DNA Structure
DNA is the chemical that determines inherited characteristics and it contains vast amounts of information in the form of the genetic code. DNA is made up of just three basic components that combine to form a nucleotide.
- A sugar called a deoxyribose
- A phosphate group
- An organic base belonging to one of two different groups:
- Single-ring bases – cytosine (C) and thymine (T)
- Double-ring bases – adenine (A) and guanine (G)
- The deoxyribose sugar, phosphate group and organic base are combined as a result of
- condensation reactions, to give a mononucleotide.
- Two mononucleotides may in turn, be combined as a result of a condensation reaction between the deoxyribose of one mononucleotide and the phosphate group of another. The new structure is called a dinucleotide.
- The continued linking of mononucleotides in this way forms a long chain known as a polynucleotide.
- DNA is made up of two long strands of nucleotides, joined together by hydrogen bonds formed between certain bases.
- Adenine always pairs with thymine by means of two hydrogen bonds. Guanine always pairs with cytosine by means of three hydrogen bonds. They are complementary to one another.
- The uprights of phosphate and deoxyribose wind around one another to form a double helix. For each complete turn of this helix, there are ten base pairs.
Function of DNA
DNA is the hereditary material responsible for passing genetic information from cell to cell and generation to generation. The DNA molecule is adapted to carry out its function in a number of ways:
- It is very stable and can pass from generation to generation without change
- Its two separate strands are joined by hydrogen bonding which allow them to separate during DNA replication and protein synthesis
- It is an extremely large molecule and therefore carries and immense amount of genetic information
- By having the base pairs within the helical cylinder of the sugar-phosphate backbone, the genetic information is, to some extent, protected from being corrupted by outside chemical and physical forces.
The Triplet Code
- In IBDP Biology, polypeptides is one of the important protein.
- Genes are sections of DNA that contain the coded information for making polypeptides.
- The coded information is in the form of a specific sequence of bases along the DNA molecule.
- Polypeptides combine to make proteins and so genes determine the proteins of an organism.
- Since enzymes are proteins, genes determine the nature and development of all organisms.
- A gene is a sequence of DNA bases that determines a polypeptide, and a polypeptide is a sequence of amino acids.
Scientists suggested that there must be a minimum of three bases that coded for each amino acid because:
- Only 20 amino acids occur regularly in proteins
- Each amino acid must have its own code of bases on the DNA
- Only four different bases are present in DNA
- If each base coded for a different amino acid, only four amino acids could be coded for
- Using a pair of bases (42), the number of codes is still inadequate
- Three bases produce 64 different codes, more than enough to satisfy the requirements of 20 amino acids
As the code has three bases, it is called the triplet code. Some amino acids have more than one code (64 codes only 20 amino acids).
The start of a sequence is always the same triplet code, this codes for the amino acids methionine. If this first methionine molecule does not form part of the final polypeptide, it is later removed.
DNA and Chromosomes
- In prokaryotic cells such as bacteria, the DNA molecules are smaller, form a circle and aren’t associated with protein molecules. They therefore do not have chromosomes.
- In eukaryotic cells, the DNA molecules are larger, form a line rather than a circle, and occur in association with proteins to form structures called chromosomes.
- In almost all species there is an even number of chromosomes because they occur in pairs known as homologous pairs. This is because sexually produced organisms are the result of the fusion of a sperm and an egg, each of which contributes a set of organisms to the offspring.
- One of each pair is derived from the chromosomes provided by the mother and the other half derived from the father.
- A homologous pair is always two chromosomes that determine the same genetic characteristics but this is not the same as being identical.
During meiosis, the halving of the number of chromosomes is done in a manner which ensures that each daughter cell receives one chromosome from each homologous pair. In this way, each cell receives one set of information for each characteristic of the organism. When these haploid cells combine, the diploid state, with pair homologous chromosomes, is restored.
Genes
- Genes are sections of DNA that contain coded information in the form of specific sequences of bases. Each gene exists in two, occasionally more, different forms.
- Each of these forms is called an allele. Each individual inherits one allele from each of its parents, which may be the same or different.
Any differences in the base sequence of an allele of a single gene may result in a different sequence of amino acids being coded for. This different amino acid sequence will lead to the production of a different polypeptide, and hence a different protein.
This is the end of the topic
Drafted by Eva (Biology)
Photo references:
- http://sophiesasbiology.blogspot.com/2016/02/nucleotides.html
- https://www.ashg.org/discover-genetics/building-blocks/
- https://theyoungsci.wordpress.com/2017/06/30/dna-mutations/
- https://www.sciencelearn.org.nz/resources/206-dna-chromosomes-and-gene-expression
- https://eschool.iaspaper.net/what-is-a-gene/gene-chromosome/