The Genetic Code

Concept 1: Cells, Chromosomes & DNA

Success Criteria

  • Explain the relationship between cells, nuclei, chromosomes, and DNA.

  • Describe chromosomes in terms of their number, location, type, and structure.

Vocabulary

Energy

Joule

Transformation

Genetic Material in Cells

Success Criteria:
I can explain the relationship between cells, nuclei, chromosomes, and DNA.

Cells are the 'building blocks' of organisms. Each cell has a 'control centre' called the nucleus. Each cell nucleus contains a fixed number of structures called chromosomes (human body cells have 46 chromosomes).

A chromosome is a rod-like structure formed from a tightly wound DNA molecule. Each chromosome carries hundreds or thousands of genes, each found at a fixed location along the chromosome.

A gene is a specific section of a chromosome (a specific section of the DNA molecule). Each of these DNA sections (genes) carries an instruction that informs the cell how to construct a specific protein.

DNA is an extremely long molecule bearing genetic (inherited) instructions for the organism. The cell's total DNA (genome)contains all the instructions needed to grow, function, and reproduce.

Chromosomes

Success Criteria:
I can describe chromosomes in terms of their number, location, type, and structure.

Each chromosome consists of a DNA molecule that is coiled up tightly around special packaging proteins. Each species has a fixed number of chromosomes in the body cells of all individuals.

The body cells of an organism have an even number of chromosomes, which can be sorted into similar-shaped (homologous) pairs. One member of each pair is inherited from each parent at fertilisation.

In many species, the sex of an individual is determined by a single pair of chromosomes. For humans, it is the 23rd pair (X and Y).

The karyotype diagrams show the complete chromosome set in a body cell.

  • For example, in the picture below, 46 chromosomes in human body cells are sorted into 23 homologous pairs. 23 chromosomes inherited from the father, 23 inherited from the mother.

  • For example, 14 chromosomes in pea plant body cells sorted into 7 homologous pairs. 7 chromosomes inherited from parent 1, 7 chromosomes inherited from parent 2.

Concept 2: Structure of DNA

Success Criteria

  • Draw and describe a labelled diagram of a DNA molecule.

  • Use the complementary base-pairing rule to help explain the process of DNA replication.

Vocabulary

Energy

Joule

Transformation

DNA

Success Criteria:
Draw and describe a labelled diagram of a DNA molecule.
Use the complementary base-pairing rule to help explain the process of DNA replication.

DNA (deoxyribose nucleic acid) is a very long molecule that carries the genetic instructions an organism needs to grow, function, and reproduce.

A DNA molecule is made up of two linked strands, twisted into a double helix (like a twisted ladder).

Each side of the ladder is a chain of alternating sugar (deoxyribose) and phosphate units. Each sugar unit has one of four bases (A, T, G, or C) attached to it.

The two strands are linked by attraction of particular base pairs. This is called complementary base pairing.

Concept 3: Traits

Success Criteria

  • I can describe the different sections of the electromagnetic spectrum and their energy.

  • I can identify examples of light sources and light reflectors.

  • I can explain that light energy can be reflected, transmitted, or absorbed by objects.

  • I can compare transparent, translucent, and opaque objects.

  • I can explain how shadows are formed.

Vocabulary

Energy

Joule

Transformation

Genes, Alleles & Proteins

Success Criteria:
Describe the relationship between DNA, genes, alleles, and proteins.
Describe the difference between dominant and recessive alleles.

The sequence of bases along a DNA molecule encodes the genetic instructions (genes) for the organism. A gene is a specific section of this base sequence (often thousands of bases long). Each gene carries the instruction for making a specific protein, which in turn determines a particular feature (trait) of the organism.

Each individual inherits two copies of the gene - on from mum, and one from dad.

But the actual sequence of bases making up a particular gene can vary. These variants of the gene are called alleles. They are responsible for the different proteins, and so cause variations in a particular trait ('inherited' variation) between individuals

For example, a gene for pea shape in a plant species has these 2 alleles on a particular homologous chromosome pair:

  • 'round' allele codes for round peas.

  • 'wrinkled' allele codes for wrinkled peas.

For example, a gene for coat colour in rabbits has these 2 alleles:

  • 'brown' allele codes for brown coat colour on a particular homologous chromosome pair..

  • 'white' allele codes for white coat colour.

A dominant allele (given a capital letter) is always expressed even if only one copy is present. A recessive allele (given a lower case letter) is expressed only if the dominant allele is not present).

The organism's genotype determines the phenotype. This means that the alleles present (genotype) code for specific proteins, which then determine the observable trait (phenotype). Both recessive alleles must be present for that trait to be expressed.

E.g. the plant with the alleles above will have smooth peas as one dominant S allele is present.

E.g. Possible genotypes (and the resulting phenotypes) for the two pea shape alleles shown above.

The rabbit with the alleles above will have a brown coat as the dominant B allele is present.

E.g. Possible genotypes (and the resulting phenotypes) for the two coat colour alleles shown above