Topic 2- Genes and Health

A summary of topic 2, genes and health

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  • Created by: R_Hall
  • Created on: 15-12-12 12:49

Gas Exchange

  • Lungs allow rapid gas exchange between the atmosphere and the lungs
  • Air is drawn into the lungs via the trachea (low pressure- created by movement of ribs and diaphragm), trachea divides into 2 bronchi, which divides into many bronchioles with alveoli at the end
  • There is a thin layer of mucus in the gas exchange system. The mucus (made by goblet cells) traps any dirt or micro-organisms and is beaten up the system to the nose/mouth by cilia that cover the epithelial cells
  • Substances that diffuse in or out of cells move down a concentration gradient (from high to low conc), which is constantly maintained
  • The larger the organism, the more exchange needs to take place. As an organism gets larger, the surface area per unit of volume ration gets smaller
  • Alveoli are adapted to efficient gas exchange because of-
  • 1. Large surface area of alveoli-
  • 2. Concentration gradient (maintained by ventilation and circulation)
  • 3. Short diffusion path to many capillaries
  • 4. Thin walls of alveoli and capillaries
  • 5. Lined with liquid surfactant (things can dissolve)
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Cell Transport

  • Diffusion- high to low concentration until equilibrium reached. Used to transport hydrophobic or small molecules through cell membrane. Passive process (no ATP)
  • Faciliated diffusion- high to low concentration until equilibrium reached. Used to transport hydrophilic molecules or ions through channel protein/ carrier proteins (change shape). Passive process
  • Osmosis- diffusion involving movement of free water molecules. High to low concentration of free water molecules until equilibrium reached. Through phospholipid bilayer. Passive process
  • Active transport- against a concentration gradient (low to high). Through a carrier protein that changes shape. Energy required (active)
  • Exocytosis- Used for bulk transport of substances out of the cell. Vesicles fuse with the cells surface membrane, releasing contents
  • Endocytosis- Used for bulk transport of substances into the cell. Vesicles are created from the cell surface membrane, bringing contents into cell
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Amino Acid

  • Proteins are composed of amino acids (20 different). All amino acids contain an amine group (-NH2), a carboxylic acid group (-COOH) and a hydrogen (-H), but each different amino acid has an individual residual group (-R)
  • Primary structure
  • Two amino acids join in a condensation reaction to form a dipeptide, with a peptide bond joining them. The sequence of amino acids in the polypeptide chain (made up of several bonded dipeptides) is the primary structure
  • Secondary structure
  • The chain may twist to form an α-helix or a β-pleated sheet. Within the alpha-helix spring, hydrogen bonds form between carboxylic and amine groups, to stabilise. Within the beta-pleated sheet, several chains link together with hydrogen bonds
  • Tertiary and quaternary structure
  • A polypeptide chain bends and folds to produce a 3D shape (tertiary). Chemical bonds(ionic, disulphide or hydrogen bonds) and hydrophobic interactions between R groups maintain structure. A quaternary is made up of several polypeptide chains
  • Proteins can be fibrous or globular. Fibrous proteins (eg. keratin and collagen) remain as long chains that can be cross linked for strength. Globular proteins (eg. enzymes and haemoglobin) are folded into a spherical shape and are soluble
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Cell Membranes

