DNA Structure, Replication, Mutations and Protein Synthesis: Key Genetics Concepts

Key Genetics Definitions

Definitions:

• Dominant (an allele that hides the effect of a weaker allele)
• Allele (different forms of the same gene)
• Gene (a fragment of DNA that codes for one protein)
• Recessive (an allele whose effect is hidden by a dominant allele)
• Genotype (the types of genes in an individual)

DNA Structure

History

James Watson and Francis Crick discovered the structure of DNA. They proposed it because of the work of others, notably Rosalind Franklin, who had been researching the structure of a number of substances using X-ray diffraction techniques. Watson and Crick proposed the double helix structure for DNA.

Structure

A DNA molecule is made from a polynucleotide molecule that contains two strands of nucleotides. DNA has 4 bases: adenine and guanine (purines), cytosine and thymine (pyrimidines).

Structure of a Nucleotide

A nucleotide consists of a sugar (deoxyribose in DNA, ribose in RNA), a phosphate group, and a base (DNA: A, T, C, G; RNA: A, C, G, U).

The DNA Code

Only the sense strand actually codes for the manufacture of proteins in a cell. The proteins manufactured can be intracellular enzymes, extracellular enzymes, structural proteins or hormones. Proteins are made of chains of amino acids.

The DNA code is a triplet code, because 3 bases are needed to code for 1 amino acid. A triplet involving 4 bases has 64 possible combinations and could therefore code for 64 amino acids. To manufacture the different proteins, some amino acids have more than 1 code; because of this we say the code is degenerate.

Down Syndrome

Cells do not divide properly, and sometimes sex cells do not form properly and contain more or fewer chromosomes than normal. One relatively common chromosome mutation results in ova containing 2 copies of chromosome 21. When such an ovum is fertilised by a normal sperm, the zygote will have 3 copies of chromosome 21.

Symptoms: Children may die in infancy from lung problems, and they cannot achieve good social and intellectual growth or achieve personal self-sufficiency.

Mutations

A mutation is a change in the order of bases in a gene. Types include addition (extra nucleotide), deletion, substitution, inversion and duplication. The effect of a mutation depends on how much the code is disrupted.

Consequences

A change in the code caused by a mutation may mean that a different polypeptide, and hence a different protein, is produced by a gene. A mutation can produce a different form of the gene and also a new allele. When the mutated DNA replicates, the new form is copied, so the mutation passes to other cells.

Cancer

When mutations occur in the genes that control cell division, unchecked irregular growth takes place and a tumour develops.

Mutagenic Agents

• X-rays and gamma rays (these have cumulative effects)
• Radioactive substances (release particles with energy levels higher than radiation)
• Organic compounds (e.g., tobacco tar)

DNA Replication

When a cell divides, it makes a complete copy of its DNA to preserve the information.

1. The hydrogen bonds that connect bases are broken by an enzyme, so the strands separate easily.
2. Another enzyme attaches free nucleotides to the exposed bases on each strand.
3. A new strand is built on each of the original strands.

mRNA passes from the nucleus to the cytoplasm and is then used as a guide to manufacture the protein encoded. Its strand is used to assemble amino acids.

Transcription

Transcription is the process of copying the code of DNA to form RNA:

1. Transcription starts when an enzyme catalyses a reaction that makes the DNA untwist.
2. Another enzyme assembles the RNA nucleotides along one side of the DNA molecule.
3. The sense strand produces a single-stranded molecule of mRNA.
4. The mRNA detaches from the DNA and passes out of the nucleus through the nuclear pores and the endoplasmic reticulum.
5. The mRNA attaches to ribosomes.

Translation

The code on the mRNA is used to assemble amino acids into proteins in the correct order. Molecules of tRNA are present as amino acid complexes in the endoplasmic reticulum.

Anticodon: the triplet of bases on the tRNA that binds to the codon.