Gene expression

• Cell differentiation is the process by which a less specialised cell becomes more specialised for a particular function. These differentiated cells produce more proteins which are specific to their cell type. Their shape and arrangements of the organelles also differ
• In multicellular organisms, every cell contains the same genetic information but different cells perform different functions. All cells also have proteins in common, like the structural proteins of the membranes and the enzymes needed
• The genotype and the organism’s environment determine an organisms phenotype
• It is more difficult to investigate the impact of environment on the phenotype with animals becayse of the difficulty of producing large numbers of cloned, genetically identical organisms and the ethical aspects of experimentation
• A unit of linked genes which is thought to regulate other genes responsible for protein synthesis is called an operon. Operon systems give very sensitive control of the metabolism of the cell, allowing it to change in response to changes in its environment. If scientists change the nutrients given to the organism, the phenotype may change because of the operon system allowing them to investigate the causes
• Many characteristic, including our intelligence and our appearance, are the result of polygenic inheritanc. This makes it difficult to determine the extent of the effect of the environment on the phenotype
• An example of the environment affecting the phenotype:

  The environment in the uterus affects the development of the fetus before birth. For example, if the mother is malnourished, the fetus may be deprived of vital nutrients and not achieve its full potential growth.
• One strategy of studying variation is to consider genetically identical individuals and try to separate the effect of genes and environment. Identical twins have the same genetic material. Ordinary twins are useful as a control group. From one study, it showed body mass was affected by environmental factors like eating factors, wheras height was little influenced. IQ was a mix of both.
• Epidemiological studies are often used to measure variation in a population - it can help to decide if a characteristic is inherited by a single gene or is polygenic
• Discontinuous variation is shown by features that are either present or not such as blood group or sex, and these features are generally determined by one or two genes
• Characteristics that show continuous variation include weight and height. In continuous variation, factors are determined by multiple genes, and very much affected by the environment
• When studying continuous variation in a population, you need to take large samples. You need to collect your results randomly because chance can affect results. Data can be displayed in a bar graph or histogram
• Different combinations of expressed genes are expressed in different cells. This creates a variety of structure and function seen in cells of different tissues. The expression of a gene involves transcription and translation
• Transcription refers to the process by which a DNA sequence is copied into an RNA sequence by the enzyme RNA polymerase
• Translation is the process by which the sequence of nucleotides present in mRNA is read by the ribosomes and is translated into the sequence of amino acids in a polypeptide chain
• Exerting controls at of the stages of the transcription and translation processes gives control over the expression of the genes
• The different proteins present in a cell, and the quantities of those proteins determines the type of cell and its function in the body
• The most common way of controlling gene expression is by switching on and off the transcription of certain genes. mRNA transcription is an effective point at which to control gene expression becayse a single mRNA molecule results in the production of many genes
• Transcription factors are proteins that bind to the DNA in the nucleus and affect the the process of transcribing the genetic material
• All transcription factors have regions that enable them to bind to specific regions of DNA known as promoter sequences. Promoter sequences are found just above the starting point for transcription on the gene.
• Some transcription factors stimulate the transcription of a region of DNA by binding to a DNA sequence. This stimulates the start of transcription of that area of DNA
• Transcription factors known as enhanced sequences regulate the DNA by changing the structure of chromatin, making it more or less open to RNA polymerase. This means transcription factors can either stimulate or prevent the transcription of certain genes
• Often several transcription factors will be involved in the expression of a single gene which leads to many levels of control. A single transition may stimulate the expression of one gene and suppress the expression of another which leads to many levels of control
• The control of transcription factors allow genes to be expressed (switched on) or repressed (switched off) at different stages of the organisms. Transcription factors work in the nucleus
• The mRNA produced in the nucleus is the result of transcription and it is made of introns and exons. Introns are the non-coding DNA. The mRNA is refereed as pre-mRNA as there are still several process before it can become mature. Enzyme complexes called spliceosomes join the exons that are to be transcribed together to produce the mature functional DNA
• The spliceosome may join the same exons in a variety of ways known as RNA splicing. As a result, a single gene may produce several versions of functional DNA. These different versions code for different arrangements or amino acids, which im turn codes for different proteins. These post-transcriptional changes lead to more variety in the phenotype then is coded for directly in the genotype
• Further modifications may happen after the proteins have been synthesised. A gene coding for a protein may be shortened or lengthened by enzymes to give a variety of other proteins