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3.1.2Genetics

DNA and the Genome

5 min read 9 parts Lesson 3 of 9 2 quizzes

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Section 1 of 9From nucleus to gene

Nucleus, Chromosome and DNA
Nucleus Chromosome DNA Organization

Most cells contain a nucleus in which the genetic material is found as chromosomes.

DNA
Each chromosome is a long molecule of DNA (deoxyribonucleic acid) coiled up.
Gene
A gene is a short section of DNA that codes for a particular protein.
The different proteins made control the characteristics of an organism, including enzymes that determine how the body works.
Each human body cell has 23 pairs of chromosomes, with one chromosome of each pair from each parent.

The order is

  • genome > chromosome > gene > DNA bases.
  • Each chromosome contains many genes.

Section 2 of 9The genome

The genome of an organism is the entire genetic material of that organism.

Human Genome Project
The human genome has been studied extensively and was sequenced in the Human Genome Project, completed in 2003,
  • which worked out the order of the DNA bases of all human chromosomes.
The project showed that humans have around 20 000 genes.
Much of the DNA does not code for proteins, but some of it controls when genes are switched on or off.

Section 3 of 9How genes code for proteins

Amino Acids to Proteins — Step-by-Step Simulation
Amino Acids to Protein
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From Gene to Protein — Step-by-Step Mechanism Simulation

DNA contains four bases, abbreviated A, C, G and T.

Amino acid
Each set of three bases in a row, a triplet, codes for one particular amino acid.
The order of the triplets determines the order of the amino acids in the protein,
  • and this in turn determines how the protein folds into its shape and what it does.
So a change in the order of the bases may change the protein formed.
Proteins include enzymes, hormones such as insulin, and structural proteins such as collagen.

Protein synthesis takes place on ribosomes in the cytoplasm. Because DNA cannot leave the nucleus, a copy of the gene is taken to the ribosome in the form of a molecule of messenger RNA.

Section 4 of 9Mutations

Gene Duplication Mutation - Step-by-Step Simulation
Ionising Radiation DNA Mutation — Semantic SVG Redraw
Substitution mutation changes one codon/amino acid

A mutation is a change in the sequence of DNA bases.

1
Most mutations have no effect on the protein, because the genetic code has some tolerance, or only a small effect.
2
Some mutations change the amino acid sequence,
  • so the protein folds differently and may not work, for instance when an enzyme's active site no longer fits its substrate.
3
In rare cases a mutation may lead to a new feature that benefits the organism, which is why mutations are a source of variation.
4
Mutations in non-coding DNA can alter the expression of genes.
5
Mutations occur spontaneously, and their rate is increased by ionising radiation such as UV light and by some chemicals.

Section 5 of 9Importance of the genome

Knowing the human genome has several uses.

1Searching for genes linked to inherited disorders
Searching for genes linked to inherited disorders and understanding how mutations cause disease.
2
Understanding and treating inherited disorders, and predicting a person's risk of certain diseases.
3
Tracing human migration patterns by comparing the genomes of populations.
4Personalised medicine
Developing personalised medicine, where drugs are chosen according to the patient's genes.

It raises ethical questions, such as who should have access to a person's genetic information, whether insurers could use it, and how it should be protected. These issues should be weighed in discussion.

The topic links to DNA structure (6.1.5), inheritance (6.1.6) and inherited disorders (6.1.7).

Section 6 of 9Gene expression and protein shape

A Gene is a Sequence of Bases
pH & enzyme shape — Interactive
Genetic Code Chart
generalized amino acid structure
Eukaryotic and Prokaryotic Gene Expression

Every cell in the body of an organism has the same genome, yet a muscle cell and a nerve cell look and behave very differently.

Gene expression and protein shape
This is because only certain genes are switched on in each type of cell.
The proteins that are made in a cell determine its structure and function: a red blood cell makes large amounts of haemoglobin,
  • while a pancreas cell makes insulin.
Switching genes on and off is controlled by regions of non-coding DNA and by proteins that bind to them.

The shape of a protein is critical.

Gene expression and protein shape
The chain of amino acids folds because of attractions between the different amino acids, so a different sequence gives a different shape.
An enzyme has an active site that matches only its substrate; a protein with the wrong shape cannot do the job.
This is the link between a mutation in DNA and a genetic disorder: the base change alters the amino acid sequence, the folded protein has a different shape,
  • and a function is lost.

Calculations may ask how many bases code for a protein with a given number of amino acids: a protein of 100 amino acids needs at least 300 bases, since each amino acid is coded by three.

Section 7 of 9Exam-style questions

DNA, chromosome, gene and cell relationship

  1. Describe the relationship between a gene, a chromosome and DNA. [3]3 marks

  2. Explain how a change in a gene can lead to a non-functioning protein. [3]3 marks

  3. Give two reasons why it is useful to know the human genome. [2]2 marks

Section 8 of 9Key terms

TermMeaning
GenomeThe entire genetic material of an organism.
GeneA section of DNA that codes for a protein.
ChromosomeA coiled molecule of DNA containing many genes.
BaseOne of A, C, G or T in DNA.
MutationA change in the DNA sequence.
Amino acidUnit that makes up proteins.
TripletA group of three bases coding for one amino acid.

Section 9 of 9Exam technique

1
Saying a gene is a whole chromosome.
2
Saying a gene codes for a characteristic directly. It codes for a protein which affects the characteristic.
3
Describing the genome as the genes of one chromosome.
4
Believing all mutations are harmful.
5
Mixing up amino acids and bases.
البيولوجيا الجزيئية — أسرار تخليق البروتين مقارنة اتجاهات الإنزيمات

قواعد اتجاهات البلمرة ومضاد الكودون

  • إنزيم بلمرة RNA: يبني دائماً في الاتجاه 5′ → 3′، ويقرأ شريط القالب فقط في الاتجاه 3′ → 5′.
  • كودون البدء: AUG يقابله مضاد كودون UAC ويشفر للميثيونين أول حمض أميني.
  • كودونات الوقف: UAA, UAG, UGA ليس لها tRNA يقابلها، بل يرتبط بها بروتين عامل الإطلاق (Release Factor) لإنهاء الترجمة.