13.3.1

Genes

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Genes

DNA consists of millions of genes. Genes are a section of DNA that code for a polypeptide or RNA molecule.

Base sequence

Base sequence

  • DNA is made from four different bases (A, T, C, G).
  • A gene is a sequence of bases in DNA.
  • Each gene has a specific base sequence.
Locus

Locus

  • The specific location of a gene in the DNA is called the locus.
  • The locus is a fixed position.
Diagram
Codons

Codons

  • Three bases in a gene are called a triplet or a codon.
  • A specific codon encodes a specific amino acid.
  • The codons present in a gene and the sequence of the codons are translated into a sequence of amino acids.
  • Together these amino acids form a polypeptide.
Diagram
mRNA

mRNA

  • The sequence of bases in a gene is made into a polypeptide by copying DNA into a molecule called messenger RNA (mRNA).
  • mRNA is then copied into the corresponding polypeptide.
Functional RNA molecules

Functional RNA molecules

  • Some genes encode a functional RNA molecule instead of a polypeptide.
  • Functional RNA molecules are necessary for protein synthesis.
    • E.g. Transfer RNA (tRNA) helps find the correct amino acids for a specific codon.
    • E.g. Ribosomal RNA (rRNA) are a part of ribosomes, the centre of protein synthesis.

Non-Coding Genes

A large proportion of the genes in eukaryotic DNA do NOT code for any polypeptides. These genes have no biological function and are referred to as non-coding genes.

Exons

Exons

  • The coding regions in a gene are called exons.
    • Sequences of coding genes are interrupted at regular intervals by non-coding regions.
Introns

Introns

  • The non-coding regions in a gene are called introns.
    • When a gene is translated for protein synthesis, the introns are removed from the sequence.
Multiple repeats

Multiple repeats

  • There are also regions of non-coding multiple repeats in eukaryotic DNA.
  • Multiple repeats are random sequences in the DNA that will be repeated many times (e.g. CGCCGCCGCCGC).

The Genetic Code

The genetic code is the combination of triplet codes in the genome. There are three important features of the genetic code:

Degenerate

Degenerate

  • The genetic code is degenerate.
  • Some amino acids are encoded by more than one codon (e.g. tyrosine is encoded by TAT and TAC).
    • There are 64 possible triplet codons and only 20 amino acids.
Non-overlapping

Non-overlapping

  • The genetic code is non-overlapping.
  • The non-overlapping nature of DNA means that there is no overlap between triplet codes.
  • Each triplet code is separate from the other triplets in the chromosome.
Universal

Universal

  • The genetic code is universal.
  • The same four bases (A, T, G, C) are used in the DNA of every organism.
  • The same codons encode the same amino acids in every organism.
  • Codons in DNA are transcribed into mRNA and translated into amino acids in every organism.
Jump to other topics
1

Unity & Diversity - Molecules

1.1

Water

1.1.1Water & Hydrogen Bonding1.1.2IB Multiple Choice - Water & Hydrogen Bonding1.1.3Extended Response - Water
1.2

DNA Structure & Replication

1.2.1DNA1.2.2RNA1.2.3DNA Replication1.2.4DNA Replication - Summary1.2.5IB Multiple Choice - DNA Replication
1.3

Transcription & Gene Expression

1.3.1Introduction to Transcription & Protein Synthesis1.3.2Types of RNA1.3.3Transcription1.3.4RNA Processing in Eukaryotes1.3.5IB Multiple Choice - Transcription
1.4

Translation

1.4.1Translation1.4.2Retroviruses1.4.3Scientific Practices: Coronavirus1.4.4IB Multiple Choice - Translation
2

Unity & Diversity - Cells

2.1

The Origin of Cells

2.1.1Origins - Endosymbiosis
2.2

Introduction to Cells

2.2.1Introduction to Cells2.2.2Cell Size & Surface Area2.2.3Surface Area to Volume Ratio2.2.4Eukaryotic vs Prokaryotic Cells2.2.5Stem Cells2.2.6Stem Cells in Disease
2.3

Ultrastructure of Cells

2.3.1Golgi, Lysosomes & Ribosomes2.3.2Cell Walls & Vacuole2.3.3Prokaryotes2.3.4Binary Fission
2.4

Cell Division

2.4.1The Cell Cycle2.4.2Mitosis2.4.3Regulation of the Cell Cycle2.4.4End of Topic Quiz - The Cell Cycle2.4.5IB Multiple Choice - The Cell Cycle2.4.6Extended Response - Mitosis
2.5

