Molecular Biology

Nucleic Acids Dna Expression and Rna Synthesis

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The majority of human genes are expressed in forms of proteins. In order for these proteins to be produced, DNA has to replicate, and than transcribe for RNA. DNA is a double stranded helix which is wound tightly. Making up this helix is a sugar(deoxyribose) and phosphate backbone attached to bases ( Adenine, Guanine, Cytosine and Thymine).

DNA strands must uncoil in order to replicate. An enzyme DNA helicase is needed to do that. Once the two DNA strands are apart, each one then binds to another strand with complementary bases. Guanine binds to Cytosine and Adenine to Thymine. An enzyme called DNA polymerase helps bind the complementary strands together.

The transcription of RNA from DNA is similar to DNA replication itself. In the case of transcription, the DNA strands uncoil again, but only one strand is used for transcription. The strand is then used as a template and RNA polymerase contributes to the synthesis of RNA. The DNA template bases serve as complements to the incoming RNA bases. The main difference is the Adenine on DNA binds to Uracil on RNA, instead of Thymine. Again, Guanine will bind to Cytosine.

The actual transcription process takes place in three steps. The first step is initiation, the second elongation of the chain and the third is termination. Initiation starts with an RNA polymerase-promoter sequence. This promoter sends a signal which starts the whole replication process. This is triggered by the sequence on the single strand of DNA not being used for transcription purposes.

Once the process gets going, the elongation of the RNA strand takes place. For this to occur, the RNA polymerase needs to "read" the DNA template. The strand is then copied as a complementary strand to the DNA. RNA polymerase continues to go down the DNA strand until all the complementary bases are present and a "terminator" sequence is reached. Once that terminating sequence is reached, the RNA polymerase releases the RNA polymer and it detaches from DNA. The type of RNA just produced is called mRNA, or messenger RNA. It performs a vital function. This mRNA contains sections known as coding regions known as exons. In between the exons are introns, sequences of no use to proteins synthesis. Simply put, these introns are removed and the remaining exons are joined together to form the mRNA which then codes for a protein.

Eventually, tRNA (transfer RNA) kicks along with other enzymes in order for proteins to be translated. tRNA binds to any of 20 amino acids in order to then form the proteins necessary for life. This is a brief explanation for a complicated process which goes on continually in the cells of humans and other eucaryotes.

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