Another aspect of the tertiary structure of the protein is addition of small molecules to the chain. For instance, phosphate groups may be attached to the protein giving it additional energy. Also, short chains of sugars are usually bound to the tail-end of the protein.
These sugar chains serve as "ZIP-code tags" for the protein, informing carrier molecules exactly where in the cell this protein needs to be carried to usually within vesicles that bud off the Rough Enodplasmic Reticulum or the Golgi apparatus.
The elements of the cytoskeleton are used as conduits "elevators and escalators" to shuttle proteins to where in the cell they are needed. Many proteins are composed of more than one polypeptide chain. For instance, hemoglobin is formed by binding together four subunits.
Each subunit also has a heme molecule attached to it, and an ion of iron attached to the heme this iron is where oxygen binds to hemogolobin. This larger, more complex structure of the protein is its quaternary structure. The views expressed are those of the author s and are not necessarily those of Scientific American. Follow Bora Zivkovic on Twitter. Already a subscriber? Sign in. Thanks for reading Scientific American.
Create your free account or Sign in to continue. See Subscription Options. Go Paperless with Digital. Transcription For a gene to be expressed, i.
Physics of sperm vs. Get smart. Sign up for our email newsletter. Sign Up. Read More Previous. Support science journalism. The resulting protein chains can be hundreds of amino acids in length, and synthesizing these molecules requires a huge amount of chemical energy Figure 8. Figure 8: The major steps of translation 1 Translation begins when a ribosome gray docks on a start codon red of an mRNA molecule in the cytoplasm.
A second tRNA molecule, bound to two, connected amino acids, is attached to the 4 th , 5 th , and 6 th nucleotide from the left. It no longer has amino acids bound to its terminus. In step 4, the tRNA molecule that formerly had two connected amino acids attached to its terminus, has now accumulated four amino acids total.
Different colored spheres represent different amino acid types, and the four spheres are connected end-to-end in a chain. A tRNA to the right has one amino acid attached to its terminus.
A tRNA molecule carrying a single amino acid is shown approaching the ribosome from the cytoplasm. In step 5, the ribosome is shown to have moved along the length of the mRNA molecule from left to right.
A long chain of approximately 19 amino acids is connected to the end of the tRNA molecule. Five tRNA molecules carrying a single amino acid each are seen floating freely in the cytoplasm surrounding the mRNA molecule. In step 6, the ribosome is disassociated from the mRNA molecule. The amino acid chain has disassociated from the tRNA and is floating freely in the cytoplasm as a complete protein molecule.
The illustrated ribosome is translucent and looks like an upside-down glass jug. The mRNA is composed of many nucleotides that resemble pegs aligned side-by-side along the molecule, in parallel. Each type of nucleotide is represented by a different color yellow, blue, orange, or green.
The first three nucleotides, bound to the ribosome, are highlighted in red to represent the stop codon. In step 2, a tRNA molecule is bound to the stop codon. At the end of the tRNA molecule opposite this point of attachment is an amino acid, represented as a sphere. In step 3, a tRNA bound to a single amino acid is attached to the 7 th , 8 th , and 9 th nucleotide from the left.
In eukaryotic cells, however, the two processes are separated in both space and time: mRNAs are synthesized in the nucleus, and proteins are later made in the cytoplasm. This page appears in the following eBook. Aa Aa Aa. Ribosomes, Transcription, and Translation. Figure 1: DNA replication of the leading and lagging strand. The helicase unzips the double-stranded DNA for replication, making a forked structure.
Figure 3: RNA polymerase at work. What Is the Function of Ribosomes? This Escherichia coli cell has been treated with chemicals and sectioned so its DNA and ribosomes are clearly visible.
Figure 7: The ribosome and translation. A ribosome is composed of two subunits: large and small. Figure 8: The major steps of translation. As a result, each new cell has its own complete genome.
This process is known as DNA replication. Replication is controlled by the Watson-Crick pairing of the bases in the template strand with incoming deoxynucleoside triphosphates, and is directed by DNA polymerase enzymes.
It is a complex process, particularly in eukaryotes, involving an array of enzymes. A simplified version of bacterial DNA replication is described in Figure 2. DNA biosynthesis proceeds in the 5'- to 3'-direction.
This makes it impossible for DNA polymerases to synthesize both strands simultaneously. A portion of the double helix must first unwind, and this is mediated by helicase enzymes. The leading strand is synthesized continuously but the opposite strand is copied in short bursts of about bases, as the lagging strand template becomes available.
The resulting short strands are called Okazaki fragments after their discoverers, Reiji and Tsuneko Okazaki. Strangely, DNA polymerases cannot initiate DNA synthesis de novo , but require a short primer with a free 3'-hydroxyl group. Pol III can then take over, but it eventually encounters one of the previously synthesized short RNA fragments in its path. The gap is filled by DNA ligase, an enzyme that makes a covalent bond between a 5'-phosphate and a 3'-hydroxyl group Figure 3.
The initiation of DNA replication at the leading strand is more complex and is discussed in detail in more specialized texts. DNA replication is not perfect. This leads to mismatched base pairs, or mispairs. DNA polymerases have proofreading activity, and a DNA repair enzymes have evolved to correct these mistakes. Occasionally, mispairs survive and are incorporated into the genome in the next round of replication.
These mutations may have no consequence, they may result in the death of the organism, they may result in a genetic disease or cancer; or they may give the organism a competitive advantage over its neighbours, which leads to evolution by natural selection. Transcription is the process by which DNA is copied transcribed to mRNA, which carries the information needed for protein synthesis. Transcription takes place in two broad steps. The mechanism of transcription has parallels in that of DNA replication.
The mRNA molecule is elongated and, once the strand is completely synthesized, transcription is terminated. The newly formed mRNA copies of the gene then serve as blueprints for protein synthesis during the process of translation. Further Exploration Concept Links for further exploration translation transcription unit gene expression frameshift mutation nonsense mutation RNA DNA enhancer promoter differentiation gene expression transcription factor intron exon chromatin histones mutation helicase transcriptome phosphate backbone poly-A tail nuclear pore primase TATA box hairpin loop mRNA DNA polymerase mRNA chromatin remodeling cis-regulatory element RNA polymerase catabolite repression methylation.
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