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9.7 Option – Genetics: The Code Broken: 1. Gene structure

Syllabus reference (October 2002 version)
1. The structure of a gene provides the code for a polypeptide	Students learn to: describe the processes involved in the transfer of information from DNA through RNA to the production of a sequence of amino acids in a polypeptide	Students: choose equipment or resources to perform a first-hand investigation to construct a model of DNA   process information from secondary data to outline the current understanding of gene expression

Extract from Biology Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW.
[Edit: 12 Jun 09]

Prior learning: Stage 4-5 Syllabus, 4.8.3(b) 5.8.2(a, b, c and d).
H.S.C. module 9.3 (subsections 3 and 5).

Background information: Deoxyribose nucleic acid (DNA) is the genetic material in all cells. In eukaryotic cells it is found mainly in nuclei and in smaller amounts in mitochondria and chloroplasts. DNA is a double helix structure made of nucleotides. Each nucleotide consists of a deoxyribose sugar, a phosphate group, and one of four bases: adenine, thymine, guanine and cytosine. These nucleotides are joined in two ways: sugars and phosphates join to make a double backbone and bases are joined by hydrogen bonds. Adenine only joins with thymine and guanine with cytosine.

choose equipment or resources to perform a first-hand investigation to construct a model of DNA

• Refer back to the work you did in HSC module 9.3.3 – Chromosomal structure provides the key to inheritance. Use the Background information and diagrams in books or commercial models to gain a thorough understanding of the parts of DNA.
• Consider materials to use to construct the model. Plasticene or play dough could be used to represent the sugar, the phosphate and the bases and pipe cleaners or thin wire could be used for the bonds. You will need to decide on a scale for your model. Decide how many nucleotides you will include. You need enough so that you will have at least one twist of the helix to get a good 3D impression.
• When you have made the components of your DNA construct the model by assembling them so they form a 3D structure. If you use different shapes for each of the bases, include a key with your model.

describe the processes involved in the transfer of information from DNA through RNA to the production of a sequence of amino acids in a polypeptide

• Sections of DNA (genes) determine the make-up of polypeptides. The genetic code of DNA is the sequence of bases. These bases, in triplets in DNA, are transcribed into codons in messenger RNA (mRNA) and anti-codons in transfer RNA (tRNA). Each codon specifies an amino acid.
• The production of polypeptides originates from a portion of DNA. This portion of DNA “unzips” to provide the template on which mRNA nucleotides line up to form the complementary single strand of mRNA molecule needed. The enzyme RNA polymerase catalyses this process. <link to 9.3.4 'outline, using a simple model, the process by which DNA controls the production of polypeptides.
  
  Click here for an image of this process , Access Excellence @the National Health Museum, USA.

• The mRNA molecule moves through the nuclear membrane pores to attach to ribosomes in the cytoplasm. A number of ribosomes attach, in turn, to the mRNA strand and each ribosome produces one polypeptide.

• Transfer RNA (tRNA) molecules are found in the cytoplasm and carry amino acids on one end. The other end of a tRNA molecule has a complementary triplet of bases to the codons of mRNA. These tRNA triplets are called anti-codons. The anti-codon of the tRNA attaches to the codon of the mRNA. The binding of two tRNA anti-codons allows two amino acids at the opposite ends of the tRNA molecules to come together. These amino acids join together by forming a peptide bond. The tRNA molecules then detach from the mRNA and move off to pick up more amino acids. Another tRNA molecule (with its specific amino acid) then moves in and its anti-codon attaches to the codon of the mRNA and more peptide bonds join the amino acids. This continues until a “stop” codon on the mRNA dictates that the polypeptide synthesis is finished. The base sequences of the mRNA are translated into amino acid sequences in polypeptides.
  
  More information on RNA and coding for amino acids , AP Biology, Conestoga High School, Berwyn, Pennsylvania, USA.

Example:

DNA triplets	TAC	CCG	ATA	TGG	TCT	AGC	ATT
mRNA codon (transcription)	AUG	GGC	UAU	ACC	AGA	UCG	UAA
tRNA anti-codon	UAC	CCG	AUA	UGG	UCU	AGC	AUU
Amino acids (translation)	methionine (start)	glycine	tyrosine	threonine	arginine	serine	stop

process information from secondary data to outline the current understanding of gene expression

• The best place to find up to date information on gene expression is to look in science journals such as Nature or New Scientist or genetics journals such as Cytogenetic and Genome Research or the European Journal of Human Genetics, or look on the internet. Be aware though that anyone can put information on the Internet and it isn’t necessarily checked for accuracy but information in journals must be peer reviewed before it is printed. Sometimes the language is quite difficult so you may need your teacher to help you use simpler language wherever possible.
• When you have enough information either from your teacher or from journals and the Internet, sort through the material and select that which is most useful. This could be information that is supported in several articles and that can be understood. Discard any information that is outdated.