Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

2.3Topic 2 · Genes and Health

Proteins — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

20 min read 30 parts Lesson 15 of 35 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read
Lesson summary

Transcription, Translation and the Genetic Code — EDEXCEL A LEVEL BIOLOGY A (SNAB) 9BI0 — REVISION NOTES notes: Describe how genes are transcribed into mRNA and translated into polypeptides, and the properties of the genetic code.A gene is a sequence of nucleotide bases in a DNA …

  • Triplet code
  • Degenerate
  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Universal code

Section 1 of 30Genes and protein synthesis

A gene is a sequence of nucleotide bases in a DNA molecule that codes for a specific sequence of amino acids, which makes a specific polypeptide (protein).

Transcription
Protein synthesis has two stages: transcription (DNA is copied into an mRNA molecule in the nucleus) and translation (mRNA is used to build a polypeptide at a ribosome in the cytoplasm).
Do not forget that transcription occurs in the nucleus but translation occurs in the cytoplasm.

Section 2 of 30Transcription

The enzyme RNA polymerase attaches to the DNA, and the hydrogen bonds between the two strands break locally so the bases are exposed.

Free RNA nucleotides pair with the exposed bases of one strand only, by complementary base pairing (A with U, T with A, C with G, G with C).
RNA polymerase joins the RNA nucleotides by phosphodiester bonds to build the mRNA strand.
Antisense
The strand that is transcribed is the antisense (template) strand; the sense (non-transcribed) strand has the same base sequence as the mRNA, with U for T.
DNA polymerase is the enzyme for replication; RNA polymerase is for transcription - do not confuse them.
The finished mRNA leaves the nucleus through a nuclear pore.

Section 3 of 30Translation

In the cytoplasm the mRNA attaches to a ribosome.

TRNA
Free tRNA (transfer RNA) molecules each carry a specific amino acid and have an anticodon that is complementary to an mRNA codon.
A tRNA whose anticodon matches the first codon binds, then a second tRNA binds the next codon.
The ribosome catalyses a peptide bond between the two amino acids, the first tRNA is released, the ribosome moves along the mRNA by one codon and the process repeats until a stop codon is reached and the polypeptide is released.

Section 4 of 30Nature of the genetic code

Triplet code
Triplet code each sequence of three bases (a triplet in DNA, a codon in mRNA) codes for one amino acid.
Degenerate
Degenerate there are 20 different amino acids but 64 possible triplets, so most amino acids are coded for by more than one triplet.
Universal
Universal almost every organism uses the same code (a few rare minor exceptions exist),
  • so the same triplets code for the same amino acids in all living things;
    • this means genetic information can be transferred between species.
Some triplets act as start or stop signals.

Section 5 of 30Worked example: from gene to polypeptide

Suppose the antisense (template) DNA strand reads 3' TAC GGA CTT 5'.

1
The mRNA is built complementary to it: 5' AUG CCU GAA 3'.
2
The first codon AUG is the start codon.
3
During translation, tRNA molecules with anticodons UAC, GGA and CUU bring the amino acids that these codons specify,
  • and peptide bonds form between them at the ribosome.
4
The sense strand of the DNA would read 5' ATG CCT GAA 3', the same as the mRNA but with T instead of U.

Remember that the amino acid sequence produced is the primary structure of the protein. Because the code is degenerate, some changes in the DNA (for example, a change in the third base of a triplet) may not change the amino acid, which is why not every mutation alters a protein.

Section 6 of 30Worked examples

  1. The template DNA strand reads TAC GGT CAA. Give the mRNA sequence and name the process. [2]2 marks

  2. Explain what is meant by a degenerate code. [2]2 marks

  3. State where transcription and translation occur. [2]2 marks

Section 7 of 30Genes control protein structure

Protein molecules are chains of amino acids.

Genes control protein structure
The type, number and sequence of the amino acids determine the protein's shape and therefore its function.
The genes in DNA control this because a gene is a sequence of nucleotides that codes for a specific polypeptide,
  • so it fixes the exact order in which amino acids join during protein synthesis.
Primary structure
This order is the primary structure.
Proteins form many cell components, such as enzymes, membrane carriers, antibodies, hormones and structural fibres.

Section 8 of 30Amino acids and peptide bonds

Every amino acid has a central carbon bonded to an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom and a variable R group.

Condensation reaction
Two amino acids join by a condensation reaction that releases a molecule of water and forms a peptide bond; the bond can be broken by hydrolysis, adding water.
Two joined amino acids form a dipeptide and many form a polypeptide.
To find a peptide bond on a diagram, look for a nitrogen bonded to a carbon that has a double bond to oxygen (-CO-NH-); the R group is not involved.
You do not need to learn the structures of specific amino acids.

