DNA (deoxyribonucleic acid) is the code for life; it makes up the genetic material of living organisms. DNA is a long molecule made up of many subunits, or monomers, called nucleotides. Nucleotides are made up of three parts: a sugar, a phosphate group, and a nitrogenous base. Nucleotides contain a sugar-phosphate backbone and bases. There are four bases in DNA: adenine, cytosine, guanine, and thymine. A (adenine) always pairs with T (thymine), and C (cytosine) always pairs with G (guanine). These pairings are joined by hydrogen bonds and are called complementary base pairs. [Stephen Taylor, i-bilogy.net]
Nucleotide Molecule Double Helix
When forming a DNA molecule, nucleotides link together through covalent bonds. Together they form a double-helix, meaning the two strands twist around each other. The strands are antiparallel; they run in opposite directions. There can be millions of base pairs in each strand. The double-helix looks like a ladder, twisted as if it was a spiral staircase.
Replication is an essential function of DNA. It must be able to replicate itself accurately so that as a cell divides, it will be able to pass its hereditary information and its genetic code to its daughter cells. Replication precisely copies DNA so that new molecules contain the same exact sequence of bases as the original strands. DNA replication occurs in the nucleus. It takes place during the S phase of interphase as DNA. The Steps of Replication:
1) First, helicase moves along the double helix unzipping the two strands. The strands separate as the hydrogen bonds between the bases break. 2) The unpaired nucleotides act as a template for the formation of a new complementary strand. As free nucleotides move into place, A will pair with T and C will pair with G. 3) Free nucleotide bases form complementary pairs with the single DNA strands. The enzyme DNA polymerase links the nucleotides. Lastly, a new double helix is formed as the two new DNA strands are rewound. Every double helix has one old and one new strand. The two new DNA strands are identical to the original strands. Because no DNA molecule is ever completely new, DNA replication is considered semi-conservative. [Biology for the IB Diploma page 49]
Transcription and Translation:
The central function of DNA is to control and direct the activities of the cell. This is accomplished by controlling the proteins produced by the cell. Enzymes and hormones are proteins that help determine how the cell functions, making protein synthesis very important. There are two steps to protein synthesis: transcription and translation.
Genes are the sections of DNA that code for certain proteins. They contain triplets, which are specific sets of sequences in sets of three. Some of these triplets determine where transcription begins and ends.
The first stage in the synthesis of protein is the production of messenger RNA, or mRNA. mRNA is an intermediate molecule that carries the coded message of DNA into the cytoplasm. RNA stands for ribonucleic acid and its building blocks are RNA nucleotides. As in DNA, RNA contains complementary base pairs. However instead of thymine, adenine pairs with uracil. The base pairs are A with U, and C with G. Transcription results in the copying of a section of the DNA molecule, but not the entire length.
The process goes as follows:
1) The enzyme DNA polymerase unzips DNA and the strands uncoil and separate. 2) Free nucleotides move into place along the one of the strands. 3) RNA polymerase assembles free nucleotides through complementary base pairing. As the nucleotides link, a single strand of mRNA is formed. This strand is a copy of just one section of DNA – a gene. The mRNA separates and the double helix zips up again. 4) Once an mRNA molecule has been transcribed, it moves through the pores in the nuclear envelope to the cytoplasm where translation can occur. [Biology for the IB Diploma page 51]
The sequence of bases along the mRNA molecule corresponds to the sequence of the original DNA molecule. Each triplet corresponds to a specific amino acid. Therefore, the order of triplets determines how amino acids will be assembled into polypeptide chains. Translation is the process in which mRNA uses its coded information to construct polypeptide chains. A codon is a triplet of mRNA bases that code for a particular amino acid. Translation is carried out in the cytoplasm by ribosomes and tRNA, or transfer RNA. Ribosomes have binding sites for mRNA and tRNA molecules. The ribosome binds to the mRNA and draws in tRNA molecules using anticodons that match mRNA codons. Anticodons match codons and are linked by hydrogen bonds. A peptide bonds forms between two amino acids when two tRNA molecules are in place at the ribosome. Together they form a dipeptide. After a dipeptide has been formed, the first tRNA molecule detaches from the amino acid and from the ribosome. From here, the ribosome moves one triplet among the mRNA to the next codon. The process is repeated over and over until a polypeptide is formed. Finally, a codon will reach a stop codon. A stop codon does not code for an amino acid therefore telling the ribosome to detach from the mRNA, thus ending the process.
Walpole, Brenda, Ashby Davies, and Leighton Dann. "The Chemistry of Life." Biology for the IB diploma coursebook. Cambridge: Cambridge University Press, 2011. 46-51. Print. "3.3 DNA Structure | i-Biology." i-Biology | international, independent, illuminated. N.p., n.d. Web. 25 Sept. 2013. . "Higher Biology." Higher Biology. N.p., n.d. Web. 25 Sept. 2013. . " double-helix-dna - Paco Villa Corrections Blog." Paco Villa Corrections Blog » Paco has your back!. N.p., n.d. Web. 25 Sept. 2013. . "Transcription: The Transcription Process." Prentice Hall Bridge page. N.p., n.d. Web. 26 Sept. 2013.