Biology Unit 2

DNA replication

Genetic screening, therapy

Phospholipid bilayer

Cystic fibrosis

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How many layers is the cell membrane composed of? What are the membrane's three main components?
2 - Phospholipid bilayer. Lipid (phospholipid), proteins and carbohydrates.
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Why is the cell membrane 'fluid'?
The phospholipid molecules are moving continuously.
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What is the difference between a glycoprotein and a glycolipid? What are the functions of glycoproteins, glycolipids, partially integral proteins and fully integral proteins?
Glycoprotein - polysaccharide and protein, glycolipid - polysaccharide and lipid. These structures are used for cell recognition and signalling.
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What is the function of the cytoskeleton?
It supports the phospholipid bilayer.
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What is the effect of temperature on the phospholipid bilayer?
As the temperature increases above 45 degrees Celcius the bilayer melts increasing the permeability causing the cell membrane to expand increasing the pressure on the membrane. The proteins then denature due to the increased permeability.
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What are the main methods of transport across the cell membrane? Explain these processes.
Diffusion - particles move down a concentration gradient. Osmosis - the movement of water molecules down a concentration gradient. Active transport - movement of larger molecules against a concentration gradient.
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How do you increase the rate of diffusion?
Increase the surface area, short diffusion path, maintain a steep concentration gradient.
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How is the rate of gas exchange in the lungs increased?
Many alveoli - large surface area. Alveolar epithelium, capillary endothelium are one cell thick - short diffusion pathway. Capillaries provide a good blood supply to the alveoli which maintains the concentration gradient.
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Describe facilitated diffusion.
Larger molecules such as glucose and amino acids are moved into/out of cell by carrier proteins. Molecules attach to carrier proteins causing it to change shape and then the molecule is released.
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What is endocytosis?
Some molecules (lipis, proteins) are too large to be transported by carrier/channel proteins. The cell surrounds the substance with a section of the cell membrane forming a vesicle. The molecule is released outside the cell.
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What is exocytosis?
Substances produced by cells (hormones, digestive enzymes) are released from the cell. Vesicles containing the substance pinch off and fuse with the cell membrane.
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What is a dipeptide bond?
A dipeptide is formed by the joining of two amino acids.
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What groups make up an amino acid?
Carboxyl group, amine group and R groups
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How are amino acids joined?
They are formed via condensation reactions where a peptide bond forms and a water molecule is released. Can be reversed by a hydrolysis reaction where a water molecule is added.
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What is the primary structure of a protein?
The sequence of amino acids in a polypeptide chain formed by peptide bonds.
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What is the secondary structure of a protein?
Hydrogen bonds form causing the chain to coil into an alpha helix or fold into a beta pleated sheet.
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What is the tertiary structure of a protein?
The chain is coiled/folded further into a three dimensional shape.
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What is the quarternery structure of a protein?
The joining together of several protein/peptide chains into a final specific shape.
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What bonds are holding protein structure together?
Hydrogen bonding, ionic interactions, disulfide bonding, hydrophilic/hydrophobic bonding.
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What are the properties of a globular protein?
They are: round/compact, contain multiple polypeptide chains, have the hydrophilic part facing outwards and the hydrophobic facing the inside, soluble. (Haemoglobin)
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What are the properties of a fibrous protein?
Long chain, insoluble, tightly coiled, strong. Used for supportive tissue - collagen
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What are enzymes?
Proteins used as biological catalysts. Catalyse metabolic reactions - digestion, respiration. Intracellular/extracellular. Globular. Active site with specific shape.
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Why do enzyme-substrate complexes help speed up a reaction?
When two substrate molecules are join the enzyme keeps them close - reducing repulsion - bond easier. Fitting into active site it puts strain on the substrate's bonds - breaking them easier.
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What effect does increasing the concentration of either the substrate or the enzyme have on the rate of reaction?
Increased chance of a collision to form an enzyme-substrate complex, therefore increasing a reaction.
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What are nucleotides?
They are the building blocks of DNA and RNA. Made of a sugar, one nitrogenous base and one or more phosphate groups.
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What are the complementary base pairings?
Adenine - Thymine. Guanine - Cytosene. Uracil replaces thymine in RNA.
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How do two complementary DNA strands join?
Hydrogen bonding. The sugar of one mononucleotide joins to a phosphate group of another via a condensation reaction.
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What is semi-conservative replication? Outline the process.