Structure of DNA & RNA

2.5.1DNA & RNA
2.6

DNA Replication, Transcription & Translation

2.6.1Protein Synthesis2.6.2Transcription & Translation2.6.3End of Topic Test - DNA, Genes & Protein Synthesis2.6.4Exam-Style Question - Protein Synthesis2.6.5End of Topic Test - Coronavirus Translation2.6.6IB Multiple Choice - Transcription2.6.7IB Multiple Choice - Translation
2.7

Cell Respiration

2.7.1Overview of Respiration2.7.2Anaerobic Respiration2.7.3End of Topic Test - Anaerobic Respiration2.7.4Respiration Experiments2.7.5End of Topic Test - Respiration2.7.6IB Multiple Choice - Respiration
2.8

Photosynthesis

2.8.1Overview of Photosynthesis
2.9

Viruses

2.9.1Viruses
3

Unity & Diversity - Organisms

3.1

Diversity of Organisms

3.1.1Chromosomal Theory of Inheritance3.1.2Chromosomal Basis of Inherited Disorders3.1.3IB Multiple Choice - Chromosomal Inheritance
3.2

Evidence for Evolution

3.2.1Evidence for Evolution - Fossils & DNA3.2.2Evidence for Evolution - Anatomy & Geography3.2.3IB Multiple Choice - Evidence for Evolution3.2.4Extended Response - DNA & Evolution
3.3

Natural Selection

3.3.1Introduction3.3.2Evolutionary Fitness3.3.3Natural Selection & Variation3.3.4End of Topic Quiz - Natural Selection3.3.5IB Multiple Choice - Natural Selection3.3.6Speciation
4

Unity & Diversity - Ecosystems

4.1

Classification

4.1.1Introduction to Biodiversity4.1.2Keystone Species4.1.3Courtship & Ancestry4.1.4Classification
4.2

Cladistics

4.2.1Evolutionary Relationships4.2.2IB Multiple Choice - Species & Taxonomy
4.3

Evolution & Speciation

4.3.1Evidence for Evolution - Fossils & DNA4.3.2Evidence for Evolution - Anatomy & Geography4.3.3IB Multiple Choice - Evidence for Evolution4.3.4Extended Response - DNA & Evolution4.3.5Populations4.3.6Mutations, Genetic Drift, & Gene Flow4.3.7Speciation4.3.8Rate of Speciation4.3.9Allopatric & Sympatric Speciation
4.4

Conservation of Biodiversity

4.4.1Coming Soon!
5

Form & Function - Molecules

5.1

Carbohydrates & Lipids

5.1.1Monosaccharides5.1.2Disaccharides & Polysaccharides5.1.3Triglycerides & Phospholipids5.1.4Cellulose5.1.5Starch & Glycogen
5.2

Proteins

5.2.1Structure & Functions of Proteins
6

Form & Function - Cells

6.1

Membranes & Membrane Transport

6.1.1Passive Movement Across Membranes6.1.2Active Movement Across Membranes6.1.3Endocytosis & Exocystosis6.1.4Cell Membrane Structure & Permeability
6.2

Organelles & Compartmentalization

6.2.1Eukaryotic Cells & Organelles6.2.2Subcellular Components & Organelles
6.3

Cell Specialization

6.3.1Cell Specialization
7

Form & Function - Organisms

7.1

Gas Exchange

7.1.1Lung Structure & Alveoli7.1.2Ventilation7.1.3Lung Disease7.1.4Lung Disease Data
7.2

Transport

7.2.1The Circulatory System7.2.2The Heart7.2.3Blood Vessels7.2.4Cardiovascular Disease7.2.5Xylem Structure7.2.6Cohesion-Tension Theory7.2.7Investigating Transpiration Rate7.2.8Phloem Structure7.2.9Translocation
7.3

Muscle & Motility

7.3.1Skeletal Muscle7.3.2Sliding Filament Theory7.3.3Contraction
8

Form & Function - Ecosystems

8.1

Species, Communities & Ecosytems

8.1.1Population Dynamics8.1.2Communities & Measuring Diversity8.1.3Relationships Between Populations8.1.4Overview of Ecosystems8.1.5Nutrient Cycles8.1.6Fertilisers8.1.7Eutrophication8.1.8Chi-Squared Test
8.2

Energy Flow

8.2.1Energy8.2.2Energy Flow8.2.3Biomass & Food Chains8.2.4Heterotrophs & Autotrophs
8.3

Carbon Cycle

8.3.1Carbon Cycle
8.4

Climate Change

8.4.1Carbon Dioxide8.4.2The Greenhouse Effect8.4.3Global Warming8.4.4Maintaining Biodiversity
9