Section 9 of 30Levels of protein structure

Three levels relate to a single polypeptide chain; the fourth relates to a protein with two or more chains.

Primary
Primary the sequence of amino acids joined by peptide bonds.
Secondary
Secondary the chain folds into an alpha helix or beta-pleated sheet because weakly negative nitrogen and oxygen atoms form hydrogen bonds with weakly positive hydrogen atoms in the amino and carboxyl groups of the backbone.
  • Many fibrous proteins, e.g. collagen and keratin, are mostly secondary structure.
  • These hydrogen bonds are broken by high temperatures and pH changes.
Tertiary
Tertiary the whole chain folds into a specific 3D shape held by interactions between R groups: hydrogen bonds, ionic bonds, disulphide bonds and hydrophobic interactions.
Quaternary
Quaternary found in proteins made of more than one polypeptide chain working as one macromolecule, e.g. haemoglobin,
  • which has four subunits that carry oxygen.
  • The same types of bond as in tertiary structure hold it together.

Section 10 of 30Why sequence matters: a worked link

The sequence of amino acids dictates where the bonds between R groups can form.

1
For example, two cysteine residues that come close during folding can form a disulphide bond; a polar R group will lie on the outside in water while a non-polar R group faces the inside.
2
A single change of amino acid can therefore change the tertiary structure,
  • which is why a single base substitution in a gene can produce a protein with a different shape and a changed function, as in sickle cell haemoglobin.

Do not confuse bond strengths.

1
Peptide and disulphide bonds are strong covalent bonds.
2
Hydrogen bonds and ionic bonds are individually weak,
  • but there are very many of them so together they hold the protein in shape;
    • hydrophobic interactions are weak attractions between non-polar R groups.
3
Heat, extremes of pH and some chemicals disrupt the weaker bonds and denature the protein, leaving the primary structure intact.

Section 11 of 30Worked examples

  1. Describe how a peptide bond forms between two amino acids. [3]3 marks

  2. Name the bonds that maintain tertiary structure. [2]2 marks

  3. Explain how a change in the DNA base sequence could change a protein's function. [3]3 marks

Section 12 of 30Globular proteins

Globular proteins are roughly spherical. They fold into this shape in their tertiary structure with hydrophobic R groups pointing to the inside and hydrophilic R groups on the outside, which makes them soluble in water and suitable for roles such as enzymes, hormones and transport.

Section 13 of 30Haemoglobin

Haemoglobin is a globular protein with quaternary structure: four polypeptide subunits (two alpha-globin and two beta-globin) each with a prosthetic haem group.

Oxyhaemoglobin
The haem group contains an iron(II) ion (Fe2+) that reversibly combines with an oxygen molecule to form oxyhaemoglobin, which is bright red.
None of the amino acids is well suited to binding oxygen, so the iron is essential.
Each haemoglobin can carry four oxygen molecules (eight oxygen atoms).
When the first oxygen binds, the shape of the protein changes so that the remaining subunits have a higher affinity for oxygen and bind more easily.

A change in the amino acid sequence of a subunit can change the properties of haemoglobin. In sickle cell anaemia a base substitution causes the non-polar amino acid valine to replace the polar glutamic acid, making haemoglobin less soluble.

Section 14 of 30Fibrous proteins and collagen

Fibrous proteins are long strands of polypeptide chains with cross-linkages formed by hydrogen bonds.

Collagen
They have little or no tertiary structure. Collagen is the most common structural protein in vertebrates and provides support in connective tissue such as tendons, ligaments, skin, cartilage, bone and the walls of arteries.
Three polypeptide chains wind together as a triple helix; the many hydrogen bonds within the helix give great tensile strength so collagen withstands large pulling forces without stretching or breaking.
Collagen molecules form fibrils and then fibres, and the molecule is long, so it is insoluble in water.

Section 15 of 30Comparison

FeatureHaemoglobinCollagen
TypeGlobularFibrous
ShapeCompact, sphericalLong triple helix
SolubilitySolubleInsoluble
Structure levelQuaternary (4 chains)Triple helix of 3 chains
FunctionOxygen transportStructural support, tensile strength

Section 16 of 30Exam application: linking structure to function

When asked to link structure to function, use the pattern 'feature - consequence - function'.

Exam application: linking structure to function
Haemoglobin: quaternary structure with four haem groups means it can bind up to four oxygen molecules; hydrophilic R groups on the outside make it soluble in the cytoplasm of red blood cells; the shape change on binding makes loading in the lungs easier.
Collagen: three chains twisted together with hydrogen bonds between them gives strength; staggered, cross-linked molecules in fibres reduce weak points; a long molecule is insoluble so it stays in tissues.
Also remember that the haem group is a prosthetic group, a non-protein part that is a permanent component of the protein.