  • The cell membrane is a bilayer of phospholipids.
  • The phosphate head of the molecule is polar, meaning that is attracts other polar molecules (eg. water) and is therefore hydrophilic
  • The fatty acid tails are non-polar, and therefore hydrophobic.
  • When added to water, the phospholipids arrange themselves so the tails do not come into contact with water. They may form a cluster (micelle) or a bilayer, with tails facing inwards. This prevents contact with cytoplasm inside, and tissue fluid outside the cell
  • The cell surface membrane contains proteins, cholesterol, glycoproteins (proteins+ polysaccharides) and glycolipids (lipids+ polysaccharides)
  • It is thought that some proteins are fixed within the membrane, but others are free to move around in the phospholipid bilayer. This is known as the fluid mosaic model of membrane structure
  • Evidence to support this theory was found by a number of different research groups. Electron micrographs showed phosphate heads and lipid tails in a bilayer. Experiments showed that there were two types of proteins (those that are easily dissociated from the membrane, and those that aren't)
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  • Enzymes are globular proteins which act as biological catalysts. They speed up chemical reactions that would otherwise occur very slowly at the temperature within the cells
  • Each type of enzyme has an active site with a specific 3D shape. Single molecules or multiple molecules with complementary shapes fit into the active site to be broken down or joined up. Each enzyme will therefore only catalyse one reaction
  • These substrate molecules form temporary bonds with the amino acids of the active site to produce an enzyme-substrate complex. When the reaction finishes, the products are released and the enzyme remains unchanged
  • The substrate is known as the "key" and the enzyme is a "lock"- the lock-and-key theory if enzyme action
  • The active site is often flexible; it changes shape slightly when a substrate molecule enters the active site. This is the induced fit theory
  • The energy needed to break bonds in the substrate and start the reaction is known as the activation energy. Enzymes reduce the amount of activation energy needed to start a reaction
  • The rate of reaction is measured by determining the quantity of a substrate used or the quantity of product formed in a given time. With an enzyme, initially the rate of reaction is high (steep curve) but levels off (as all active sites of the enzyme are full)
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The Structure Of DNA

  • DNA is the genetic code which dictates all inherited characteristics of an organism by controlling protein production
  • A gene is a sequence of bases on a DNA molecule coding for a sequence of amino acids in a polypeptide chain. Chromosomes contain genes. The genome is all the genes in an individual
  • DNA is made up of deoxyribonucleic acid; it is a long chain of nucleotides. A nucleotide contains 3 molecule linked by a condensation reaction; deoxyribose (a sugar), phosphate group and an organic base (containing nitrogen)
  • There are 4 different bases- adenine (A),  thymine (T), guanine (G) and cytosine (C)
  • In a DNA molecule, there are two strands of nucleotides twisted together to form a double helix. The sugar and phosphate groups form the backbone, and the bases are the rungs
  • The two strands are antiparallel (run in opposite directions) and are held by hydrogen bonds between bases
  • The bases only pair in certain ways- adenine with thymine, cytosine with guanine- this is complementary base paring. This is because A and G have a two-ring structure, and T and C only have one ring. Each rung is 3 rings long, so the "rungs" of the ladder have a uniform length.
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How Does DNA Code for Proteins?

  • The code carried by the DNA is a three-base or triplet code (each group of 3 bases codes for an amino acid)
  • A copy of the "target" DNA sequence is made from ribonucleic acid, or RNA. An RNA molecule is a single strand of RNA nucleotides (which allows it to pass through nuclear membrane to the cytoplasm, where proteins are made).
  • RNA and DNA are very similar, but RNA is made of ribose sugar (not deoxyribose) and in RNA, the base uracil (U) replaces thymine
  • There are 3 types of RNA in protein synthesis- messenger RNA (mRNA), transport RNA (tRNA) and ribosomal RNA (rRNA)
  • Transcription
  • Takes place in the nucleus. The target DNA strand unwinds, and H bonds between bases break. The mRNA is built from RNA nucleotides lining up with the template strand (antisense strand). Every triplet code on the DNA gives rise to a complementary codon on the mRNA. The mRNA leaves via a pore in the nuclear envelope
  • Translation
  • Takes place on ribosomes (found free or on endoplasmic reticulum). A tRNA carrying an amino acid has 3 bases- an anticodon- which pairs with the mRNA codon. The amino acids join by means of peptide bonds
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DNA Replication

