The Structure of the Genome DNA and the Genome Key Area 5 The Structure of the Genome

The structure of the genome Learning Intentions By the end of this topic you should be able to: The structure of the genome Define ‘genome’ Define ‘gene’ Describe the structure of the genome Explain the difference between coding and non-coding sequences of DNA Describe the functions of non-coding sequences

The Genome An organisms genome is its entire hereditary information encoded in DNA A genome is made up of genes and other DNA sequences that do not code for proteins. Genes are sequences of that DNA that code for a protein However, in eukaryotes the genes only make up a small portion of the DNA. The majority of an organisms DNA is composed of non- protein coding sequences

Non-Protein Coding DNA The non-protein coding regions are composed of sequences that are repeated thousands of times over The function of lots of these sequences are still unknown but there are some sequences whose role is known: Regulation of Transcription Transcription of Non-Translated RNA

Regulation of Transcription The process of transcription requires RNA polymerase but this enzyme alone is unable to initiate the process on its own. RNA Polymerase requires the assistance of transcription factors Some of these factors are called activators that are bound to non- coding DNA sequences called regulator sequences that can be found some distance from the coding sequence they control The DNA strand bends so that these activators are brought close to promoter sequence of the gene to be transcribed

Activators on regulator sequence away from gene to be transcribed Looped DNA that brings the regulator sequence close to the activator sequence of the gene to be transcribed Activators on regulator sequence away from gene to be transcribed Transcription factors RNA Polymerase associating with transcription factors at activator sequence and about to begin transcription

Activators are bound to sequences of DNA called regulator sequences Activators are bound to sequences of DNA called regulator sequences. Often these can be at some distance from the coding sequence. Promoter Gene to be transcribed DNA RNA polymerase Transcription factors Activator Regulator sequence

The DNA must bend to bring the regulator and the activators bound to it in contact with other transcription factors close to the promoter. Regulator sequence RNA polymerase ready to start transcription

The DNA strand bends so that the regulator sequence (bearing the activators) is brought into contact with the transcription factors close to the promoter of the gene to be transcribed By this means, a non-protein coding DNA sequence has regulated the process of transcription

Transcription of Non-Translated RNA We have looked previously at the different types of RNA. 2 of these are non-translated forms of RNA: rRNA tRNA These molecules are essential for protein translation but they are not themselves proteins The sequence of DNA that forms these molecules is non-protein coding DNA

Genes code for proteins Genes code for proteins. The genome is made up of genes and other DNA that does not code for proteins: eg Gene regulatory sequences, which control transcription, DNA, which is transcribed into transfer RNA (tRNA) or ribosomal RNA (rRNA), and small pieces of RNA and DNA sequences that have no known function. Coding and non-coding sequences make up the genome. some DNA makes RNA some DNA makes RNA makes protein some DNA does not appear to do anything One gene can code for many different proteins depending on how many exons or which exons are spliced together. Proteins are mostly enzymes that carry out the ‘instructions’ of the cell, giving us our characteristics. Often one characteristic is controlled by more than one gene so it is a complex business. Introns are common in eukaryotes and the number and length varies a lot between species. Genes are inherited from both parents and are passed on from one generation to the next.