Dna and protein synthesis test pdf
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- DNA, RNA, Protein Synthesis Unit Test for Grades 8-12
- 6: DNA and Protein Synthesis
- Central dogma of molecular biology
DNA, RNA, Protein Synthesis Unit Test for Grades 8-12
NCBI Bookshelf. Molecular Biology of the Cell. New York: Garland Science; Transcription and translation are the means by which cells read out, or express, the genetic instructions in their genes. Because many identical RNA copies can be made from the same gene , and each RNA molecule can direct the synthesis of many identical protein molecules, cells can synthesize a large amount of protein rapidly when necessary.
But each gene can also be transcribed and translated with a different efficiency, allowing the cell to make vast quantities of some proteins and tiny quantities of others Figure Moreover, as we see in the next chapter, a cell can change or regulate the expression of each of its genes according to the needs of the moment—most obviously by controlling the production of its RNA. Genes can be expressed with different efficiencies.
Gene A is transcribed and translated much more efficiently than gene B. This allows the amount of protein A in the cell to be much greater than that of protein B. The first step a cell takes in reading out a needed part of its genetic instructions is to copy a particular portion of its DNA nucleotide sequence—a gene —into an RNA nucleotide sequence.
The information in RNA, although copied into another chemical form, is still written in essentially the same language as it is in DNA—the language of a nucleotide sequence.
Hence the name transcription. Like DNA , RNA is a linear polymer made of four different types of nucleotide subunits linked together by phosphodiester bonds Figure It is not uncommon, however, to find other types of base pairs in RNA: for example, G pairing with U occasionally. The chemical structure of RNA. Uracil forms base pairs with adenine.
The absence of a methyl group in U has no effect on base-pairing; thus, U-A base pairs closely resemble T-A base pairs see Figure RNA chains therefore fold up into a variety of shapes, just as a polypeptide chain folds up to form the final shape of a protein Figure As we see later in this chapter, the ability to fold into complex three-dimensional shapes allows some RNA molecules to have structural and catalytic functions. RNA can fold into specific structures.
RNA is largely single-stranded, but it often contains short stretches of nucleotides that can form conventional base-pairs with complementary sequences found elsewhere on the same molecule. These interactions, along more Transcription begins with the opening and unwinding of a small portion of the DNA double helix to expose the bases on each DNA strand.
When a good match is made, the incoming ribonucleotide is covalently linked to the growing RNA chain in an enzymatically catalyzed reaction. The RNA chain produced by transcription—the transcript —is therefore elongated one nucleotide at a time, and it has a nucleotide sequence that is exactly complementary to the strand of DNA used as the template Figure Transcription, however, differs from DNA replication in several crucial ways. Instead, just behind the region where the ribonucleotides are being added, the RNA chain is displaced and the DNA helix re-forms.
A DNA molecule in a human chromosome can be up to million nucleotide -pairs long; in contrast, most RNAs are no more than a few thousand nucleotides long, and many are considerably shorter. The enzymes that perform transcription are called RNA polymerases. Like the DNA polymerase that catalyzes DNA replication discussed in Chapter 5 , RNA polymerases catalyze the formation of the phosphodiester bonds that link the nucleotides together to form a linear chain.
The RNA polymerase moves stepwise along the DNA, unwinding the DNA helix just ahead of the active site for polymerization to expose a new region of the template strand for complementary base -pairing. When RNA polymerase molecules follow hard on each other's heels in this way, each moving at about 20 nucleotides per second the speed in eucaryotes , over a thousand transcripts can be synthesized in an hour from a single gene.
Transcription of two genes as observed under the electron microscope. The micrograph shows many molecules of RNA polymerase simultaneously transcribing each of two adjacent genes. Although RNA polymerase catalyzes essentially the same chemical reaction as DNA polymerase , there are some important differences between the two enzymes. First, and most obvious, RNA polymerase catalyzes the linkage of ribonucleotides, not deoxyribonucleotides.
This difference may exist because transcription need not be as accurate as DNA replication see Table , p. RNA polymerases make about one mistake for every 10 4 nucleotides copied into RNA compared with an error rate for direct copying by DNA polymerase of about one in 10 7 nucleotides , and the consequences of an error in RNA transcription are much less significant than that in DNA replication.