  • The process of copying DNA is called semi- conservative replication
  • 1. Hydrogen bonds between the bases break, allowing the DNA strand to "unzip"
  • 2. DNA nucleotides pair up with their complementary bases. DNA polymerase (enzyme)  links adjacent nucleotides
  • 3. Two identical daughter strands are created
  • Each of the two DNA molecules contain one "old" strand and one "new" strand; they are identical to each other and the original molecule
  • Messelson and Stahl conducted an experiment that proved that replication is semi-conservative. There were 3 models- conservative (daughter strand made of new nucleotides), fragmentary (all DNA strands are a mix of parent and new) and semi-conservative (each DNA molecule has one new and one old strand)
  • They made heavy of DNA by growing bacteria in a nitrogen isotope (N15) . They then transferred the bacteria to a medium containing lighter N14. They allowed the bacteria to grow and replicate.
  • The DNA was spun in a centrifuge with a density-gradient solution. A band of medium density was formed in the middle. The conservative model was rejected- no heavy DNA
  • The DNA was extracted and replicated 2x before centrifuging again. It showed one light band and one medium. Ruled out fragmentary (would be one band of light-heavy mix)
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  • Gene- A sequence of bases on a DNA molecule that codes for a protein
  • Allele- A different version of a gene. Humans and most plants and animals have two copies of each gene
  • Genotype- The alleles that a person has
  • Phenotype- The characteristics that the alleles produce
  • Recessive- An allele whose characteristic only appears in the phenotype if two copies are present
  • Dominant- An allele whose characteristic appears in the phenotype even when there is only one copy
  • Homozygote- An organism who carries two copies of the same allele
  • Heterozygote- An organism that carries to different alleles
  • Carrier- Someone who carries one dominant and one recessive allele (if recessive allele causes disease). Won't have the disease, but will carry the allele
  • Monohybrid Inheritance- Where a characteristic is controlled by only one gene (eg. Cystic fibrosis)
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  • Mutations are changes to the base sequence of DNA. They can be caused by errors in replication
  • Can be substitution, deletion, insertion, duplication or inversion (where order is reversed)
  • The order of bases in  a gene determines the order of amino acids in a protein. If a mutation occurs, then the primary structure of the amino acids could be altered. This could change the final 3D shape so it doesn't work properly
  • If a mutation occurs on a gene it can cause a genetic disorder, which is passed on
  • The CF gene is located on chromosome 7 and carries the code to make the CTFR protein (a channel protein which is involved in mucus production). However, hundreds of mutations on the CF gene have been identified.
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CF Problems

  • Respiratory system
  • Microorganisms and pathogens become trapped in the thick mucus in the lungs, and breed. The mucus layer is so thick that it cannot be removed by cilia, and the mucus becomes thicker as WBCs are made to combat infection. This repeated infection damages the system
  • The mucus blocks the bronchioles, preventing ventilation and reduces surface area of alveoli (as less are able to receive gases).
  • Digestive system
  • CF sufferers have problems with the digestion and absorption of nutrients. The pancreatic duct becomes blocked by mucus, impairing release of digestive enzymes and reducing digestion (malabsorption syndrome). Also the enzymes trapped behind the mucus can cause damage to the pancreas and insulin released can be blocked
  • Reproductive system
  • Females have a reduced chance of pregnancy if a mucus plug develops in the cervix
  • Males can either have sperm ducts blocked by mucus, or cannot have them at all
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Gene Therapy and Genetic Screening

  • In gene therapy the genotype (hence phenotype) are altered-
  • 1. Normal alleles are inserted into target cells (via virus or liposome)
  • 2. The normal form of the gene is transcribed and translated
  • 3. A functioning protein is produced in the target cells
  • All gene therapy today affects somatic cells (body cells), so the changed genes are not passed on to children. There are ethical objections to altering germ cells (sperm or egg) because of concerns about effects in future generations when the new gene is inserted
  • Genetic screening can be used to confirm a diagnosis, to identify carriers (blood sample or cheek swab) or to test embryos
  • The amniocentesis test involves collecting cells from the placenta and foetus via using a needle to collect amniotic fluid. At 15-17 weeks and 0.5-1% miscarriage risk
  • Chronic Villus Sampling is where a sample of placental tissue is removed through abdomen wall or vagina. At 8-10 weeks and 1-2% miscarriage risk
  • Pre-Implantation Genetic Diagnosis (PGD) is used in IVF to test an embryo before implantation. At 8-16 cell division. Very expensive and fairly unreliable, but will not damage the embryo
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  • Against
  • Testing can provoke parents to abort the foetus, which is murder. Everything has the right to life
  • The results are not totally accurate, a false positive result could mean that a healthy baby is terminated
  • We don't have the right to know
  • There are discrepancies with confidentiality and cost implications
  • Can cause miscarriage and extra stress to baby and parents
  • For
  • The test can allow parents to make an informed decision about their child
  • Parents are able to plan for the future of their child
  • If a condition is found in a foetus, the treatment can start as early as possible
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