Concept 3.1: main point Acids room Informational Macromolecules

Nucleic acids space polymers the store, transmit, and also express hereditary (genetic) information. This details is encoded in the sequences of monomers that consist of nucleic acids. There space two species of main point acids: DNA (deoxyribonucleic acid) and also RNA (ribonucleic acid). DNA stores and also transmits genetic information. With RNA intermediates, the info encoded in DNA is used to specify the amino acid sequences that proteins. Together you will see later on in this chapter, protein are necessary for both metabolism and structure. Certain committed RNA molecules also play duties in metabolism. Ultimately, nucleic acids and the protein encoded by them identify the metabolic features of an organism.

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Nucleotides space the structure blocks of nucleic acids

Nucleic acids space polymers composed of monomers called nucleotides. A nucleotide consists of three components: a nitrogen-containing base, a pentose sugar, and also one to three phosphate groups (FIGURE 3.1). Molecules consisting of a pentose sugar and a base—but no phosphate group—are dubbed nucleosides. The nucleotides that consist of nucleic mountain contain simply one phosphate group—they room nucleoside monophosphates.



Figure 3.1: Nucleotides have actually Three Components Nucleotide monomers are the building blocks of DNA and also RNA polymers. Nucleotides may have one to three phosphate groups; those in DNA and also RNA have actually one.

The bases that the main point acids take one of two chemical forms: a six-membered single-ring structure dubbed a pyrimidine, or a unify double-ring structure dubbed a purine (see number 3.1). In DNA, the pentose street is deoxyribose, which different from the ribose discovered in RNA through the absence of one oxygen atom (see figure 2.9).

During the formation of a nucleic acid, brand-new nucleotides are included to an present chain one in ~ a time. The pentose street in the last nucleotide the the currently chain and the phosphate top top the new nucleotide undergo a condensation reaction (see number 2.8) and also the resulting affiliation is referred to as a phosphodiester bond. The phosphate ~ above the brand-new nucleotide is attached come the 5′ (5 prime) carbon atom that its sugar, and also the bond occurs between it and the 3’ (3 prime) carbon ~ above the critical sugar that the existing chain. Due to the fact that each nucleotide is included to the 3′ carbon the the critical sugar, nucleic acids are said to grow in the 5′ to 3 direction (FIGURE 3.2).



Figure 3.2: Linking Nucleotides Together growth of a nucleic mountain (RNA in this figure) native its monomers wake up in the 5′ (phosphate) come 3′ (hydroxyl) direction.

Nucleic acids deserve to be oligonucleotides, with a few to about 20 nucleotide monomers, or longer polynucleotides:

Oligonucleotides encompass RNA molecule that function as “primers” to start the duplication of DNA; RNA molecule that control the expression the genes; and synthetic DNA molecules used for amplifying and analyzing other, longer nucleotide sequences.Polynucleotides, much more commonly referred to as nucleic acids, encompass DNA and most RNA. Polynucleotides deserve to be an extremely long, and also indeed room the longest polymers in the life world. Some DNA molecule in humans contain thousands of millions the nucleotides.

Base pairing occurs in both DNA and also RNA

In addition to differing in your sugar groups, DNA and RNA also differ in your bases and general structures (TABLE 3.1). Four bases are discovered in DNA: adenine (A), cytosine (C), guanine (G), and also thymine (T). RNA also contains adenine, cytosine, and guanine, but the 4th base in RNA is uracil (U) fairly than thymine. The lack of a hydroxyl group at the 2′ position of the deoxyribose sugar in DNA provides the structure of DNA much less flexible 보다 that that RNA. As we explain below, DNA is written of 2 polynucleotide strands vice versa, RNA is normally single-stranded. However, a lengthy RNA deserve to fold up on itself, forming a range of structures.



The an essential to expertise the structure and function of both DNA and RNA is the rule of complementary basic pairing. In DNA, adenine and thymine always pair (A-T), and also cytosine and guanine constantly pair (C-G):



In RNA, the basic pairs room A-U and C-G. Base pairs are held together primarily by hydrogen bonds. Together you can see, there room polar C

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O and N—H covalent bonds in the nucleotide bases (see ide 2.2 because that a discussion of polar covalent bonds). Hydrogen bonds kind between the partial an adverse charge (δ−) on one oxygen or nitrogen atom of one base, and also the partial positive charge (δ+) top top a hydrogen atom of an additional base. Complementary base pairing occurs because the arrangements of polar bond in the nucleotide bases donate the pairing that bases together they happen (C v G, and A v U or T).

Individual hydrogen bond are relatively weak, however there are so countless of lock in DNA and RNA that collectively they provide a substantial force of attraction. However, this attraction is no as solid as that detailed by lot of covalent bonds. This way that basic pairs are fairly easy to different with a modest input of energy. As you will check out in Chapters 9 and also 10, the breaking and also making of hydrogen binding in main point acids is crucial to their functions in living systems. Let’s now look in a little an ext detail at the structures of RNA and DNA.

RNA Usually, RNA is single-stranded (FIGURE 3.3A). However, numerous single-stranded RNA molecules fold up right into three-dimensional structures, due to the fact that of hydrogen bonding between nucleotides in separate parts of the molecules (FIGURE 3.3B). An RNA strand can also fold ago on chin to form a double-stranded helix. This results in a three-dimensional surface for the bonding and also recognition of other molecules. That is important to realize the this folding occurs by complementary base pairing, and the framework is thus identified by the specific order of bases in the RNA molecule.



Figure 3.3: RNA (A) RNA is generally a solitary strand. (B) once a single-stranded RNA folds ago on itself, hydrogen bonds in between complementary sequences have the right to stabilize it right into a three-dimensional shape with unique surface characteristics.

