OCR A level Biology Module 6

• What is a gene mutation?

  Change in the DNA'S base sequence. It can occur spontaneously during DNA replication meaning it occurs without any outside influence. Outside factors called the mutagenic agents increase rate of mutations e.g. certain chemicals such as tar in tobacco smoke and ionising radiation such as UV light, X-rays and gamma rays.
• What are the 3 types of gene mutation?

  1. Deletion
  2. Substitution
  3. Insertion
• What is deletion?

  Occurs when a nucleotide is removed from a DNA sequence. This causes a frameshift which is a change to the sequence of triplets and this leads to changes the sequence of amino acid that are coded for results in a large change to the primary structure of resulting protein.
• What is substitution?

  Occurs when a base is swapped out for a different one. This mutation only affects a single triplet in the DNA sequence. Can either result in a different amino acid changing the primary structure or the same amino acid so doesn't change the primary structure having a neutral effect and can occur as the genetic code is degenerate.
• What is insertion?

  Occurs when one or more nucleotides is added to a DNA sequence. This either results in a frameshift or the addition of an extra amino acid.
• How do mutations change a proteins primary structure?

  Alter the bonds within its tertiary structure resulting in a non-functional protein which may be harmful for the organism or a new protein which is beneficial for the organism.
• What is control of gene expression?

  When a gene is transcribed and translated, Biologists say that this gene is expressed. Organisms are able to control gene expression, so that genes are only expressed when they’re needed. One level of control is the control at the transcriptional level.
• What’s the difference between gene expression and protein synthesis?

  Gene expression refers to the entire process of transcription and translation, protein synthesis just refers to the part of the translation process, where amino acids are assembled into a polypeptide, forming a protein.
• What genes are involved in the transcriptional level?

  Structural genes are genes that code for proteins that have a function in the cell. Regulatory genes are genes that code for proteins that control structural genes.
• What is a promoter gene?

  A promoter is a DNA sequence that sits between the regulatory gene and the first structural gene and is the binding site for RNA polymerase which transcribes the structural genes.
• How does RNA polymerase interact slightly differently with promoters in prokaryotes than promoters in eukaryotes?

  In prokaryotes, RNA polymerase binds straight away to the promoter and transcribes the structural genes. However, in order to control gene expression at the transcriptional level, prokaryotes have another DNA sequence that sits between the promoter and the first structural gene.

  This sequence is called the operator.
• What is transcription factors in prokaryotes?

  A regulatory gene codes for a protein that controls the transcription of structural genes. This protein is called a transcription factor. In prokaryotes, a transcription factor that inhibits transcription is called a repressor protein. A repressor protein binds to a DNA sequence called the operator. As a result, the repressor protein prevents RNA polymerase from transcribing the structural genes.
• What is the operon in a DNA sequence?

  An operon is a cluster of structural genes that is under the control of one promoter. An operon contains a promoter, operator and cluster of structural genes
• What is an example of an operon?

  The lac operon is only active when lactose is present. The regulatory gene (lacI) produces a represser

  protein. When lactose is present, it binds to the represser protein and changes its tertiary structure. As a result, the represser protein is unable to bind to the operator (lacO). This means that RNA polymerase can bind to the promoter and transcribe the structural genes (lacZ, lacY and lacA).
• Why isn't the lac operon always on?

  Lactose is a source of ATP, like glucose. Expressing a gene costs ATP. Glucose is always present in bacteria, but lactose isn't. The structural genes coding for glucose digestion are always active. This ensures a steady production of ATP through respiration. The structural genes coding for lactose digestion are only active when lactose is present. This ensures that ATP isn't wasted on expressing the genes coding for lactose digestion when lactose isn't present.

  
• What happen if there is not lactose present?

  The regulatory gene produces a represser protein. When lactose is not present, the represser protein is able to bind to the operater. As a result, RNA polymerase is unable to bind to the promoter, so it is unable to transcribe the structural genes.
• How do transcription factors in eukaryotes control gene expression?

  Transcription factors stimulate gene expression, produced by regulatory genes and bind to promoters.
• How do transcription factors in eukaryotes control gene expression?

  To control the transcription of one or more structural genes in eukaryotes, first, the 

  regulatory gene produces an inactive transcription factor. Next, a hormone binds to the transcription factor and activates it. Third, the active transcription factor binds to the promoter. Fourth, this enables the RNA polymerase to also bind to the promoter, and to start transcribing the structural genes.
• Operons are far more common in prokaryotes than eukaryotes. Suggest why.

  Prokaryotes are much smaller organisms that have smaller genomes than eukaryotes. They are made up of one cell that only needs to respond to changes in its environment.In contrast, most eukaryotes are made up of many cells, forming tissues and organs. Coordination between many cells requires a more complex transcription control than operons can provide.
• Lactose-intolerant adults, who do not produce lactase into adulthood, experience digestive issues if they consume milk. However, lactose-intolerant adults do not experience digestive issues if they consume yoghurt. Yoghurt is produced from milk that has been fermented by bacteria carrying out anaerobic respiration. Suggest why lactose-intolerant adults can safely consume yoghurt.

  There is no lactose present in yoghurt because it has been digested by the bacteria during anaerobic respiration into other sugars.
• What is the second level of control after the gene has been transcribed?

  Control at the post-transcriptional level - this describes the changes mRNA goes through after or post transcription. These changes ensure that mRNA is ready for translation.
• What is primary mRNA and what does it undergo?

  Transcription in eukaryotes produces an mRNA called primary mRNA. This primary mRNA is not ready for translation. There are 2 process that the primary mRNA needs to go through before it can be translated.

  1. RNA Splicing non-coding introns removed
  2. RNA Editing - some RNA bases are substituted, deleted or added in As a result mature mRNA is created which is now ready for translation.
• What is the third level of control taking place after the gene has been translated?

  Control at post-translational level. Post-translational control describes the changes of proteins go through after or post-translation ensuring the protein is ready to perform a specific function in the cell.
• What do translated proteins need to be in order to perform its specific function?

  Activated. Activation allow the protein to perform a specific function in the cells such as catalysing a reaction.
• How are proteins activated?

  Phosphorylation. To active a protein through phosphorylation :

  1. Cyclic AMP (cAMP) binds to and activates an enzyme called protein kinase. This protein kinase then phosphorylates the protein
  2. Changes the proteins tertiary structure and activates it as a result the activated protein can now perform its specific function in the cell.
• What is an organism's body plan?

  The observable, spatial arrangement of their body parts.
• What is a homeobox gene?


  Control the initial development of an organism’s body plan, are active during the embryonic stage and can be found in animals, plants and fungi. Homeobox genes code for transcription factors which control the expression of structural genes, so homeobox genes are a type of regulatory gene. In the case of homeobox genes, these structural genes ensure the correct development of an organism’s  body plan.
• 
  

  A gene that contains a homeobox is a homeobox gene. Homeoboxes in all organisms are 180 base pairs in length coding for 60 amino acids. A DNA sequence inside the homeobox gene is a homeobox. A specific part of the transcription factor coded for by the homeobox is the homeodomain. The homeobox found in all plants, animals and fungi is remarkably similar. So, biologists say that both homeobox genes and homeoboxes are highly conserved by natural selection.
• Homeobox genes are highly conserved between most animal species. Suggest why.

  Mutations in homeobox genes, which alter the body plan, are likely to be lethal. As a result, mutations in homeobox genes are likely to be selected against during evolution.
• What are Hox genes?

  A type of homeobox gene and contain a homeobox. That is only found in animals control the development of symmetry in animals.
• EXAM QUESTION: Homeoboxes in all organisms are 180 base pairs in length. 1.1% of the base pairs in the homeobox of a chimpanzee are different to that of a human.Using your knowledge of the genetic code, how many amino acids coded for by the homeobox would be different between chimpanzees and humans?

        1.1% of 180=1.98≈ 2 base pairs 

  So, a minimum of 0 and a maximum of 2 amino acids will be different between chimpanzees and humans.
• What 2 process control the development of body plans?

  Mitosis and apoptosis
• What happens during mitosis?

  During cell cycle mitosis causes cells to divide this produces new cells for growth, repair and reproduction.
• What happens during apoptosis?

  Programmed cell death removing old cells that are no longer needed.
• How do mitosis and apoptosis control the development of body plans?

  In early embryo mitosis produces 5 fingers but also produces a layer of skin that holds all 5 fingers together. When humans embryo matures apoptosis removes the layer of skin between the fingers as a result their hands develop separate fingers. Both mitosis and apoptosis are controlled by hox genes that respond to internal stimuli e.g. DNA damage or external stimuli such as stress.
• What are the 2 types of intraspecific variation?

  Genetic and phenotypic variation
• What is genetic variation?

  Difference in DNA between organisms of the same species. Results from organisms having different genetic makeup such as due to different allele & mutations. So genetic variation is caused by genetic factors.
• What 2 events does genetic variation occur?

  1. Meiosis where different combinations of gamete alleles are produced
  2. Sexual reproduction where 2 gamete randomly fuse at fertilisation
• What is phenotypic variation?

  Difference in phenotypes between organisms of the same species. Genetic factors like different alleles cause phenotypic variation but so do environmental factors.
• What are examples of phenotypic variations?

  • Chlorosis - Plant in regular environment it's genetic factors causing it to produce chlorophyll as a result it's phenotype shows green leaves so plant is able to photosynthesise. However, when an environmental factor changes the plant may stop producing chlorophyll called chlorosis phenotype shows yellow leaves.
  • Diet - Fruit fly with regular diet has black phenotype but when given diet of silver salts they produce a yellow phenotype.