Just as saw means one thing in English and was means another, the sequence of bases CGT means one thing, and TGC means something different. Although there are only four letters—the four nucleotides—in the genetic code of DNA, their sequencing along the DNA strands can vary so widely that information storage is essentially unlimited.
There are three key differences between replication and transcription:. The DNA sequence that is transcribed to make RNA is called the template strand , while the complementary sequence on the other DNA strand is called the coding or informational strand. Thymine in DNA calls for adenine in RNA, cytosine specifies guanine, guanine calls for cytosine, and adenine requires uracil. Only a short segment of the RNA molecule is hydrogen-bonded to the template strand at any time during transcription.
The nucleotide sequence of the RNA strand formed during transcription is identical to that of the corresponding coding strand of the DNA, except that U replaces T. The representation of RNA polymerase is proportionately much smaller than the actual molecule, which encompasses about 50 nucleotides at a time.
What is the sequence of nucleotides in the RNA that is formed from this template? The molecular dimensions of the mRNA molecule vary according to the amount of genetic information a given molecule contains. After transcription, which takes place in the nucleus, the mRNA passes into the cytoplasm, carrying the genetic message from DNA to the ribosomes, the sites of protein synthesis.
Elsewhere, we shall see how mRNA directly determines the sequence of amino acids during protein synthesis. Ribosomes are cellular substructures where proteins are synthesized. Molecules of tRNA, which bring amino acids one at a time to the ribosomes for the construction of proteins, differ from one another in the kinds of amino acid each is specifically designed to carry.
A set of three nucleotides, known as a codon, on the mRNA determines which kind of tRNA will add its amino acid to the growing chain. Each of the 20 amino acids found in proteins has at least one corresponding kind of tRNA, and most amino acids have more than one.
The nucleotides that are not specifically identified here are slightly altered analogs of the four common ribonucleotides A, U, C, and G. On one loop is a sequence of three nucleotides that varies for each kind of tRNA. This foundation is a short stretch of nucleotides called a primer Figure 2. As DNA polymerase makes its way down the unwound DNA strand, it relies upon the pool of free-floating nucleotides surrounding the existing strand to build the new strand.
The nucleotides that make up the new strand are paired with partner nucleotides in the template strand; because of their molecular structures, A and T nucleotides always pair with one another, and C and G nucleotides always pair with one another. This phenomenon is known as complementary base pairing Figure 4 , and it results in the production of two complementary strands of DNA.
Base pairing ensures that the sequence of nucleotides in the existing template strand is exactly matched to a complementary sequence in the new strand, also known as the anti-sequence of the template strand. Later, when the new strand is itself copied, its complementary strand will contain the same sequence as the original template strand. Thus, as a result of complementary base pairing, the replication process proceeds as a series of sequence and anti-sequence copying that preserves the coding of the original DNA.
In the prokaryotic bacterium E. In comparison, eukaryotic human DNA replicates at a rate of 50 nucleotides per second. In both cases, replication occurs so quickly because multiple polymerases can synthesize two new strands at the same time by using each unwound strand from the original DNA double helix as a template. One of these original strands is called the leading strand, whereas the other is called the lagging strand. The leading strand is synthesized continuously, as shown in Figure 5.
In contrast, the lagging strand is synthesized in small, separate fragments that are eventually joined together to form a complete, newly copied strand. This page appears in the following eBook. Aa Aa Aa. How is DNA replicated? What triggers replication? Figure 1: Helicase yellow unwinds the double helix.
The initiation of DNA replication occurs in two steps. First, a so-called initiator protein unwinds a short stretch of the DNA double helix. Then, a protein known as helicase attaches to and breaks apart the hydrogen bonds between the bases on the DNA strands, thereby pulling apart the two strands. As the helicase moves along the DNA molecule, it continues breaking these hydrogen bonds and separating the two polynucleotide chains Figure 1.
How are DNA strands replicated? Figure 3: Beginning at the primer sequence, DNA polymerase shown in blue attaches to the original DNA strand and begins assembling a new, complementary strand.
Figure 4: Each nucleotide has an affinity for its partner. A pairs with T, and C pairs with G. The color of the rectangle represents the chemical identity of the nitrogenous base.
A grey horizontal cylinder is attached to one end of the rectangle in each nucleotide and represents a sugar molecule. The nucleotides are arranged in two rows and the nitrogenous bases point toward each other. Annunziato, A. Split decision: What happens to nucleosomes during DNA replication? Journal of Biological Chemistry , — Bessman, M.
Enzymatic synthesis of deoxyribonucleic acid. General properties of the reaction. Kornberg, A. The biological synthesis of deoxyribonucleic acid. Nobel Lecture, December 11, Biological synthesis of deoxyribonucleic acid. Science , — Lehman, I. Preparation of substrates and partial purification of an enzyme from Escherichia coli. Losick, R. DNA replication: Bringing the mountain to Mohammed. Mackiewicz, P. Where does bacterial replication start? Rules for predicting the oriC region.
Nucleic Acids Research 32 , — Ogawa, T. Molecular and General Genetics , — Okazaki, R. Mechanism of DNA chain growth. Possible discontinuity and unusual secondary structure of newly synthesized chains. Proceedings of the National Academy of Sciences 59 , — Restriction Enzymes. Genetic Mutation.
Functions and Utility of Alu Jumping Genes. Transposons: The Jumping Genes. DNA Transcription. What is a Gene? Colinearity and Transcription Units. Copy Number Variation. Copy Number Variation and Genetic Disease. Copy Number Variation and Human Disease. Tandem Repeats and Morphological Variation. Chemical Structure of RNA. Eukaryotic Genome Complexity. RNA Functions. Pray, Ph. Citation: Pray, L. Nature Education 1 1 Arthur Kornberg compared DNA to a tape recording of instructions that can be copied over and over.
How do cells make these near-perfect copies, and does the process ever vary? Aa Aa Aa. Initiation and Unwinding. Primer Synthesis. The Challenges of Eukaryotic Replication.
References and Recommended Reading Annunziato, A. Journal of Biological Chemistry , — Bessman, M. Journal of Biological Chemistry , — Kornberg, A. Science , — Lehman, I. Journal of Biological Chemistry , — Losick, R.
Science , — Mackiewicz, P. Nucleic Acids Research 32 , — Ogawa, T. Molecular and General Genetics , — Okazaki, R. Article History Close. Share Cancel. Revoke Cancel. Keywords Keywords for this Article. Save Cancel.
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