Pattern of inheritance:
• The skin colour in human beings as we observe is not just black or white or dark or light. There is a wide range of skin colour.
• Hence, so instead of single gene, the skin colour is governed by three different genes with total six alleles: 3 being dominant and 3 being recessive.
• The dominant alleles are denoted by A, B and C whereas recessive alleles are denoted by a, b and c.
• More the dominant alleles present in the individual, darker will be the skin colour and similarly, more the recessive alleles present in the individual, lighter will be the skin colour.
• If an individual has three dominant and three recessive alleles, the skin colour is intermediate.
This type of inheritance is called polygenic inheritance.
Other examples of polygenic inheritance are human height, flower size, human hair colour etc.
Transcription: It is defined as the process by which the synthesis of RNA strand takes place by copying the DNA template.
• Only single strand of DNA is used in the process of transcription.
• The DNA strand with the polarity 3’ to 5’ is used as the template strand.
• The 3’ end of the template strand has a promoter region from which the transcription starts. The promoter region consists of a promoter called sigma factor.
• The enzyme DNA dependent RNA polymerase binds to the promoter and the process of transcription starts.
• The nucleoside triphosphate is used as the substrate. It helps in polymerization of RNA by following the rule of complimentarity.
• Hence, in this way the RNA strand keeps forming. This step is also known as elongation.
• The 5’ end of the DNA template strand has a terminator known as rho factor.
• When the enzyme reaches the termination region, it binds with terminating factor and stops it work.
• The formed RNA falls off and the process of transcription stops.
• In this way, the mRNA is formed in prokaryotes and it does not require any processing to be active.
The diagram below shows the structure of transcription unit and the process of transcription in prokaryotes:
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