Question

Can both strands of DNA act as template?

Can both DNA of homologous chromosomes serve as template?

How can you predict the sequence of proteins based on DNA sequences?

In terms of transcription and translation, how can we predict if an allele will cause a dominant or a recessive condition, a loss-of-function or gain-of-function mutation?

If all cells carry the same DNA information, how is it that different cells may carry out different functions or that particular traits be expressed at different times of an organisms lifetime?

What are loss-of-function and gain-of-function mutations?

Answer 1

1. Both strands of DNA can act as template for transcriptiption provided they contain the required reading frames with the promoter regions and initiator sites. If there are transcriptional promoters on both strands of your template, then you will get RNA from both strands; BUT AT DIFFERENT INSTANCES. Transcription of RNA is complementary to the DNA sequence and is in opposite in direction to the template. This is why the SPECIFIC promoter sequences and transcription initiation sites will be present only in one strand, which acts as the template. The same gene cannot be transcribed into mRNA from the opposite strand because logically thinking their complementary strand would not carry the same sequence.
2. Both DNA of homologous chromosomes can serve as template for transcription because both of them have separate promoter regions and initiation sites as well as frame of reading.
3. DNA sequence is a string of nitrogenous bases comprising of purines (A and G) and pyrimidines (C and T). While transcription, the RNA which is formed has a complementary sequence to the DNA from which it is transcribed with the exception of Uracil in place of Thymine. Those genes that code for proteins are composed of tri-nucleotide units called codons, each coding for a single amino acid. genetic code is the set of rules by which information encoded in genetic material (DNA or RNA sequences) is translated into proteins (amino acid sequences) by living cells. While translation, tRNA with anticodons complementary to the genetic code on mRNA bring the respective amino acid coded by that particular codon to add to the polypeptide chain. Hence, if the DNA sequence of a gene is known, we can easily predict the sequence of protein by identifying the genetic triplet codons and the amino acids associated with those codons.
4. In terms of transcription and translation, we can predict if an allele will cause a dominant or a recessive condition, a loss-of-function or gain-of-function mutation by studying the gene expression of that particular allele and its regulation. In eukaryotes, this regulation of gene expression can be done during transcription of mRNA itself by upregulating or downregulating the transcription of gene, thereby establishing dominance and recessive conditions for that allele. Post translational modifications are another way of regulation by which it can be determined as to how the allele will be expressed.
5. All the cells carry the same DNA information, however, different cells may carry out different functions or that particular traits be expressed at different times of an organisms lifetime. This phenomenon can be explained through the concept of Differential Gene Expression. The three postulates of differential gene expression are as follows [Gilbert SF. Developmental Biology. 6th edition. Sunderland (MA): Sinauer Associates; 2000. Differential Gene Expression]:

• Every cell nucleus contains the complete genome established in the fertilized egg. In molecular terms, the DNAs of all differentiated cells are identical.
• The unused genes in differentiated cells are not destroyed or mutated, and they retain the potential for being expressed.
• Only a small percentage of the genome is expressed in each cell, and a portion of the RNA synthesized in the cell is specific for that cell type.

This is why, inspite of all the cells in an organism having identical DNA, it is still possible for the cells to differentiate into tissues and organs sprecific in functions.

6. Loss of Function Mutation : The mutation because of which gene product results in having little or no function is termed as loss of function mutation. It is also termed as inactivating mutations.

Gain of Function Mutation : This is the type of mutation in which altered gene expression possesses a new molecular function or a new pattern of gene expression.