If the incorrect ribonucleotide is added to the growing RNA chain, the polymerase can back up, and the active site of the enzyme can perform an excision reaction that mimics the reverse of the polymerization reaction, except that water instead of pyrophosphate is used see Figure RNA polymerase hovers around a misincorporated ribonucleotide longer than it does for a correct addition, causing excision to be favored for incorrect nucleotides. However, RNA polymerase also excises many correct bases as part of the cost for improved accuracy.
The majority of genes carried in a cell's DNA specify the amino acid sequence of proteins; the RNA molecules that are copied from these genes which ultimately direct the synthesis of proteins are called messenger RNA mRNA molecules.
The final product of a minority of genes, however, is the RNA itself. Careful analysis of the complete DNA sequence of the genome of the yeast S. These RNAs, like proteins, serve as enzymatic and structural components for a wide variety of processes in the cell. In Chapter 5 we encountered one of those RNAs, the template carried by the enzyme telomerase. Each transcribed segment of DNA is called a transcription unit. In eucaryotes, a transcription unit typically carries the information of just one gene , and therefore codes for either a single RNA molecule or a single protein or group of related proteins if the initial RNA transcript is spliced in more than one way to produce different mRNAs.
In bacteria, a set of adjacent genes is often trans-cribed as a unit; the resulting mRNA molecule therefore carries the information for several distinct proteins.
Overall, RNA makes up a few percent of a cell's dry weight. The mRNA population is made up of tens of thousands of different species, and there are on average only 10—15 molecules of each species of mRNA present in each cell. To transcribe a gene accurately, RNA polymerase must recognize where on the genome to start and where to finish. The way in which RNA polymerases perform these tasks differs somewhat between bacteria and eucaryotes.
Because the process in bacteria is simpler, we look there first. The initiation of transcription is an especially important step in gene expression because it is the main point at which the cell regulates which proteins are to be produced and at what rate.
Bacterial RNA polymerase is a multisubunit complex. RNA polymerase molecules adhere only weakly to the bacterial DNA when they collide with it, and a polymerase molecule typically slides rapidly along the long DNA molecule until it dissociates again.
However, when the polymerase slides into a region on the DNA double helix called a promoter , a special sequence of nucleotides indicating the starting point for RNA synthesis, it binds tightly to it. The transcription cycle of bacterial RNA polymerase. The polymerase unwinds the DNA at the position at which transcription more After the RNA polymerase binds tightly to the promoter DNA in this way, it opens up the double helix to expose a short stretch of nucleotides on each strand Step 2 in Figure Instead, the polymerase and DNA both undergo reversible structural changes that result in a more energetically favorable state.
With the DNA unwound, one of the two exposed DNA strands acts as a template for complementary base -pairing with incoming ribonucleotides see Figure , two of which are joined together by the polymerase to begin an RNA chain. Several structural features of bacterial RNA polymerase make it particularly adept at performing the transcription cycle just described. With the polymerase now functioning in its elongation mode, a rudder-like structure in the enzyme continuously pries apart the DNA-RNA hybrid formed.
We can view the series of conformational changes that takes place during transcription initiation as a successive tightening of the enzyme around the DNA and RNA to ensure that it does not dissociate before it has finished transcribing a gene. If an RNA polymerase does dissociate prematurely, it cannot resume synthesis but must start over again at the promoter. The structure of a bacterial RNA polymerase. This RNA polymerase is formed from four different subunits, indicated by different colors right.
How do the signals in the DNA termination signals stop the elongating polymerase? As the polymerase transcribes across a terminator , the hairpin may help to wedge open the movable flap on the RNA polymerase and release the RNA transcript from the exit tunnel.
At the same time, the DNA-RNA hybrid in the active site , which is held together predominantly by U-A base pairs which are less stable than G -C base pairs because they form two rather than three hydrogen bonds per base pair , is not sufficiently strong enough to hold the RNA in place, and it dissociates causing the release of the polymerase from the DNA, perhaps by forcing open its jaws.