Compare these proteins in a table in the exam, covering shape, solubility, structure level, bonding and role, and quote a specific example of each tissue or location where relevant (haemoglobin in red blood cells; collagen in tendons, bones and artery walls).

Section 17 of 30Worked examples

  1. Explain how the structure of haemoglobin enables it to transport oxygen. [4]4 marks

  2. Explain why collagen is suited to its function in tendons. [3]3 marks

  3. Explain how the sickle cell mutation affects haemoglobin. [2]2 marks

Section 18 of 30Enzymes as biological catalysts

Enzymes are biological because they function in living systems and catalysts because they speed up chemical reactions without being used up or permanently changed. All chemical reactions involve energy changes, and for a reaction to proceed there must be enough activation energy. Enzymes speed up reactions by reducing the activation energy, so the reaction proceeds at body temperature.

Enzymes are proteins, so anything that can denature a protein, such as extremes of temperature or pH, will denature an enzyme and make it non-functional.

Section 19 of 30Active site and specificity

An enzyme has a unique active site where specific substrates bind to form an enzyme-substrate complex.

Active site and specificity
The shape of the active site is determined by the enzyme's complex tertiary structure,
  • which depends on the primary structure; if the order is altered, the resulting three-dimensional shape changes.
    • (the order of amino acids)
If the tertiary structure is changed in any way, the active site changes shape, the substrate no longer fits, no enzyme-substrate complex forms, no product is made and the enzyme cannot function.
This is why enzymes are highly specific.

Section 20 of 30Models of enzyme action

Lock-and-key model
(Emil Fischer, 1890s): the substrate fits the rigid active site exactly, like a key into a lock.
Induced-fit hypothesis
(first proposed in 1959): modified after advances in molecular techniques.
  • The active site is not perfectly complementary at first;
    • as the substrate approaches, the enzyme and substrate interact and the enzyme changes shape.
      • (conformational change)
  • This gives an ideal binding arrangement, strains bonds in the substrate and maximises the enzyme's ability to catalyse the reaction.

Section 21 of 30Factors that affect enzyme activity

Temperatureas temperature rises, molecules have more kinetic energy, so collisions between enzyme and substrate are more frequent and the rate rises until the optimum is reached.

Factors that affect enzyme activity
Above the optimum the hydrogen and ionic bonds that maintain the tertiary structure break, the active site changes shape and the enzyme is denatured; this is irreversible. pH: each enzyme has an optimum pH; extremes alter the charges on R groups, disrupt ionic and hydrogen bonds, change the active site and denature the enzyme.
Low temperature does not denature enzymes; it only reduces the rate because there are fewer successful collisions.

Enzymes may be intracellular (working inside the cell, e.g. catalase) or extracellular (secreted, e.g. digestive amylase).

Factors that affect enzyme activity
Because the induced fit changes the shape of the active site, the enzyme can exert a strain on bonds in the substrate,
  • which is the mechanism for lowering the activation energy.
Remember that the products no longer fit the active site and are released, so the enzyme is free to catalyse another reaction.

Section 22 of 30Worked examples

  1. Explain how an enzyme lowers activation energy. [3]3 marks

  2. Explain why a change in one amino acid may stop an enzyme working. [3]3 marks

  3. Give one difference between the lock-and-key and induced-fit models. [1]1 mark

Section 23 of 30Enzyme concentration

The higher the enzyme concentration in a reaction mixture, the more active sites are available and the greater the likelihood of enzyme-substrate complexes forming, so the rate of reaction increases (provided substrate is not limiting).

Section 24 of 30Substrate concentration and saturation

More substrate molecules mean more collisions between enzyme and substrate, so an active site is more likely to be used.

Substrate concentration and saturation
This is only true up to a certain substrate concentration, the saturation point,
  • when all active sites are occupied;
  • increasing the substrate further does not affect the rate.
Over time, if no new substrate is added, the substrate concentration decreases, so the rate of reaction decreases.

Section 25 of 30Practical: catalase and hydrogen peroxide

Catalase breaks down hydrogen peroxide into water and oxygen.

Practical: catalase and hydrogen peroxide
To investigate enzyme concentration: add a set volume of hydrogen peroxide solution to a boiling tube, add a set volume of buffer solution,
  • then add catalase and measure the volume of oxygen collected (e.g. in a gas syringe or by displacement of water) over time.
    • (for example from a series of dilutions)
The volume of oxygen plateaus after the initial rate because the substrate is used up and converted into product.
Another approach for substrate concentration is to measure how fast a substrate is removed from the reaction.

Always include a control, e.g. distilled water in place of the enzyme or substrate, and keep other variables such as pH (use a buffer) and temperature constant so they do not affect enzyme activity.

