Central dogma of molecular biology

In molecular biology, there is a central dogma which describes information flow between informational macromolecules. These informational macromolecules are

2.2. Central dogma of molecular biology 5

Figure 2.1 Central dogma and macromolecules related to it. (a) Information flow in the cell in regards to the central dogma of molecular biology. (b) DNA complementary strands showing all four bases related to DNA, adenine (A), thymine (T), cytosine (C), and guanine (G), and complimentary pair for each. (c) RNA sequence, similar to DNA but instead of thymine there is uracil(U). (d) Proteins primary structure i.e., amino acid chain.

deoxyribonucleic acids (DNA), ribonucleic acids (RNA), and proteins, and they are responsible for all information stored in cells. Replication, transcription, and translation form the central dogma of molecular biology. This is the information flow in the cell and it is demonstrated in Figure 2.1(a). DNA can be replicated and the replication will result in two identical DNA molecules. During transcription, RNA is produced as an information replica of DNA. The functional unit of the cell, protein, is produced from RNA during a translation. (Madigan et al. 2006)

2.2. Central dogma of molecular biology 6

2.2.1 DNA

DNA is the central hub of information in cells. One DNA is a macromolecule, made of pentose sugar deoxyribose and four bases: adenine, cytosine, guanine, and thymine. Sugars are connected as a chain with a link between 3rd carbon and 5th carbon. Phosphate is in the middle of this link between two sugars. For future, 3rd carbon end is marked as 3’ and 5th is marked as 5’ and these are used to mark direction as well. Each sugar has one of four bases connected to it. Information in the cell is stored in two strands of DNA. These strands are complementary to each other based on bonding of bases, in DNA Cytosine-Guanine and Thymine-Adenine hydrogen bonds are possible. An example structure of a two-strand DNA is demonstrated in Figure 2.1(b). Linking between shows hydrogen bonds between DNA strands. (Madigan et al. 2006)

According to the central dogma of molecular biology, during replication, DNA strands are complemented resulting two identical DNA. Sometimes errors occur dur-ing replication, or original DNA has been altered somehow. There are mechanisms in the cell that protect the original DNA from alterations but it is possible that those fail, resulting in altered DNA. Assuming organism survives with the alteration its offspring will also inherit the altered DNA. (Madigan et al. 2006)

2.2.2 RNA

RNA has the similar structure to the single strand DNA which is shown in Figure 2.1(c). It has a ribose instead of the deoxyribose as the backbone and a uracil replaces the thymine. Even though RNA has affinities similar to the DNA, it does not typically appear as the double strand. Usually, inside of the cell has several singular RNA strands of differing sizes, all much smaller in length than DNA. During transcription, information from a part of the DNA strand is imported into an RNA strand. (Madigan et al. 2006)

RNA polymerase is a protein that constructs RNA from DNA during transcrip-tion. RNA polymerase recognizes specific region in DNA strand called a promoter sequence. After finding a promoter sequence RNA polymerase starts moving 5’ to 3’ direction of the strand from which it found the promoter sequence. A promoter sequence determines how easily RNA polymerase can bind to it, different promoters have different binding rates. RNA polymerase complements the strand it is moving along creating a replica of the other DNA strand, with uracil instead of thymine. It continues this until it reaches a terminator sequence which will stop the transcrip-tion. (Madigan et al. 2006)

2.2. Central dogma of molecular biology 7 The RNA can have several different functions and its function is usually speci-fied when talked about RNA i.e., messenger RNA (mRNA) and non-coding RNA (ncRNA). There are several different kinds of non-coding RNA e.g., transfer RNA (tRNA) and ribosomal RNA (rRNA). These are all important to cell functions, but in regards to the central dogma, mRNA is of our interest. While the DNA works as an information storage of the cell role of the mRNA is an information transmitter.

(Madigan et al. 2006)

There are many parallels in translation and transcription. Structure called ribosome is responsible on translation process. The mRNA also has regions, which ribosome attaches, starts and ends translation. However, information is not directly copied but instead, it is formulated through a genetic code.

The genetic code consists of codons, which are three base sequences (or triplets). As there are four different bases in RNA (adenine, cytosine, glycine, and uracil), this means that there are4³ = 64different codes that codons can have. We can consider there to be three types of codons: start, stop, and amino acid. Start-codon begins protein synthesis and stop-codon ends it. Other codons map only to an amino acid, a total of 20 different amino acid is coded by these. As this is a total of 22 different functions, it becomes clear that there is an overlap. There are three different codons for the stop signal, 60 mapping to 19 amino acids, and one of amino acid codon also works as the start signal. Table 2.1 shows whole codon-amino acid mapping in greater detail. This is genetic code can be considered as universal code between all cells. (Madigan et al. 2006)

2.2.3 Protein

While DNA and RNA are information mediums in the cell, proteins carry cell func-tions. Protein is constructed of several amino acids linked with peptide bonds and is thus a polypeptide. Proteins have four stages of structure, the primary being its amino acid sequence. The content of the polypeptide chain determines functions of the protein and its structures, though what structures form also contributes to pro-tein does. Propro-teins primary structure is constructed during translation in a propro-tein synthesis mediated by a ribosome. (Madigan et al. 2006)

The ribosome is a main actor in the protein synthesis. Its structure has both proteins and rRNA. The ribosome attaches itself into mRNA into a region called ribosome binding site and starts moving towards 3’ end. When it finds the AUG-sequence protein synthesis starts. During a protein synthesis, ribosomes require that tRNA carries amino acids into its structure. These amino acids ribosome attaches to a

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