Figure 1 The relationship between a gene and the protein for which it codes. DNA is shown here as a double strand with the helix unwound.
Figure 1Figure 1Video: Click to view clip on the one gene-one protein hypothesis3 The flow of information from DNA to RNA to proteinThe information flow from DNA to protein is more complex than shown in Figure 1. The genetic information encoded within the DNA of a gene is carried via an intermediary molecule, RNA (ribonucleic acid). Information within a cell can therefore be seen as passing from DNA, via RNA, to a protein. This flow of information can be expressed in another way.DNA makes RNA makes protein.This statement implies that there are two separate steps in this information flow: from DNA to RNA and from RNA to protein; these are called, respectively, transcription and translation. Transcription of DNA produces RNA and the subsequent translation of this RNA produces proteins. These steps are summarized in Figure 2 (also click on the link to the video clip below).
Figure 2 Information flow from DNA to RNA to protein.
Figure 2Figure 2Video: Click to view clip from DNA to RNA to protein4 From DNA to RNA: transcriptionIn the process of transcription, the information in a gene, i.e. the DNA base sequence, is copied, or transcribed, to form an RNA molecule. RNA is therefore an intermediary in the flow of information from DNA to protein. Before we consider the details of transcription, we will first look at the structure of RNA.The name ribonucleic acid suggests that RNA is chemically related to DNA. Like DNA, RNA is a chain of nucleotides.SAQ 2What are the three component parts of a nucleotide?Each nucleotide consists of three parts: phosphate, a sugar and a base.There are some important differences between DNA and RNA. One way in which RNA and DNA differ is in the sugar component; RNA has ribose (not deoxyribose as in DNA) hence ribonucleic acid. A second difference is in the nucleotide bases.SAQ 3What are the four bases in DNA?The DNA bases are adenine (A), guanine (G), cytosine (C), and thymine (T).In contrast, the RNA bases are adenine (A), guanine (G), cytosine (C) and uracil (U). Why one of the four bases in RNA is different from the equivalent base in DNA is not fully understood.There is a third important structural difference between DNA and RNA. Recall that the basic structure of DNA is a double helix of two spiralled strands, i.e. it is double-stranded. In contrast, RNA is usually a single strand, as shown diagrammatically in Figure 2.The similarities in structure between DNA and RNA suggest that RNA is synthesized in a manner similar to DNA replication, i.e. using the DNA as a template. This is indeed what happens. The process of transcription is illustrated diagrammatically in Figure 3 (see also the link to the video clip below). As in DNA replication, the starting point is a double helix molecule of DNA (Figure 3a). The length of DNA sequence corresponding to a gene unwinds and the two strands separate (Figure 3b). Here the process of transcription diverges from the familiar one of DNA replication, because synthesis of RNA molecules occurs on only one of the two strands: only one DNA strand is the template for RNA synthesis, and this is termed the template strand. The other DNA strand, which is not used as a template in RNA synthesis, is termed the non-template strand (Figure 3b). The basic mechanism of RNA synthesis is the same as that for DNA, in that pairing of complementary bases is the key to the process. The enzyme that brings about the synthesis of RNA is RNA polymerase, and is a different enzyme from the DNA polymerase that brings about DNA synthesis.Video: Click to view clip from DNA to RNA: transcriptionSAQ 4Which bases are paired together in DNA?C pairs with G, and A pairs with T.
Figure 3 The synthesis of RNA on a DNA template. (a) The DNA double helix with 10 labelled base pairs. (b) The two strands of the helix unwind; note that only one of the strands is used as the template for RNA synthesis. (c) A short length of RNA (shown in red) is being synthesized: a nucleotide with base A is about to be added to the RNA strand. In reality, the RNA molecule would be much longer than the chain of nine nucleotides shown here.
Figure 3Figure 35 Where does transcription occur in the cell?Up to now we have described the processes of transcription without considering where each occurs within the cell.SAQ 5Given that transcription — the production of mRNA — requires a DNA template, where do you think this process occurs in the cell?It must occur in the nucleus where the DNA in the cell is located.However, once mRNA is produced, it leaves the nucleus and protein synthesis – translation – occurs in the cytoplasm. Thus transcription and translation are separated both in space within the cell and in time, in that one occurs after the other, as shown schematically in Figure 11.8. Thus the role of mRNA is to carry a coded message from the nucleus where the information is stored, to the cytoplasm where the coded message is translated into a specific protein; hence it’s name – messenger RNA.Translation occurs at particular sites within the cytoplasm; it occurs on ribosomes. Ribosomes are large aggregates of proteins and ribosomal RNA (rRNA). Hence three types of RNA are involved in the process of translation but only one of them, mRNA, codes for proteins.Video: Click to view clip of conclusion to this courseConclusionThis free course provided an introduction to studying Science. It took you through a series of exercises designed to develop your approach to study and learning at a distance and helped to improve your confidence as an independent learner.Keep on learningStudy another free courseThere are more than 800 courses on OpenLearn for you to choose from on a range of subjects. Find out more about all our free courses.Take your studies furtherFind out more about studying with The Open University by visiting our online prospectus. If you are new to university study, you may be interested in our Access Courses or Certificates.What’s new from OpenLearn?Sign up to our newsletter or view a sample.For reference, full URLs to pages listed above:OpenLearn – www.open.edu/openlearn/free-coursesVisiting our online prospectus – www.open.ac.uk/coursesAccess Courses – www.open.ac.uk/courses/do-it/accessCertificates – www.open.ac.uk/courses/certificates-heNewsletter – www.open.edu/openlearn/about-openlearn/subscribe-the-openlearn-newsletterThe content acknowledged below is Proprietary (see terms and conditions) and is used under licence.Grateful acknowledgement is made to the following sources for permission to reproduce material in this course:Course image: brewbooks in Flickr made available under Creative Commons Attribution-ShareAlike 2.0 Licence.All other materials included in this course are derived from content originated at the Open University.Don't miss out:If reading this text has inspired you to learn more, you may be interested in joining the millions of people who discover our free learning resources and qualifications by visiting The Open University - www.open.edu/openlearn/free-coursesDiscussion2019012600