Transcription, Translation, and Regulation of Eukaryotic DNA - Insect Molecular Genetics - page 39

chromosome and are genetically more complex (with ≈ 15,000-35,000 genes).
Furthermore, most eukaryotic protein-coding genes are split, with one or more
noncoding introns interspersed among the coding exons . The presence of
introns requires that transcribed RNA, called pre-mRNA, must have the introns
removed before it becomes mRNA and can go from the nucleus into the cyto-
plasm, where translation can occur.
Control elements, such as promoters and enhancers and a variety of ncRNAs,
are important components of gene regulation in eukaryotes. Furthermore,
within a particular tissue, a mixture of active and inactive genes is present on
each chromosome. Research on Drosophila indicates that specific sequences
called boundary elements insulate the active from the inactive regions of chro-
mosomes ( Bell and Felsenfeld 1999, Bell et al. 2001 ). The location of specific
chromosomal regions in the nucleus affects the ease with which specific genes
are transcribed.
Finally, the nuclear membrane in eukaryotes separates the processes of tran-
scription in the nucleus and translation in the cytoplasm in both time and space.
The nuclear membrane controls which molecules enter and leave the nucleus.
The nuclear membrane contains pores with a complex structure consisting
of ≈ 400 polypeptide chains (nucleoporins) forming a cylindrical structure. The
nuclear pore allows the diffusion of ions, small molecules, and molecules < 40
kilodalton (kDa) ( Kriwacki and Yoon 2011 ). Larger molecules containing mark-
ers indicating they are destined for the nucleus are actively transported through
the membrane through mechanisms that remain under study. The intricacies
of transcription and translation of eukaryotic DNA are still being unraveled,
but molecules involved in transcription and translation must pass through this
membrane.
2.3 RNA Synthesis is Gene Transcription
Transcription is the first stage of protein-coding gene expression ( Figure 2.1 ).
During transcription, the coding strand of DNA serves as a template for synthesis
of a complementary RNA molecule. The sequence of the RNA molecule is deter-
mined by complementary-base pairing so that the RNA is a complementary tran-
script (copy) of the coding strand of DNA.
Transcription requires four ribonucleoside 5 ′ -triphosphates: ATP, GTP, CTP, and
UTP (uracil substitutes for thymine in RNA). A sugar-phosphate bond is formed
between the 3 ′ -OH group of one nucleotide and the 5 ′ -triphosphate of a sec-
ond nucleotide by the enzyme RNA polymerase. RNA polymerase can initiate
RNA synthesis without requiring a primer. The sequence of bases in the RNA