DNA helix unzips into two single strands which act as template strands. Mononucleotides on new strand join to each template - complementary base pairing. Joined using DNA polymerase - hydrogen bonds form.
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How did Meselson and Stahl prove that DNA replication was semi-conservative?
Two bacteria samples were grown in different nutrient broths - one in heavy nitrogen (N15) and the other in light nitrogen (14). DNA sample from each sample spun in centrifuge. Light DNA on top of centrifuge, heavy on bottom.
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What differences do different proteins have?
Genetic codes, amino acid sequence, codon sequence.
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What organelles produce new proteins?
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Describe transcription.
RNA copy made in cell nucleus. Hydrogen bonds between two DNA strands break - DNA uncoils. One of the template strands form and RNA copy by complementary base pairing - mRNA. mRNA moves out of nucleus through a nuclear pore to a ribosome.
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Describe translation.
mRNA attaches to a ribosome and a tRNA molecule carries amino acids to ribosome. tRNA attaches to mRNA via complementary base pairing and this process goes on with other tRNA molecules on each codon. Forms a peptide bond. tRNA moves away. Stop codon.
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What structure does a tRNA molecule have?
It is folded with a single polynucleotide strand. Has a binding site where a specific amino acid attaches.
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What is a mutation?
A change to the base sequence of DNA and the 3D structure that makes up a gene.
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What is a substitution mutation?
A type of mutation where on base is changed for another.
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What is a deletion mutation?
One base is deleted.
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What in an insertion mutation?
An extra base is added and causes a mutation.
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What is a duplication mutation?
One or more bases are repeated.
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What is an inversion mutation?
A sequence of bases are repeated.
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What can mutations in a gene cause? Give an example.
Genetic disease eg cystic fibrosis (CF)
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What are the genotypes?
The alleles that a person has.
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What are the phenotypes?
The characteristics of the alleles produced.
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Which protein has been affected by a mutation in CF and what does it cause?
CFTR - Cystic Fibrosis Transmembrane Regulator. Less transport of chloride ions therefore osmosis is reduced causing the mucus to be thick and sticky.
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How does thick mucus affect the respiratory system?
The cilia are unable to move mucus towards the throat which blocks airways. Lack of gas exchange - reduced surface area - more prone to lung infections.
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How does thick mucus affect the digestive system?
Tube connecting pancreas to small intestine blocked. Prevents digestive enzymes from entering small intestine. Cysts form in pancreas. Inhibits enzyme production. Mucus lines small intestine - prevents food digestion and nutrient absorption.
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How does thick mucus affect the reproductive system?
In men tubes connecting the testicles to the penis are blocked. In women there is thickened cervical mucus - prevents sperm reaching egg.
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What is genetic screening used for?
For carrier testing. Couples tested before conceiving to determine chances of offspring having a genetic disorder. Allows informed decisions to be made.
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What issues are there regarding genetic screening?
Emotional stress, aren't always 100% accurate, other genetic abnormalities may be found.
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What is Pre-implantation Genetic Diagnosis?
Form embryos using IVF which are screened before implantation and avoids issue of abortion.
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What are the issues regarding Pre-implantation Genetic Diagnosis?
Can be used to find out other characteristics, could provide false results.
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Give and describe to methods of pre-natal testing.
Amniocentesis - 15-16 weeks of pregnancy a sample of amniotic fluid containing cell is analysed. Choronic Villus Sampling - 8-12 weeks of pregnancy where a sample of cells are taken from the choronic villus and analysed.
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What issues are there regarding pre-natal testing?
Increased risk of miscarriage - 1%, could be false results/incorrect information, unethical to abort.
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What is genetic therapy?
Altering alleles inside cells to cure genetic disorders.
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If the disease is caused by two recessive alleles what can be done to prevent these being shown in the phenotype?
Add a working dominant allele.
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How do you prevent a dominant allele from being shown in the phenotype?
Silence the dominant allele.
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What is somatic therapy?
Changing most affected alleles in cells.
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What is germ line therapy?
Changing alleles in sex cells/gametes.
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Why is the cell membrane 'fluid'?


The phospholipid molecules are moving continuously.

Card 3


What is the difference between a glycoprotein and a glycolipid? What are the functions of glycoproteins, glycolipids, partially integral proteins and fully integral proteins?


Preview of the front of card 3

Card 4


What is the function of the cytoskeleton?


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Card 5


What is the effect of temperature on the phospholipid bilayer?


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