Section 26 of 30Calculating initial rate

The initial rate is the rate at the start of the reaction, before substrate falls.

Calculating initial rate
From a graph of volume of product (or substrate) against time: draw a tangent to the curve through the origin,
  • then calculate its gradient.
    • (change in y divided by change in x)
This gradient is the initial rate of reaction, in units such as cm3 s-1.

Section 27 of 30Planning and evaluating the investigation

Independent variableenzyme concentration (dilutions of catalase, e.g. from yeast or liver extract) or substrate concentration (dilutions of hydrogen peroxide).

Planning and evaluating the investigation
Dependent variable: volume of oxygen produced in a fixed time, or the time taken to collect a set volume.
Control variables: temperature (use a water bath), pH (use a buffer), the total volume of the mixture and the volume of enzyme or substrate.
Repeat each concentration at least three times and calculate a mean to improve reliability, and exclude anomalous results.

Sources of error include gas escaping before the bung is put on, the time delay in starting the stopwatch, and reading the syringe or measuring cylinder with limited precision.

Planning and evaluating the investigation
Improvements include using a gas syringe with a smaller scale division, adding the enzyme last through a delivery tube,
  • and increasing the number of concentrations tested.
Always state the units of rate and give an answer to a suitable number of significant figures.

Section 28 of 30Worked examples

  1. A tangent drawn at t = 0 passes through (0, 0) and (20 s, 10 cm3). Calculate the initial rate. [2]2 marks

  2. Explain why the rate levels off when substrate concentration is increased. [3]3 marks

  3. Suggest why a control with distilled water instead of enzyme is used. [2]2 marks

Section 29 of 30Key terms

TermMeaning
GeneSequence of DNA bases that codes for a specific polypeptide.
TranscriptionProduction of mRNA using a DNA template strand.
TranslationAssembly of a polypeptide at a ribosome using the mRNA code.
CodonThree mRNA bases coding for one amino acid.
AnticodonThree bases on tRNA complementary to an mRNA codon.
Degenerate codeSeveral codons can code for the same amino acid.
Universal codeThe same code is used by nearly all organisms.
TermMeaning
Amino acidMonomer of proteins with amino group, carboxyl group and R group.
Peptide bondCovalent bond between amino acids formed in condensation.
Primary structureSequence of amino acids.
Secondary structureAlpha helix or beta-pleated sheet held by hydrogen bonds.
Tertiary structure3D folding of one polypeptide held by R-group interactions.
Quaternary structureArrangement of two or more polypeptides in one protein.
TermMeaning
Globular proteinCompact, soluble protein with a spherical tertiary structure.
Fibrous proteinLong, insoluble structural protein with little tertiary structure.
Prosthetic groupNon-protein part permanently attached to a protein, e.g. haem.
OxyhaemoglobinHaemoglobin combined with oxygen.
Tensile strengthAbility to resist pulling forces.
TermMeaning
CatalystSubstance that speeds up a reaction without being used up.
Activation energyEnergy needed to start a reaction.
Active siteRegion of an enzyme where the substrate binds.
Enzyme-substrate complexTemporary structure formed when substrate binds to the active site.
Induced fitModel in which the active site changes shape as the substrate binds.
TermMeaning
Initial rateRate of reaction at the start, found from the tangent gradient at time zero.
Saturation pointSubstrate concentration at which all active sites are occupied.
ControlA test with the variable removed to show its effect.
Limiting factorThe factor that restricts the rate of reaction.

Section 30 of 30Exam technique

1
Confusing DNA polymerase (replication) and RNA polymerase (transcription).
2
Saying both DNA strands are transcribed — only one (the antisense/template strand) is.
3
Saying uracil pairs with adenine in DNA — it appears only in RNA, so the template A pairs with U in mRNA.
4
Stating translation occurs in the nucleus.
Avoid these mistakes
Saying the R group forms the peptide bond — it does not.
Saying secondary structure involves R groups — it involves hydrogen bonds in the backbone.
Saying all proteins have quaternary structure — only those with more than one polypeptide.
Avoid these mistakes
Saying iron binds oxygen permanently — the binding is reversible.
Saying collagen is soluble — it is insoluble.
Saying haemoglobin carries four oxygen atoms — it carries four molecules (eight atoms).
Avoid these mistakes
Saying enzymes are used up in the reaction — they are unchanged.
Saying high temperature 'kills' enzymes — they are not alive; they denature.
Saying enzymes give energy to reactions — they lower activation energy.
Avoid these mistakes
Forgetting to use the initial rate — later in the reaction the substrate has been used up.
Not controlling pH and temperature.
Saying more enzyme always means faster — substrate may become limiting.

Last updated: