Several metabolic stages in eukaryotes may serve as control points for gene expression. The four ways by which genes are regulated in eukaryotes are as follows:
• Transcriptional control:
This is the most important mechanism for regulating gene expression. Transcription factors have either positive or negative effect on transcription. In some cases these transcription factors act locally in the cells producing them or are transported to different parts of the body for action.
• Silencing of genes:
Silencing of genes by methylation of cytosine bases is another control mechanism. A methyl group binds to the carbon at 5 th position in the cytosine ring. It happens when cytosine is present next to a guanine; thus the bases in the complementary DNA strand would also be a cytosine and guanine. When the DNA is replicated, the enzyme recognizes the CG sequence and quickly methylates the daughter strand, thus keeping the gene inactive.
• Translational control:
Genes can be transcribed and the mRNA sequestered. This delays the translation. When needed, activation of mRNA occurs to give the gene products.
• Gene rearrangement:
Vertebrates contain lymphocytes that bear genes encoding proteins called antibodies. Each type of antibody binds specifically to the antigen. As there are a number of antigens, the number of antibodies should also be high. One source of this diversity is the rearrangement of DNA sequences coding for antibodies during development of lymphocytes.
The purines present in DNA are adenine and guanine. The pyrimidines in DNA are thymine and cytosine. Adenine pairs with thymine while guanine pairs with cytosine.
The purines present in RNA are adenine and guanine. The pyrimidines in DNA are uracil and cytosine. Adenine pairs with uracil while guanine pairs with cytosine.
RNA is similar to DNA in structure except that it consists of a single polynucleotide chain and has ribose instead of deoxyribose, and has uracil instead of thymine.
During replication, the two strands of the double helix unwind. The direction of one strand is
while the direction of the complementary strand is
. Each separated strand serves as a template for the synthesis of a complementary strand.
The enzyme DNA polymerase catalyzes the assembly of a new strand of polynucleotides based on the template. It attaches a thymine group opposite the adenine group in the template strand, a guanine group opposite the cytosine group and vice versa.
DNA polymerase synthesizes new strands only in the
Therefore, only the
strand acts as a template. The
strand cannot act as a template. Therefore, the synthesis of DNA complementary to
strand is continuous while the synthesis of DNA complementary to
strand is discontinuous in the form of fragments.