At the beginning of chapter three, Shubin talks about what makes one cell different from the other. He explains that the answer lies in the DNA and what genes are actually turned on. In unit 10 we learned that this is because of the DNA transcription and translation. Explain in detail how a strand of DNA is replicated and eventually ends up as a protein and why they types of proteins that are translated determine the function of the cell.
-Robbie Thomashow
diehardcubsfan93@comcast.net
At the beginning of Chapter 3 of Your Inner Fish, Shubin explains “the cells that make our bones, nerves, guts, and so on look and behave entirely differently” (45). Shubin further explains “when a gene is turned on, it makes a protein that can affect what the cell looks like and how it behaves” (45-46). Although every cell in our body has the same exact DNA, the genes that are active in each cell are what make cells different. This is the essential feature to how cells function: gene expression. Gene expression is when segment of DNA is transcribed and translated into a functional protein.
ReplyDeleteNow DNA replication and DNA transcription and translation are different features of cells, so I will first touch upon DNA replication. A key feature in DNA replication is the structure of DNA. DNA is a double helix shaped molecule composed of two nucleotide chains that are hydrogen-bonded to each other. In DNA, adenine is hydrogen bonded to thymine and guanine is hydrogen bonded to cytosine. This is a key feature to DNA replication because when the DNA is replicated the proteins match the bases accordingly: adenine with thymine and guanine with cytosine.
Replication of DNA begins at a specific sequence called the origin of replication. The two strands separate forming a small replication bubble. At each of the ends of the bubble, there is what is known as the replication fork. At each replication fork, the protein helicase unwinds the double helix. While helicase unwinds, topoisomerase relieves unwinding strain ahead of the replication fork by breaking, swiveling, and rejoining DNA strands. Along the leading strand, primase synthesizes an RNA primer at the 5’ end. DNA polymerase III is able to attach to the RNA primer, and synthesize in the 5’ to 3’ direction (and move in from the 3’ to 5’ ends of the template DNA). Along the lagging strand, primase lays multiple RNA primers while DNA polymerase III attaches to these primers and continues to move in the 3’ to 5’ ends of the template DNA, while synthesizing in the 5’ to 3’ direction. Because the lagging strands consists of multiple fragments of primers and lagging strands, another protein comes in (DNA polymerase I) and replaces the RNA primer with DNA nucleotides. Afterwards, DNA ligase joins the 3’ end of DNA that replaces primer to the rest of the leading stand and joins the Okazaki fragments of lagging strand. After the whole process, two DNA strands are formed in a semiconservative manner: the original strands served as the template for the synthesis of new strands, and in the end, one original strand is hydrogen bonded to one new strand.
DNA transcription is the process in which RNA polymerase (an enzyme) unzips the DNA and copies nucleotide sequences to make a strand of mRNA. The process of mRNA synthesis is much like that of DNA synthesis except the nucleotide base uracil is paired with adenine rather than thymine. After transcription in prokaryotic cells (cells that lack nuclei), the mRNA is immediately translated without additional processing. In eukaryotic cells, however, the mRNA undergoes further processing. The ends of mRNA in eukaryotes are altered. The 5’ end receives a cap (the 5’ cap), which is a modified form of guanine, and at the 3’ end, an enzyme adds 50-250 more adenine nucleotides, forming a poly-A tail. The 5’ cap is where the ribosome attaches to in order to begin translation. Both the cap and tail help export the mature mRNA out of the nucleus and they protect the mRNA from degradation by hydrolytic enzymes.
In addition, the mRNA undergoes RNA splicing in which a spliceosome made up of snRNPs and other proteins remove introns from the mRNA and leave exons. As the spliceosome cuts out the introns, it splices together the exons. After RNA splicing, the mRNA is ready for translation and exits the eukaryote nucleus.
In translation, a ribosome binds to mRNA (in eukaryotes to the 5’ cap) and reads a triplet of nucleotides known as a condon. These condons are read by the ribosome, which then binds the matching anticodon of a tRNA molecule, which has a specific amino acid attached. The first tRNA molecule enters the P site, and as the mRNA is read by the ribosome, the next tRNA molecules enter the A site. The tRNA molecules move from the A site to the P site and then exit at the E site. At the P site, the tRNA adds its amino acid to the growing polypeptide chain, the basis for the protein. It is important to note that the mRNA moves from 5’ to 3’ along the ribosome throughout this process. Once a stop codon is read, the polypeptide chain elongation is complete and the mRNA will either move onto more ribosome to be translated or will disintegrate. Now that the polypeptide chain is formed, it may be modified, affecting its three-dimensional shape.
ReplyDeleteThe reason why transcription and translation determine the function of the cell is because the cell is only going to make the proteins it needs. So much of the DNA in each type of cell will not be read, as only some of it is read and used to synthesize the mRNA to make the proteins necessary to carry out the cell’s processes.
(Bobby Muttilainen, rmuttilainen@gmail.com)