRNA and protein and have three bonding sites to promote the synthesis. The first site orients the mRNA so the codons are accessible to the tRNA, which occupy the remaining two sites as they deposit their amino acids and then release from the mRNA to search for more amino acids. Translation continues until the ribosome recognizes a codon that signals the end of the amino acid sequence. The polypeptide, when completed, is in its primary structure. It is then released from the ribosome to begin contortions to configure into the final form to begin its function.
Each codon on the mRNA specifies a particular amino acid, which is recognized by the anticodon of the complementary tRNA. There are 20 different amino acids; there are also 20 different tRNA molecules.
After the proteins are made, they are packaged and transported to their final destination in an interesting pathway that can be described in three steps involving three organelles:
- Vesicles transport the proteins from the ribosomes to the Golgi apparatus, a.k.a Golgi complex, where they are packaged into new vesicles.
- The vesicles migrate to the membrane and release their protein to the outside of the cell.
- Lysosomes digest and recycle the waste materials for reuse by the cell.
Enzymes within the Golgi apparatus modify the proteins and enclose them in a new vesicle that buds from the surface of the Golgi apparatus. The Golgi apparatus is often seen as the packaging and distribution center of the cell.
Vesicles are small, membrane-enclosed envelopes that are usually made in the endoplasmic reticulum or Golgi apparatus and are used to transport substances through the cell.
Lysosomes are a special type of vesicle that contains the digestive enzymes for the cell and are useful in breaking down leftover waste products of proteins, lipids, carbohydrates, and nucleic acids into their component parts for reassembly and reuse by the cell.
Excerpted from The Complete Idiot's Guide to Biology © 2004 by Glen E. Moulton, Ed.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.