Nucleotides and Nucleic acid
DNA or deoxyribonucleic acid is the genetic material of living organisms making it very important. It is made of simple subunits known as nucleotides. Each nucleotide consists of three parts- a sugar known as deoxyribose, a phosphate group, and a nitrogenous base. DNA nucleotides do not all have the same base. There are four different bases, adenine (A), cytosine (C), guanine (G), and thymine (T). Adenine and guanine are purines whereas cytosine and thymine are pyrimidines. In RNA, uracil is a pyrimidine.
A polymer of nucleotides is known as a nucleic acid. The carbons of sugar molecules have assigned numbers that are used to show points of attachment.
DNA molecules can be linked together by a covalent bond between the sugar of a nucleotide and the phosphate group of another molecule. DNA molecules consist of two strands of nucleotides wound together to form the double helix. Hydrogen bonds link the two strands together and are formed between the bases of two strands. Only adenine and thymine form hydrogen bonds and cytosine and guanine form hydrogen bonds. This is known as complementary base pairing. The two DNA strands are complementary, so the sequence on one strand determines the sequence on the other strand. This complementary feature allows DNA able to self-replicate and also allows DNA to serve as a guide for RNA production.
One strand of the DNA is in the direction of 5 prime to 3 prime while the other is in the direction of 3 prime to 5 prime. The two strands are coiled together which is known as the double helix. A pyrimidine is always facing a purine. While G and C have three hydrogen bonds, A and T only have two bonds.
DNA replication is a way of copying DNA to produce new molecules with the same base sequence. It is semi-conservative meaning that each molecule formed by replication consist of a old and new DNA molecule.
Central dogma of genetics
Reproduction for cells requires transmission of information from parent to offspring. This is termed heredity. A gene is unit of heredity that is coded for by a sequence of DNA bases. Most genes specify the sequence of amino acids in a particular polypeptide. Polypeptides compose proteins and these proteins often directly or indirectly determine characteristics. In transcription, the DNA sequence in a gene is used to create an RNA molecule. These RNA molecules are then decoded into the amino acid sequence of a polypeptide through the process translation.
In bacteria, DNA is naked but in eukaryotes, DNA is associated with proteins. The most common proteins are histones and they serve to help supercoil DNA to package chromatin and help regulate the expression of genes.
The Human Genome Project has allowed us to understand the many patterns observed in DNA. There are about 25,000 protein-coding genes in the human genome. Some sequences are transcribed to produce other forms of RNA besides mRNA. Because of the diversity of phenotypes it was believed that there were many more genes. It has been found that non-gene factors influence phenotype and on gene expression so this can be one source of diversity. Most of the genome is not transcribed. Originally called ‘junk DNA’, it has been recognized that elements of this ‘junk’ play roles in gene expression and affect repetitive sequences. The repeating sequence are normally between 5 and 300 base pairs long. A repeated sequence can be duplicated as most 105 times per genome.
Introns and exons
Introns are sequences of bases that are transcribed but not translated. Exons are bases of nucleotides hat are both transcribed and translated. Typically, genes of eukaryotes have exons and introns. After transcription, the introns are removed to form mature mRNA in a process known as post-transcriptional modification. Prokaryotes do not have introns.
Data-based question: Chargaff’s data
1. For adenine to guanine and thymine to cytosine humans have a larger amount of the source than wheat. However, for adenine to thymine the amount of the source is the same for humans and wheat (1.0). For guanine to cytosine the numbers are quite close (humans have 1.00 and wheat has 0.97) Also, for purines to pyrimidines the numbers are quite similar (humans have 1.0 and wheat has 0.99)
2a. This is not true because ox and human are the only ones that have 1:1 for guanine to cytosine. The other species have either larger or smaller values.
2b. This ratio is found in many species such as human, yeast, hemophilus influenza, e coli K2, and bacillus schatz. For the other species it is somewhere between 0.99 to 1.02 which is quite close to 1.0
2c. For adenine to thymine, only one speices (human) has this exact ratio. Other species have it somewhere between 0.95 and 1.12.