Interaction & Interdependence - Molecules

9.1

Enzymes

9.1.1Enzymes9.1.2Factors Affecting Enzyme Activity9.1.3Enzyme-Controlled Reactions9.1.4End of Topic Test - Enzymes
9.2

Metabolism

9.2.1Molecules to Metabolism
9.3

Cell Respiration

9.3.1Glycolysis9.3.2Aerobic Respiration
9.4

Photosynthesis

9.4.1Light-Dependent Reaction9.4.2Light-Independent Reaction9.4.3End of Topic Test - Photosynthesis Reactions9.4.4Limiting Factors9.4.5Photosynthesis Experiments9.4.6End of Topic Test - Photosynthesis9.4.7End of Topic Test - Photosynthesis
10

Interaction & Interdependence - Cells

10.1

Chemical Signalling

10.1.1Coming Soon!
10.2

Neural Signalling

10.2.1Medical Application10.2.2Speed of Transmission10.2.3Synapses10.2.4End of Topic Test - Nervous Comm&Coord10.2.5Neuron Structure10.2.6Types of Synapse10.2.7End of Topic Test - Neurones & Action Potentials10.2.8Transmission of an Impulse10.2.9End of Topic Test - Synapses
10.3

Adaptation to Environment

10.3.1Coming Soon!
10.4

Ecological Niches

10.4.1Coming Soon!
11

Interaction & Interdependence - Organisms

11.1

Integration of Body Systems

11.1.1Plant Tropisms
11.2

Defence Against Disease

11.2.1Immune System11.2.2The Immune Response11.2.3Antibodies11.2.4Primary & Secondary Response11.2.5Vaccines11.2.6HIV11.2.7End of Topic Test - Immune System
12

Interaction & Interdependence - Ecosystems

12.1

Populations & Communities

12.1.1Coming Soon!
12.2

Transfers of Energy & Matter

12.2.1Coming Soon!
13

Continuity & Change - Molecules

13.1

DNA Replication

13.1.1DNA Replication
13.2

Protein Synthesis

13.2.1Protein Synthesis13.2.2Transcription & Translation13.2.3End of Topic Test - DNA, Genes & Protein Synthesis
13.3

Mutation & Gene Editing

13.3.1Genes13.3.2Mutations13.3.3DNA Technology13.3.4PCR & Gel Electrophoresis13.3.5Genetic Modification13.3.6Cloning
14

Continuity & Change - Cells

14.1

Gene Expression

14.1.1Introduction to Transcription & Protein Synthesis14.1.2Types of RNA14.1.3Transcription14.1.4RNA Processing in Eukaryotes14.1.5IB Multiple Choice - Transcription
14.2

Water Potential

14.2.1Controlling Blood Water Potential
15

Continuity & Change - Organisms

15.1

Inheritance

15.1.1Non-Nuclear Inheritance15.1.2Linked Genes15.1.3IB Multiple Choice - Non-Mendelian Genetics15.1.4Extended Response - Inheritance15.1.5Introduction to Non-Mendelian Inheritance15.1.6Chi-Squared Test15.1.7End of Topic Quiz - Inheritance15.1.8Sex-Linked Genes15.1.9Grade 4-5 (Scientific Practices) - Inheritance
15.2

Homeostasis

15.2.1Overview of Homeostasis15.2.2Blood Glucose Concentration15.2.3Controlling Blood Glucose Concentration15.2.4End of Topic Test - Blood Glucose15.2.5Type I & Type II Diabetes15.2.6Human Reproductive Systems15.2.7Hormonal Control of the Menstrual Cycle15.2.8Kidneys & Osmoregulation
16

Continuity & Change - Ecosystems

16.1

Natural Selection

16.1.1Natural Selection
16.2

Stability & Change

16.2.1Coming Soon!
16.3

Climate Change

16.3.1Carbon Dioxide16.3.2The Greenhouse Effect16.3.3Global Warming16.3.4Maintaining Biodiversity

Practice questions on Genes

Can you answer these? Test yourself with free interactive practice on Seneca — used by over 10 million students.

  1. 1
    Which RNA molecule copies the DNA sequence into a polypeptide?Multiple choice
  2. 2
    Which of the following statements about genes are true?True / false
  3. 3
    Which of the following statements about non-coding genes are true?True / false
  4. 4
    What is the name given to coding genes?Multiple choice
  5. 5
    Which of the following statements about the genetic code are true?True / false
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End of Topic Test - DNA, Genes & Protein Synthesis

Mutations