Sex Determination, Genetic Disorders, and DNA Processes
Sex Determination
The mechanism of sex determination has always been a puzzle for geneticists. The initial clue about the genetic or chromosomal mechanism of sex determination can be traced back to some experiments carried out in insects. In fact, the cytological observations made in a number of insects led to the development of the concept of a genetic or chromosomal basis of sex determination.
Mutation
Mutation is a phenomenon that results in the alteration of DNA sequences and, consequently, results in changes in the genotype and the phenotype of an organism. In addition to recombination, mutation is another phenomenon that leads to variation in DNA.
Genetic Disorders
Pedigree Analysis
The idea that disorders are inherited has been prevalent in human society for a long time. This was based on the heritability of certain characteristic features in families. After the rediscovery of Mendel’s work, the practice of analyzing inheritance patterns of traits in human beings began. Since it is evident that control crosses that can be performed in pea plants or some other organisms are not possible in the case of human beings, the study of family history regarding the inheritance of a particular trait provides an alternative. Such an analysis of traits across several generations of a family is called pedigree analysis.
Mendelian Disorders
Broadly, genetic disorders may be grouped into two categories: Mendelian disorders and chromosomal disorders. Mendelian disorders are mainly determined by alteration or mutation in a single gene. These disorders are transmitted to the offspring.
DNA Structure
DNA is a long polymer of deoxyribonucleotides. The length of DNA is usually defined as the number of nucleotides (or a pair of nucleotides referred to as base pairs) present in it. This is also a characteristic of an organism. For example:
- A bacteriophage known as φ 174 has 5386 nucleotides.
- Bacteriophage lambda has 48,502 base pairs (bp).
- Escherichia coli has 4.6 × 106 bp.
- The haploid content of human DNA is 3.3 × 109 bp.
Let us discuss the structure of such a long polymer (DNA).
DNA Replication
While proposing the double helical structure for DNA, Watson and Crick had immediately proposed a scheme for the replication of DNA. To quote their original statement: “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material” (Watson and Crick, 1953). The scheme suggested that the two strands would separate and act as a template for the synthesis of new complementary strands. After the completion of replication, each DNA molecule would have one parental and one newly synthesized strand. This scheme was termed semiconservative DNA replication.
Transcription
The process of copying genetic information from one strand of DNA into RNA is termed transcription.
Genetic Code
During replication and transcription, a nucleic acid is copied to form another nucleic acid. Hence, these processes are easy to conceptualize on the basis of complementarity. The process of translation requires the transfer of genetic information from a polymer of nucleotides to a polymer of amino acids.