Bio+Essay+By+Yan+Rui,+Lin+Zhuo,+and+Kai+Sheng

Protein Synthesis

Transcription DNA templates are in the nucleus while ribosomes are in the cytoplasm. DNA is too large to exit the nucleus and needed to be transcripted into mRNA which is small enough to exit the nucleus and enter the cytoplasm. Preparation for DNA transcription into mRNA (messenger ribonucleic acid) begins when various transcription factors gather around a DNA. A transcription factor undo the double-helical structure of DNA --> One of the strands in the gene determines the type of protein made and is called the template. An enzyme copies the message in the template into an mRNA molecule using the base pairing rule, except that in the mRNA molecule T (thymine) is replaced by U (uracil). Transcription occurs in the nucleus. Three bases in the mRNA make up a codon. The mRNA molecule is single-stranded and contains only one gene. The mRNA carries the message into the cytoplasm where a ribosome helps to translate the message in the mRNA into a protein molecule.

Translation In the cytoplasm there are amino acids and tRNA (transfer ribonucleic acid). Each tRNA molecule have a specific amino acid attached to one end of their structure based on their anticodons. tRNA molecules contain anticodons which binds to complementary codons on mRNA. translation begins with an mRNA attaching to a ribosome. The first two tRNA molecules together with their amino acids also fits into the ribosome and attach to the codons on the mRNA according to the rule of base pairing. A peptide bond is formed between the first two amino acids molecules and the ribosome moves along one codon to the right of the mRNA. As the ribosome moves across the mRNA, the first tRNA is released (without its amino acid) and the third tRNA with its amino acid slots into the ribosome, attaches to its complementary codon and its amino acid is attached to the chain. The process continues as the ribosome moves along the mRNA until it reaches a stop codon (UGA, UAA or UAG) which does not have any tRNA with complementary codons. The ribosome then leaves the mRNA and a whole chain of polypeptide is produced. The polypeptide then folds into its specific three-dimensional shape and it is now a functional protein.

The order of amino acids in the polypeptide produced depends on the order of codons in the mRNA, which in turn depends on the DNA template in the nucleus.

 DNA is a nucleic acis that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. DNA stands for Dexoxyribonucleic acid. DNA is called the nucleic acid, together with RNA, which stands for ribonucleic acid. Both DNA and RNA are found in the nuclei of cells. The DNA consists of 2 polymers of simple units called the nucleotides. Nucleotides consists of 3 parts, a phosphoric acid, a carbon or 5 pentose sugar and a nitrogenous base. The presence of the hydroxyl group (OH) determines whether the sugar would be found in DNA or RNA. If the carbon 2 has a hydroxyl group (OH), then the sugar is ribose and found in RNA whereas if the carbon 2 has no hydroxyl group then the sugar is deoxyribose, found in DNA.

There are five different organic bases in total, which can be divided into 2 groups, purines and pyrimidines. Purines consists of two rings of carbon and nitrogen atoms. Pyrimidines consists of a single ring of carbon and nitrogen atoms. Thymine is found in DNA only, and uracil is only found in RNA. Therefore altogether there are four different bases present in one nucleic acid molecule in the same time.

Nucleotides can be joined together by a condensation reaction between the phosphate group of one nucleotide and the hydroxyl group on carbon 3 of the sugar of the other nucleotides. Phosphodiester bonds link the nucleotides together. The condesation reaction results in the removal of water. The phosphodiester bond is a group of strong covalent bonds between the phosphorus atom in a phosphate group and two other molecules over two ester bonds.

The bases do not take part in the polymerisation. Therefore, there is a sugar-phosphate backbone with the bases extending off it. 

Components of DNA:

DNA is a polymer. The monomer units of DNA are nucleotides, and the polymer is known as a "polynucleotide." Each nucleotide consists of a 5-carbon sugar (deoxyribose), a nitrogen containing base attached to the sugar, and a phosphate group. There are four different types of nucleotides found in DNA, differing only in the nitrogenous base. The four nucleotides are given one letter abbreviations as shorthand for the four bases.  The monomer of DNA is the nucleotide. The nucleotide itself is a complex molecule, consisting of three seperate parts: i. **The sugar component**: in DNA the sugar component of the nucleotide is a pentose (contains five carbon atoms) sugar called 2’-deoxyribose. ii. **The nitrogenous bases**: these are attached to the number 1 carbon sugar in the nucleotide. In DNA, any one of four nitrogenous bases can be attached to the sugar. These are adenine (A) and guanine (G), which are purines and thymine (T) and cytosine (C), which are pyrimidines. iii. **The phosphoric acid component**: also attached to the number 5 carbon of the sugar.
 *  A is for adenine
 *  G is for guanine
 *  C is for cytosine
 *  T is for thymine

The nucleotides are linked together by their phosphate groups to form a polymer, the polynucleotide. The phosphate group of one nucleotide joins to the carbon ring of the sugar in the next nucleotide. The linkage between the nucleotides is a phosphodiester bond. An important feature of polynucleotides is that they have chemically distinct ends. This chemical distinction between the two ends means that polynucleotides have a direction. The direction of the polynucleotide is very important in molecular genetics. Polynucleotides can be any length and have any sequence. For example within the human nucleus, the smallest chromosome contains 50 x 10 6 nucleotide pairs (stretched full-length this molecule would extend 1.7 cm) and there are up to 250 x 10 6 nucleotide pairs in the largest chromosome (which would extend 8.5 cm). In addition, there are no restrictions on the order in which the nucleotides can join together in a single strand. At any point in the polynucleotide theoretically the nucleotide could be A, T, C or G. If we consider a polynucleotide just ten nucleotides in length, it could have any one of 4 10 = 1 048 576 different sequences. Imagine the number of different sequences possible for a polynucleotide a thousand nucleotides in length or one million.

Both DNA and RNA contain a 5 carbon sugar. Sugars are molecules made up of carbon,hydrogen and oxygen with molecular formulas that are usually some multiple of CH2O. In DNA, the sugars are closed to form ring structures. DNA contains the 5 carbon sugar deoxyribose (hence the name deoxyribonucleic acid)

Resources www.massey.ac.nz/~wwbioch/**DNA**/**DNA**make/framset.htm www.blc.arizona.edu/Molecular.../**DNA**.../**DNA**_Tutorial.HTML genetics.suite101.com/article.cfm/the_structure_of_**dna** 