DNA Usually, DNA is double-stranded; the is, it is composed of two different polynucleotide strands the the same size (FIGURE 3.4A). The two polynucleotide strands space antiparallel: they run in opposite direction so that their 5′ ends room at opposite ends of the double-stranded molecule. In contrast to RNA’s diversity in three-dimensional structure, DNA is remarkably uniform. The A-T and also G-C basic pairs are about the same size (each is a purine paired v a pyrimidine), and also the two polynucleotide strands type a “ladder” that twists right into a double helix (FIGURE 3.4B). The sugar–phosphate groups type the political parties of the ladder, and also the bases with their hydrogen bonds type the rungs on the inside. The twin helix is virtually always right-handed:


Figure 3.4: DNA (A) DNA usually consists of two strands to run in opposite directions the are held together by base pairing in between purines and also pyrimidines the contrary one an additional on the 2 strands. (B) The two antiparallel strands in a DNA molecule room twisted right into a dual helix.

DNA carries information and is expressed through RNA

DNA is a purely informational molecule. The information is encoded in the sequence of bases carried in the strands. Because that example, the information encoded in the sequence TCAGCA is various from the info in the succession CCAGCA. DNA has actually two features in terms of information:

DNA have the right to be reproduced specifically by DNA replication. DNA is replicated by polymerization utilizing an currently strand together a base-pairing template.Some DNA sequences deserve to be copied into RNA, in a process called transcription. The nucleotide sequences in most RNA molecules have the right to then be offered to clues sequences the amino acids in protein (polypeptides). This process is referred to as translation.

The details of these important processes are defined in Chapters 9 and also 10, but it is essential to realize several things in ~ this point:


DNA replication and transcription depend on the base pairing nature of main point acids. In both replication and also transcription, the hydrogen bonds in between two DNA strands room broken, so the complementary basic pairing deserve to occur between an existing DNA strand and also a newly creating strand the DNA or RNA. The resulting brand-new DNA or RNA strand is complementary to the existing DNA design template strand. Recall that the hydrogen-bonded base pairs space A-T and also G-C in DNA and also A-U and also G-C in RNA. Now, think about this double-stranded DNA region:
Transcription the the lower strand will an outcome in a single strand that RNA with the sequence 5′-UCAGCA-3′. Deserve to you figure out what RNA sequence the optimal strand would certainly produce?DNA replication usually entails the whole DNA molecule. Since DNA holds necessary information, it have to be replicated fully so the each brand-new cell or new organism receive a complete collection of DNA indigenous its parent (FIGURE 3.5A).
Figure 3.5: DNA Replication and also Transcription DNA is completely replicated during cell reproduction (A), however it is only partially transcribed (B). In transcription, the DNA password is replicated to RNA. The succession of the last determines the amino mountain sequence that a protein. Transcription of the gene for plenty of different protein is activated at different times and, in multicell organisms, in various cells of the body.
Gene expression is the transcription and also translation of specific DNA sequences. Order of DNA the encode details proteins and also are transcribed into RNA are dubbed genes (FIGURE 3.5B). The complete collection of DNA in a living organism is called its genome. However, not every one of the details in the genome is essential at all times and also in every tissues. For example, in humans, the gene the encodes the significant protein in hair (keratin) is expressed just in skin cells. The hereditary information in the keratin-encoding gene is transcribed into RNA and also then analyzed into the protein keratin. In other tissues such together the muscles, the keratin gene is not transcribed, yet other genes are—for example, the genes that encode proteins existing in muscles but not in skin.

The DNA basic sequence reveals evolution relationships

Because DNA carries hereditary information from one generation come the next, a theoretical series of DNA molecule stretches back through the lineage of every biology to the start of biological evolution ~ above Earth, about 3.8 billion years ago. The genomes the organisms slowly accumulate transforms in their DNA base sequences over evolutionary time. Because of this closely associated living types should have more similar base sequences than species that are an ext distantly related.

Over the previous two decades there have been remarkable developments in innovations for determining the order of nucleotides in DNA molecules (DNA sequencing), and also in computer technologies to analysis these sequences. These advances have permitted scientists to recognize the entire DNA basic sequences of totality organisms, consisting of the person genome, which contains about 3 billion basic pairs. This studies have confirmed numerous of the evolutionary relationships that were inferred from more traditional to compare of human body structure, biochemistry, and physiology. Timeless comparisons had indicated the the closest living family member of humans (Homo sapiens) is the chimpanzee (genus Pan). In fact, the chimpanzee genome shares practically 99 percent that its DNA basic sequence through the human genome. Increasingly, scientists rotate to DNA analyses to figure out evolution relationships as soon as other comparisons room not possible or are not conclusive. For example, DNA researches revealed a nearby relationship between starlings and also mockingbirds the was not expected on the communication of your anatomy or behavior.

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For more on the use of DNA sequences to reconstruct the evolutionary history of life, see Concept 16.2


CHECKpoint ide 3.1

perform the crucial differences between DNA and RNA and also between purines and also pyrimidines. What space the differences between DNA replication and also transcription? If one strand of a DNA molecule has actually the succession 5′-TTCCGGAT-3′, what is the succession of the various other strand of DNA? If RNA is transcribed indigenous the 5′-TTCCGGAT-3′ strand, what would be that sequence? and also if RNA is transcribed from the other DNA strand, what would certainly be its sequence? (Note the it is typical to create these sequences through the 5′ finish on the left.) How can DNA molecule be so varied when they show up to it is in structurally similar?

Nucleic mountain are greatly informational molecules the encode proteins. Us will now turn come a conversation of proteins—the most structurally and also functionally diverse course of macromolecules.