Help
Options
Didn't know it?
click below
click below
Knew it?
click below
click below
Don't Know
Remaining cards (0)
Know
retry
shuffle
restart
0:00
Embed Code - If you would like this activity on your web page, copy the script below and paste it into your web page.
Normal Size Small Size show me how
Normal Size Small Size show me how
4.2c
| Question | Answer |
|---|---|
| We can now move on to an understanding of how DNA and RNA collaborate to produce proteins. Before studying the details, however, it will be helpful to consider the big picture. | In brief, the genetic code in DNA specifies which proteins a cell can make. |
| All the body’s cells except the sex cells and some immune cells contain more or less the same genes. | However, different genes are activated in different cells; for example, genes for digestive enzymes are active in stomach cells but not in muscle cells. |
| Any given cell uses only one-third to two-thirds of its genes; the rest remain dormant in that cell, | but may be functional in other types of cells. |
| When a gene is activated, a messenger RNA (mRNA) is made—a mirror image of the gene, more or less. Most mRNA migrates from the nucleus to the cytoplasm, where it serves as a code for assembling amino acids in the right order to make a particular protein. | In summary, you can think of the process of protein synthesis as DNA → mRNA → protein, with each arrow reading as “codes for the production of.” |
| The step from DNA to mRNA is called transcription, and the step from mRNA to protein is called translation. | Transcription occurs in the nucleus, where the DNA is. Most translation occurs in the cytoplasm, but 10% to 15% of proteins may be synthesized in the nucleus, with both steps occurring there. |
| DNA is too large to leave the nucleus and participate directly in cytoplasmic protein synthesis. It is necessary, therefore, to make a small mRNA copy that can migrate through a nuclear pore into the cytoplasm. | Just as we might transcribe (copy) a document, transcription in genetics means the process of copying genetic instructions from DNA to RNA. |
| An enzyme called RNA polymerase (po-LIM-urase) | binds to the DNA and assembles the RNA. |
| It opens up the DNA helix about 17 base pairs at a time, reading the bases from one strand of the DNA and making a corresponding RNA. Where it finds a C on the DNA, it adds G to the RNA; where it finds an A, it adds U; and so forth. | The enzyme then rewinds the DNA helix behind it. Another RNA polymerase may follow closely behind the first one; thus, a gene may be transcribed by several polymerase molecules at once, and numerous copies of the same RNA are made. |
| At the end of the gene is a base sequence that serves as a terminator, which signals the ----- to stop. | polymerase |
| The RNA produced by transcription is an “immature” form called pre-mRNA. This molecule contains segments called exons that will be translated into a protein, and segments called introns that are removed before translation (Fig. 4.6). | Enzymes cut out the introns and splice the exons together into a functional mRNA. |
| It may help you in remembering these if you think of introns being removed while still in the nucleus and the exons being exported from the nucleus to undergo translation in the cytoplasm. | The introns aren't entirely useless, but code for a variety of regulatory ncRNAs. |
| Through a mechanism called alternative splicing, one gene can code for more than one protein. Suppose a gene produced a pre-mRNA containing six exons separated by noncoding introns. | As shown in figure 4.6, these exons can be spliced together in various combinations to yield codes for two or more proteins. This is a partial explanation of how the body can produce millions of different proteins with little more than 22,000 genes. |
| Just as we might translate a work from Spanish into English, genetic translation converts the language of nucleotides into the language of amino acids. This job is carried out by the following participants: | Messenger RNA (mRNA), Transfer RNA (tRNA), Ribosomes |
| Messenger RNA (mRNA), which carries the genetic code from the nucleus to the cytoplasm. | During its synthesis in the nucleus, mRNA acquires a protein cap that acts like a passport, permitting it to pass through a nuclear pore into the cytosol. The cap also acts as a recognition site that tells a ribosome where to begin translation. |
| Transfer RNA (tRNA), a relatively small RNA whose job is to bind a free amino acid in the cytosol and deliver it to the ribosome to be added to a growing protein chain. | tRNA is a single-stranded molecule that turns back and coils on itself to form an angular L shape (fig. 4.7). |
| One loop of the molecule includes an anticodon, a series of three nucleotides complementary to a specific codon of mRNA. For the codon AUG, for example, the anticodon is UAC. | The other end of the tRNA has an amino acid-accepting end that binds a specific amino acid corresponding to that codon. |
| The first tRNA to bind to a ribosome at the start of translation is called the initiator tRNA. | It always has the anticodon UAC and always carries the amino acid methionine. |
