different genes are expressed
as the embryo develops cells differentiate and become specialised for one function or a group of functions depending on the proteins that it synthesises.
scientists have discovered that different genes are expressed in different cells meaning cells have different functions.
scientists igor dawid and thomas sargent discovered this by extracting mRNA from differentiated and undifferentiated cells . they found that complientary DNA (cDNA) strands were produced for all the mRNA in differentiated cells using an enzyme called reverse transcriptase , which reverses transcription - making DNA from mRNA.
these cDNA srands were mixed with the mRNA from the undifferentiated cell. complimentary strands of cDNA and mRNA combined to produce double hybrids.
when these hybrids were separated out, there remained a range of cDNA stands that had not been hybridised - meaning the two cells were expressig some of the same genes but also some different genes.
switching genes on
cells become specialised because only some genes are switched on snd produce active mRNA which is translated into proteins within the cell.
scientists proposed a theory for gene exression. they studied to control of genes in prokaryote escherichia coli which produce the enzyme B-galatosidase to breakdown lacose. this enzyme converts the disaccharide lactose to the monosccharides glucose and galactose.
when this lactose in not present a lactose repressor molecule binds to the DNA and prevents the transcription of the B-galactosidase gene being expressed.
if lactose is present the repressor molecule is prevented from binding to the DNA and the B-glactosidase gene is expressed. the mRNA coding for B-galactosidase is transcribed and the tranlation of mRNA produces the enzyme.
swithcing on genes in eukaryotes
genes in uncoiled, accessible areas of a eukaryotes DNA can be transcribed into mRNA. the enzyme RNA polymerase binds to a section of the DNA adjacent to the gene to be transcribed.
this section is known as the promoter region.
only when the enzyme has attached to the DNA will transcription proceed.
the gene remains swiched off untill the enzyme attaches to the promoter region successfully. the attchment of a regualtor protein is usually required to start transcription.
trnascription of a gene can be prevented by protein repressor molecules attaching to the DNA of the promoter region. blocking the attachment site.