Describe how the information in DNA is used during transcription and translation to construct polypeptides, including the role of messenger RNA (mRNA), transfer RNA (tRNA) and the ribosomes.

RNA Is synthesised in the nucleus from a DNA template. During synthesis of RNA, the two strands of the DNA molecule separate temporarily; one of these strands is used as a template for synthesis of an RNA molecule - this is transcription. The code triplets in the DNA cause formation of complementary code triplets (called codons) in the RNA. These codons, in turn, will control the sequence of amino acids in a protein to be synthesised in the cell cytoplasm - translation. Basically DNA makes RNA, and RNA makes protein.  Transcription produces single-stranded RNA that is complementary to one strand of DNA. In the DNA strand immediately ahead of the gene to be transcribed is a sequence of nucleotides called the promoter. The enzyme RNA polymerase becomes attached to the promoter which is then initiates the formation of the RNA molecule. After the RNA polymerase attaches to the promoter, the polymerase causes unwinding of the DNA helix and separation of the two strands. The polymerase then moves along the DNA strand, temporarily unwinding and separating the two DNA strands at each stage of its movement. As it moves along, it adds a new RNA nucleotide to the end of the newly forming RNA chain by causing a hydrogen bond to form between the complementary bases of the DNA strand and the RNA nucleotide. When the RNA polymerase reaches the end of the DNA gene, it encounters a terminating sequence, which causes the polymerase and the newly formed RNA chain to break away from the DNA strand. 
Messenger RNA (mRNA) carries the genetic code to the cytoplasm for controlling the type of protein formed. Only mRNA encodes proteins. Transfer RNA (tRNA) transports activated amino acids to the ribosomes to be used in assembling the protein molecule during translation. mRNA is decoded in sets of three nucleotides (codons) that are exactly complementary to the code triplets of the DNA genes. Each tRNA has a triplet of bases called an anticodon and carries an amino acid corresponding to this anticodon. Then is the process of translation, which takes place in the cytoplasm. Ribosomes are the functional machine in translation, it is composed of two subunits, a large (60S) subunit and a small (40S) subunit.The first step in translation is Initiation. mRNA binds to a site on the small sub-unit of the ribosome which causes this is where the ribosome subunits to assemble together on the mRNA. tRNA molecules are present in large numbers around the ribosome. There are three binding sites for tRNA molecules on the large sub-unit of the ribosome but only two ever bind at once. A tRNA can only bind if it has the anticodon that is complementary to the codon on the mRNA. The amino acids carried by the tRNA molecules are bonded together by a peptide linkage. A dipeptide is formed, attached to the tRNA on the right. The tRNA on the left detaches. The ribosome moves along the mRNA to the next codon. Another tRNA carrying an amino acid binds. A chain of three amino acids is now formed. These stages are repeated until a polypeptide is formed.