| Ribosomes, the little “reading machines” found in the cytosol and on the outside of the rough ER and nuclear envelope. | Inactive ribosomes occur in the cytosol in two pieces—a small subunit and a large subunit. Each is composed of several enzymes and ribosomal RNA (rRNA) molecules. The two subunits come together only when translating mRNA. |
| A ribosome has three pockets that serve as binding sites for tRNA. In the course of translation, a tRNA usually binds first to the A site on one side of the ribosome, then shifts to the P site in the middle, and finally to the E site on the other side. | To remember the order of these sites, it may help you to think of A for the site that accepts a new amino acid; P for the site that carries the growing protein; and E for exit. (A and P actually stand for aminoacyl and peptidyl sites.) |
| Translation | occurs in three steps called initiation, elongation, termination |
| Initiation. mRNA passes through a nuclear pore into the cytosol and forms a loop. | A small ribosomal subunit binds to a leader sequence of bases on the mRNA near the cap, then slides along the mRNA until it recognizes the start codon AUG. |
| An initiator tRNA with the anticodon UAC pairs with the start codon and settles into the P (middle) site of the ribosome with its cargo of methionine (Met). | The large subunit of the ribosome then joins the complex. The assembled ribosome now embraces the mRNA in a groove between the subunits sliding along it, reading its bases. |
| Elongation. The next tRNA arrives, carrying another amino acid; it binds to the A site of the ribosome and its anticodon pairs with the second codon of the mRNA–GCU, for example. | A tRNA with the anticodon CGA would bind here, and according to the genetic code, it would carry alanine (Ala). |
| An enzyme in the ribosome transfers the Met of the initiator tRNA to the Ala delivered by the second tRNA and creates a peptide bond between them, giving us a dipeptide. Met–Ala. | The ribosome then slides down to read the next codon. This shifts the initiator tRNA (now carrying no amino acid) into the E site, where it leaves the ribosome. |
| The second tRNA (now carrying Met–Ala) shifts into the P site. | The now-vacant A site binds a third tRNA. |
| Suppose the next codon is CAU. A tRNA with anticodon GUA would bind here, and would carry histidine (His). (This is the state shown in the figure.) | The ribosome transfers the Met–Ala to the His, creates another peptide bond, and we now have a tripeptide, Met–Al–His. By repetition of this process, a larger and larger protein is produced. |
| As the protein elongates | , it folds into its three-dimensional shape. |
| Each time a tRNA leaves the E site, it goes off to pick up another amino acid from a pool of free amino acids in the cytosol. | One ATP molecule is used in binding the amino acid to the tRNA; therefore, protein synthesis consumes one ATP for every amino acid added to the chain |
| All new proteins, as we can see, begin with the amino acid methionine, carried by the initiator tRNA. | This is often cleaved off in later processing, however, so not every finished protein starts with methionine. |
| Codon–anticodon pairing is less precise than just depicted; it tolerates some mismatches, especially at the third base of the codon. Therefore, UGC isn’t necessarily the only anticodon that can pair with ACG. | Due to this imprecision or “wobble” in the system, as few as 48 different tRNAs are needed to pair up with the 61 codons that represent the amino acids. |
| Termination. When the ribosome reaches a stop codon, its A site binds a protein called a release factor instead of a tRNA. The release factor causes the finished protein to break away from the ribosome. | The ribosome then dissociates into its two subunits, but since these are now so close to the mRNA’s leader sequence, they often reassemble on the same mRNA and repeat the process, making another copy of the same protein. |
| ) Making proteins for packaging or export. If a protein is to be packaged into a lysosome or secreted from the cell (such as a digestive enzyme), | the ribosome docks on the rough endoplasmic reticulum and the new protein spools off into the cistern of the ER instead of into the cytosol. The ER modifies this protein and packages it into transport vesicles whose destiny we will examine later. |
| One ribosome can work very rapidly, adding about two to twenty amino acids per second. Most proteins take from 20 seconds to several minutes to make | However, a ribosome doesn't work alone. |
| After one ribosome moves away from the start of an mRNA molecule, another often binds and begins translation, followed by another and another. Consequently, a single mRNA is commonly translated by 10 to 20 ribosomes at once. | This cluster of ribosomes all translating the same mRNA is called a polyribosome. The farther along the mRNA a ribosome is, the longer the protein chain it has produced. |
| Not only is each mRNA translated by multiple ribosomes at once, but a cell may contain up to 300,000 identical mRNA molecules, each being translated simultaneously by about 20 ribosomes. | With so many “factory workers” doing the same task, a cell may produce over 100,000 protein molecules per second—a remarkably productive factory! |
| As much as 25% of the dry weight of liver cells, which are highly active in protein synthesis, | consists of ribosomes. |
Created by:
Russells3709