What advantages might there be to having them occur together? Eukaryotic and prokaryotic cells : A eukaryote contains a well-defined nucleus, whereas in prokaryotes, the chromosome lies in the cytoplasm in an area called the nucleoid. The size of the genome in one of the most well-studied prokaryotes, E. So how does this fit inside a small bacterial cell? The DNA is twisted by what is known as supercoiling.
Supercoiling means that DNA is either under-wound less than one turn of the helix per 10 base pairs or over-wound more than 1 turn per 10 base pairs from its normal relaxed state. Some proteins are known to be involved in the supercoiling; other proteins and enzymes such as DNA gyrase help in maintaining the supercoiled structure.
Eukaryotes, whose chromosomes each consist of a linear DNA molecule, employ a different type of packing strategy to fit their DNA inside the nucleus. At the most basic level, DNA is wrapped around proteins known as histones to form structures called nucleosomes.
The histones are evolutionarily conserved proteins that are rich in basic amino acids and form an octamer. The DNA which is negatively charged because of the phosphate groups is wrapped tightly around the histone core. This nucleosome is linked to the next one with the help of a linker DNA. This is further compacted into a 30 nm fiber, which is the diameter of the structure. At the metaphase stage the chromosomes are at their most compact, approximately nm in width, and are found in association with scaffold proteins.
Eukaryotic chromosomes : These figures illustrate the compaction of the eukaryotic chromosome. In interphase, eukaryotic chromosomes have two distinct regions that can be distinguished by staining. The tightly packaged region is known as heterochromatin, and the less dense region is known as euchromatin. Heterochromatin usually contains genes that are not expressed, and is found in the regions of the centromere and telomeres. The euchromatin usually contains genes that are transcribed, with DNA packaged around nucleosomes but not further compacted.
RNA is the nucleic acid that makes proteins from the code provided by DNA through the processes of transcription and translation. DNA is the genetic material found in all living organisms and is found in the nucleus of eukaryotes and in the chloroplasts and mitochondria. In prokaryotes, the DNA is not enclosed in a membranous envelope. Each nucleotide is made up of three components: a nitrogenous base, a pentose five-carbon sugar called ribose, and a phosphate group.
RNA Structure : A nucleotide is made up of three components: a nitrogenous base, a pentose sugar, and one or more phosphate groups. Figure 3. The phosphate backbone is located on the outside, and the bases are in the middle. Adenine forms hydrogen bonds or base pairs with thymine, and guanine base pairs with cytosine. A mutation occurs, and cytosine is replaced with adenine.
What impact do you think this will have on the DNA structure? RNA is usually single-stranded and is made of ribonucleotides that are linked by phosphodiester bonds. A ribonucleotide in the RNA chain contains ribose the pentose sugar , one of the four nitrogenous bases A, U, G, and C , and the phosphate group.
In the cytoplasm, the mRNA interacts with ribosomes and other cellular machinery Figure 4. Figure 4. A ribosome has two parts: a large subunit and a small subunit. The mRNA sits in between the two subunits. A tRNA molecule recognizes a codon on the mRNA, binds to it by complementary base pairing, and adds the correct amino acid to the growing peptide chain.
The mRNA is read in sets of three bases known as codons. Each codon codes for a single amino acid. In this way, the mRNA is read and the protein product is made. The rRNA ensures the proper alignment of the mRNA and the ribosomes; the rRNA of the ribosome also has an enzymatic activity peptidyl transferase and catalyzes the formation of the peptide bonds between two aligned amino acids.
It carries the correct amino acid to the site of protein synthesis. It is the base pairing between the tRNA and mRNA that allows for the correct amino acid to be inserted in the polypeptide chain. One other difference bears mention. There is only one type of DNA. There are mutliple types of RNA: Messenger RNA is a temporary molecule that transports the information necessary to make a protein from the nucleus where the DNA remains to the cytoplasm, where the ribosomes are.
It is present even in the smallest organisms like viruses and bacteria. The metabolic pathways of ribose hold prime importance in the living organisms.
