Since a chromosome is a series of genes, linked end to end, how does this DNA structure fit in with the concept of genes? Genes control traits. We have genes for eye color, for hair color, for producing lactose (an enzyme needed to digest milk sugar), etc. Humans are estimated to have approximately 30,000 genes, each one controlling a biochemical process, such as producing eye color, producing hair color, producing the enzyme lactase, etc. Each gene is a sequence of hundreds or thousands of base pairs, and each chromosome must have hundreds or thousands of genes (we have twenty-three different shapes of chromosomes and 30,000 genes…) Note, since we have two of each shape of the chromosome (one from our father and one from our mother) we have two copies (doses) of each gene.
HOW DOES A GENE WORK?
The information in a gene is contained in the sequence of its bases. That information is used to produce a pattern, a sequence of amino acids linked together. Each gene codes for a different sequence of amino acids.
For each gene, the bases of only one backbone (the template strand or backbone) are used to make the protein. The template sequence of DNA bases, through the processes of transcription and translation, specifies the precise sequence of amino acids in the protein.
The language of DNA consists of DNA bases used in groups of three (a triplet) with no spaces between them. The first three bases in the sense backbone of a molecule of DNA (first triplet) specify amino acid number 1 in the protein, the second three DNA bases (second triplet) specify amino acid number 2 in the protein, the third triplet of DNA bases specify amino acid number three, etc. For example, where we have the triplet GAA in the gene, the amino acid, leucine is found at that corresponding position in the protein. Similarly, the DNA triplet CAA specifies the amino acid valine, the triplet TCT specifies the amino acid arginine, the triplet AGA specifies serine, etc. This ‘genetic code’ has been worked out so that if I know the base sequence of a gene, I can work out the amino acid sequence in its protein. Similarly, if I know the amino acid sequence of a protein, I can work out a possible base sequence for that gene.
One other fact emerges from our knowledge of the genetic code – it is non-overlapping. Given the base sequence of the template backbone G A A C A A T C T A G A. The DNA molecule would be
ROW - + - + - + - + - + - + - + - + - + - + - + - + 1 G A A C A A T C T A G A 2 C T T G T T A G A T C T 3 + - + - + - + - + - + - + - + - + - + - + - + - 4 note the template backbone is rows 1 & 2
This sequence of bases would specify the amino acid sequence leucine-valine-arginine-serine in the protein. Note the code is non-overlapping and each base is used once and only once, i.e., is part of only one triplet. The triplets are GAA, CAA, TCT, AGA. (An overlapping code of the same region could have the triplets: GAA, AAC, ACA, CAA, AAT, ATC, TCT, CTA, TAG, etc.)
There are sixty-four different combinations of triplets, AAA, AAG, AAC, AAT, ATA, ATG, etc. However, there are only twenty different amino acids used to make proteins. Therefore, some amino acids are specified by more than one triplet; a phenomenon called degeneracy. For example, the DNA triplets, TTT and TTC both specify the amino acid Lysine and the DNA triplets AGA, AGG, AGT, AGC, TCA, and TCG specify the amino acid serine.
To convert the sequence of bases in the DNA of a gene to a sequence of amino acids in a protein, we need to understand the roles of messenger RNA and transfer RNA.
|Two sugar-phosphate backbones||singles sugar-phosphate backbone|
|Has base pairs||No base pairs|
|Five-carbon Sugar = Deoxyribose||Five-carbon-Sugar = Ribose|
In the nucleus a molecule of RNA is made from the template backbone of DNA. RNA is composed of a single sugar (ribose) – phosphate backbone with the RNA bases (complementary to those in the DNA) attached to the riboses. The RNA is then transported from the nucleus to the cytoplasm where the base sequence in messenger RNA is used to assemble an amino acid sequence corresponding to groups of three mRNA bases. Each group of three mRNA bases is called a codon. The first mRNA codon (sequence of three bases) specifies the first amino acid in the protein. The second codon specifies the second amino acid, etc.
Each mRNA codon is recognized by a complementary group of three bases on another type of RNA called transfer RNA (tRNA). These three bases in the tRNA are called the anticodon as they are complementary to the bases in a mRNA codon. Each tRNA also carries the amino acid corresponding to its anticodon. Thus each tRNA anticodon specifies a specific amino acid, and each tRNA anticodon is complementary to a single mRNA codon.
In this fashion the DNA sequence C-T-C codes for G-A-G in the mRNA. The GAG codon in the mRNA is recognized by the anticodon, C-U-C of the tRNA that carries that amino acid glutamine.
ROW - + - + - + - + - + - + - + - + - + - + - + - + 1 G A A C A A T C T A G A 2 C U U G U U A G A U C U 3 O---O---O---O---O---O---O---O---O---O---O---O--- 4 G A A C A A U C U A G A 5 -O---O---O---O---O---O---O---O---O---O---O---O 6 += deoxyribose, -O= ribose, --=phospate Rows 1 and 2 are using the template backbone from above. Rows 3 is the corresponding mRNA sequence. Rows 5 is the corresponding tRNA sequence.
In Section 3 as you learn about the processes of transcribing DNA to RNA, you will find the following table useful. It is on a separate page for easy reference and printing.
Section 2 provides a clear understanding of how creativity is used in problem-solving.
Use the following link to go to Section 2: Creativity