Thus, in some respects, transcription termination seems to involve a reversal of the structural transitions that happen during initiation. The process of termination also is an example of a common theme in this chapter: the ability of RNA to fold into specific structures figures prominantly in many aspects of decoding the genome. As we have just seen, the processes of transcription initiation and termination involve a complicated series of structural transitions in protein , DNA , and RNA molecules.
It is perhaps not surprising that the signals encoded in DNA that specify these transitions are difficult for researchers to recognize. Indeed, a comparison of many different bacterial promoters reveals that they are heterogeneous in DNA sequence.
These common features are often summarized in the form of a consensus sequence Figure In general, a consensus nucleotide sequence is derived by comparing many sequences with the same basic function and tallying up the most common nucleotide found at each position.
Consensus sequence for the major class of E. A The promoters are characterized by two hexameric DNA sequences, the sequence and the sequence named for their approximate location relative to the start point of transcription designated more One reason that individual bacterial promoters differ in DNA sequence is that the precise sequence determines the strength or number of initiation events per unit time of the promoter.
Evolutionary processes have thus fine-tuned each promoter to initiate as often as necessary and have created a wide spectrum of promoters. Promoters for genes that code for abundant proteins are much stronger than those associated with genes that encode rare proteins, and their nucleotide sequences are responsible for these differences.
Like bacterial promoters, transcription terminators also include a wide range of sequences, with the potential to form a simple RNA structure being the most important common feature. Since an almost unlimited number of nucleotide sequences have this potential, terminator sequences are much more heterogeneous than those of promoters. We have discussed bacterial promoters and terminators in some detail to illustrate an important point regarding the analysis of genome sequences. Although we know a great deal about bacterial promoters and terminators and can develop consensus sequences that summarize their most salient features, their variation in nucleotide sequence makes it difficult for researchers even when aided by powerful computers to definitively locate them simply by inspection of the nucleotide sequence of a genome.
When we encounter analogous types of sequences in eucaryotes, the problem of locating them is even more difficult. Often, additional information, some of it from direct experimentation, is needed to accurately locate the short DNA signals contained in genomes.
Promoter sequences are asymmetric see Figure , and this feature has important consequences for their arrangement in genomes. However a gene typically has only a single promoter , and because the nucleotide sequences of bacterial as well as eucaryotic promoters are asymmetric the polymerase can bind in only one orientation.
The choice of template strand for each gene is therefore determined by the location and orientation of the promoter. The importance of RNA polymerase orientation. Directions of transcription along a short portion of a bacterial chromosome.
Some genes are transcribed using one DNA strand as a template, while others are transcribed using the other DNA strand. The direction of transcription is determined by the promoter more Having considered transcription in bacteria, we now turn to the situation in eucaryotes, where the synthesis of RNA molecules is a much more elaborate affair.
6: DNA and Protein Synthesis
Practice Quiz for Molecular Level of Genetics. The largest molecules in our bodies are: a nucleic acids b chromosomes c proteins d amino acids 5. Which of the following statements is true? Which of the following statements is true about DNA? Which of the following statements is true about the protein synthesis process? Which of the following occurs at the ribosomes?
DNA RNA Protein Synthesis Review pdf (#10). CP Biology. DNA, RNA & Protein Synthesis.
Central dogma of molecular biology
Worksheet December 19, A Protein Synthesis Worksheet is a vital step in the overall plan of growth, development and athletic performance of an athlete. B Multiple choice Directions: Answer the following questions based on the following diagram. The resulting software tool allows us to perform near-optimal quantification of in vitro protein-DNA interaction specificity for all eight Drosophila Hox proteins and Exd-Hox
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We present a flexible, real-time-coupled transcription—translation assay that involves the continuous monitoring of fluorescent Emerald GFP formation. Along with numerical simulation of a reaction kinetics model, the assay permits quantitative estimation of the effects on full-length protein synthesis of various additions, subtractions or substitutions to the protein synthesis machinery. Since the assay uses continuous fluorescence monitoring, it is much simpler and more rapid than other assays of protein synthesis and is compatible with high-throughput formats.
According to the central dogma, which of the following represents the flow of genetic information in cells? Which of the following enzymes involved in DNA replication is unique to eukaryotes? Which of the following components is involved in the initiation of transcription?