In this article, we will discuss different aspects of ribose like its structure, isomers, properties, sources, metabolism, etc. Keep reading to completely understand the concepts related to this sugar. Ribose is a monosaccharide having five carbons, thus called a pentose sugar. Ribose is the most important pentose present in living organisms. It is an aldose sugar, having an aldehydic functional group. Its molecular formula is represented as C 5 H 10 O 5. Both these structural forms of ribose coexist in equilibrium with each other in an aqueous solution.
They are readily interconvertible. The aliphatic or open chain form of ribose is made up of five carbon atoms that are arranged in the form of a linear chain. As monosaccharides are polyhydroxy aldehydes or ketones, each carbon atom in ribose is having a hydroxyl group except the first carbon. Like all other aldose sugar, the first carbon of ribose is also a part of aldehydic functional group.
Ribose is also called an aldopentose. Ribofuranose is a five cornered ring. Four corners of this pentagon are occupied by carbon atoms, while the fifth corner, the apical one, is formed by an oxygen atom of the carbonyl functional group.
One carbon hangs outside the ring. This is the least abundant closed ring form of ribose in aqueous solutions. Ribopyranose ring has a hexagonal structure. Its structure is similar to glucose with five corners occupied by carbon atoms and one corner by an oxygen atom. However, unlike glucose, none of its carbon atom hangs outside the ring as all the five carbon atoms participate in ring formation. The property of isomerization is common is all monosaccharides.
The number of isomers of a monosaccharide is dependent on the number of chiral carbons present in it. Beyond the ladder-like structure described above, another key characteristic of double-stranded DNA is its unique three-dimensional shape.
The first photographic evidence of this shape was obtained in , when scientist Rosalind Franklin used a process called X-ray diffraction to capture images of DNA molecules Figure 5. Although the black lines in these photos look relatively sparse, Dr. Franklin interpreted them as representing distances between the nucleotides that were arranged in a spiral shape called a helix.
Around the same time, researchers James Watson and Francis Crick were pursuing a definitive model for the stable structure of DNA inside cell nuclei. Watson and Crick ultimately used Franklin's images, along with their own evidence for the double-stranded nature of DNA, to argue that DNA actually takes the form of a double helix , a ladder-like structure that is twisted along its entire length Figure 6. Franklin, Watson, and Crick all published articles describing their related findings in the same issue of Nature in Most cells are incredibly small.
For instance, one human alone consists of approximately trillion cells. Yet, if all of the DNA within just one of these cells were arranged into a single straight piece, that DNA would be nearly two meters long!
So, how can this much DNA be made to fit within a cell? The answer to this question lies in the process known as DNA packaging , which is the phenomenon of fitting DNA into dense compact forms Figure 7. During DNA packaging, long pieces of double-stranded DNA are tightly looped, coiled, and folded so that they fit easily within the cell.
Eukaryotes accomplish this feat by wrapping their DNA around special proteins called histones , thereby compacting it enough to fit inside the nucleus Figure 8.
Together, eukaryotic DNA and the histone proteins that hold it together in a coiled form is called chromatin. It is impossible for researchers to see double-stranded DNA with the naked eye — unless, that is, they have a large amount of it. Modern laboratory techniques allow scientists to extract DNA from tissue samples, thereby pooling together miniscule amounts of DNA from thousands of individual cells. When this DNA is collected and purified, the result is a whitish, sticky substance that is somewhat translucent.
To actually visualize the double-helical structure of DNA, researchers require special imaging technology, such as the X-ray diffraction used by Rosalind Franklin. However, it is possible to see chromosomes with a standard light microscope, as long as the chromosomes are in their most condensed form. To see chromosomes in this way, scientists must first use a chemical process that attaches the chromosomes to a glass slide and stains or "paints" them.
Staining makes the chromosomes easier to see under the microscope. In addition, the banding patterns that appear on individual chromosomes as a result of the staining process are unique to each pair of chromosomes, so they allow researchers to distinguish different chromosomes from one another.
Then, after a scientist has visualized all of the chromosomes within a cell and captured images of them, he or she can arrange these images to make a composite picture called a karyotype Figure This page appears in the following eBook. Aa Aa Aa.
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