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The Law

Belief - VII

Our Cellular Biology - VI

Let us first look at the replication process in the leading strand.

A short piece of RNA called a primer (produced by an enzyme called primase) comes along and binds to the end of the leading strand. The primer acts as the starting point for DNA synthesis.

Note: Primase is an enzyme that synthesizes short RNA sequences called primers. These primers serve as a starting point for DNA synthesis.Since primase produces RNA molecules, the enzyme is a type of RNA polymerase. A polymerase is an enzyme that synthesizes long chains or polymers of nucleic acids. DNA polymerase and RNA polymerase are used to assemble DNA and RNA molecules, respectively, by copying a DNA or RNA template strand.

DNA polymerase binds to the leading strand and then walks along it adding new complementary nucleotide bases (A, C, G and T) to the strand of DNA in the 5' to 3' direction. This kind of replication is called continuous.

As mentioned earlier, the replication in the lagging strand happens differently.

In the case of the lagging strand, numerous RNA primers made by primase latch on to the lagging strand at various points on the lagging strand. Chunks of DNA, called Okazaki fragments, are then added to the lagging strand also in the 5' to 3' direction. This type of replication is called discontinuous, as the chunks of Okazaki fragments will need to be joined up later.

Once all the bases are matched (A with T and C with G) in both the fragments, the primers are stripped away by another enzyme called exonuclease. the gaps, where the primers were there, are then filled up by more complementary nucleotides.

Note: Exonucleases are enzymes that work by severing nucleotides one at a time from the end (exo) of a polynucleotide chain

The new strand is proofread to see that there is no mistake in the new DNA strands.

An enzyme called DNA ligase seals up the sequence of DNA into two continuous double strands.

Note: In molecular biology, DNA ligase is a specific type of enzyme, that facilitates the joining of DNA strands together by catalyzing the formation of a phosphodiester bond. A phosphodiester bond is a chemical bond of the kind joining successive sugar molecules in a polynucleotide. 

The result of  DNA replication is two DNA molecules consisting of one new and one old chain of nucleotides. So half of the chain is original DNA molecule and other half is new DNA molecule. following replication, the new DNA automatically winds up into a double helix.

Let us look at how the cells access their genetic information.

Transcription is the first step in decoding a cell's genetic information. During transcription, the enzymes, RNA polymerases builds RNA molecules that are complementary to to a portion of one strand of DNA double helix. As mentioned earlier, RNA molecules differ from DNA molecules in several ways. They are single stranded vis-a-vis the double stranded DNAs, their sugar component is a ribose rather than deoxyribose sugar in DNAs; and they include uracil (U) nucleotide rather than thymine (T) nucleotide in DNAs. Also RNAs, being single stranded, do not form helices; rather, they fold into complex structures that are stabilized by internal complementary base-pairing.

Note: Deoxyribose sugar is obtained from ribose sugar by loss an oxygen atom from the ribose sugar.

Three general classes of RNA molecules are involved in expressing the code encoded in DNA molecules - messenger RNAs (mRNA), ribosomal RNAs (rRNA) and transfer RNAs (tRNA). Messenger RNA (mRNA) is a large family of RNA molecules that convey genetic information from DNA to the ribosome, where they specify the amino acid sequence of the protein products of gene expression. Ribosomal RNA (rRNA) is the RNA component of the ribosome, and is essential for protein synthesis in all living organisms. It constitutes the predominant material within the ribosome, which is approximately 60% rRNA and 40% protein by weight. Transfer RNA (tRNA) is an adaptor molecule composed of RNA, typically 76 to 90 nucleotides in length, that serves as the physical link between the mRNA and the amino acid sequence of proteins. tRNA carries the amino acid molecules to the ribosome during protein synthesis. Each class of RNA has its own polymerase.

Ribosome

Note: Ribosomes are the protein builders or the protein synthesizers of the cell. They are like construction guys who connect one amino acid at a time and build long chains. Ribosomes consist of two major components: the small ribosomal subunit, which reads the RNA, and the large subunit, which joins amino acids to form a polypeptide chain. Each subunit is composed of one or more ribosomal RNA (rRNA) molecule and a variety of proteins. 

Cells can be characterized by the spectrum of mRNA molecules present within them; this spectrum is called transcriptome. Whereas each cell in a multi-cellular organism carries the same DNA or genome, its transcriptome varies widely according to cell type and function. For instance, the insulin-producing cells of the pancreas contain transcripts for insulin, but bone cells do not. Even though the bone cell carries the transcript for insulinn, this gene is not transcribed. therefore, the transcriptome functions as a catalogue of all of the genes that are being expressed in a cell.



We will continue with the process of creating a new protein in the next post.


Namaste

à bientôt

Prabir