Biology Edexcel Topic 3 (The genome)

3.4 to 3.11B

What is DNA?

A double-stranded polymer of nucleotides, wound to form a double helix

What are the monomers of DNA?

Nucleotides

What are DNA nucleotides made up of?

Common sugar, phosphate group, one of four bases - A, T, C or G

State the full name of the four bases found in nucleotides

Adenine, Thymine, Cytosine, Guanine

Describe how nucleotides interact to form a molecule of DNA

Sugar and phosphate molecules join to form a sugar phosphate backbone in each DNA strand, there is a base connected to each sugar, with complementary base pairs (A pairs with T, C pairs with G) joined by weak hydrogen bonds

Define genome

The entire genetic material of an organism

What is the chromosome?

A long, coiled molecule of DNA that carries genetic information in the form of genes

Define gene

A section of DNA that codes for a specific sequence of amino acids which undergo polymerisation to form a protein

Explain how a gene codes for a protein

A sequence of three bases in a gene forms a triplet, each triplet codes for an amino acid, and the order of amino acids determines the structure (i.e. how it will fold) and function of protein formed

Why is the 'folding' of amino acids important in proteins such as enzymes?

The folding of amino acids determines the shape of the active site which must be highly specific to the shape of its substrate

What is protein synthesis?

The formation of a protein from a gene

What are the two stages of protein synthesis?

Transcription and translation

What does transcription involved?

The formation of mRNA from a DNA template

Outline transcription

1 - DNA double helix unwinds
2 - RNA polymerase binds to a specific base sequence of non-coding DNA in front of a gene and moves along the DNA strand
3 - RNA polymerase joins free RNA nucleotides to complementary bases on the coding DNA strand
4 - mRNA formation is complete, mRNA detaches and leaves the nucleus

Describe the differences between mRNA and DNA

mRNA is single stranded whereas DNA is double stranded, and mRNA uses U whereas DNA uses T

Why is mRNA used in translation rather than DNA?

DNA is too large to leave the nucleus so cannot reach the ribosome

What does translation involve?

A ribosome joins amino acids in a specific order dictated by mRNA to form a protein

Outline translation

1 - mRNA attaches to a ribosome
2 - Ribosome reads the mRNA bases in triplets (codons), each triplet codes for one amino acid which is brought to the ribosome by a tRNA molecule
3 - A polypeptide chain is formed from the sequence of amino acids which join together

How is a tRNA molecule adapted to its function?

Each tRNA molecule has an anticodon which is specific to the codon of the amino acid that it carries

What is a mutation?

A random change in the base sequence of DNA which results in a genetic variant

Describe the effect of a gene mutation in coding DNA

If a mutation changes the coding DNA (amino acid sequence), protein structure and function may change

What is non-coding DNA?

DNA which does not code for a protein but instead controls gene expression

Describe the effect of a gene mutation in non-coding DNA

A mutation may affect how often RNA polymerase to bind to non-coding DNA, which could result in more or less protein to be produced

Outline how the work of Mendel helped scientists to develop their understanding of genetics

Mendel studied the inheritance of different phenotypes of pea plants, he established a correlation between parent and offspring phenotypes, he noted that inheritance was determined by units passed onto the descendants, and using gene crosses, he devised the terms 'dominant' and 'recessive'

Why was Mendel's work initially overlooked?

Scientists didn't understand Mendel's work as there was no knowledge of genes or DNA at the time