3.4.1 DNA, genes and chromosomes

• In prokaryotic cells, DNA molecules are short, circular and not associated with proteins.
• In the nucleus of eukaryotic cells, DNA molecules are very long, linear and associated with proteins, called histones. Together a DNA molecule and its associated proteins form a chromosome.
• The mitochondria and chloroplasts of eukaryotic cells also contain DNA which, like the DNA of prokaryotes, is short, circular and not associated with protein.
• A gene is a base sequence of DNA that codes for the amino acid sequence of a polypeptide or a functional RNA (including rRNA and tRNA).
• A gene occupies a fixed position on a chromosome called a locus.
• A sequence of 3 DNA bases is called a triplet and this codes for a specific amino acid.
• The genetic code is universal, meaning that the same triplet of bases codes for the same amino acid in all organisms.
• The genetic code is non-overlapping, meaning that each base is part of only one triplet.
• The genetic code is degenerate, meaning that more than one triplet code can code for the same amino acid
• In eukaryotes, there are non-coding regions in DNA called introns that don’t code for polypeptides.
• Exons are coding regions of DNA that code for amino acid sequences.
• Ribosomes are made up of RNA and proteins and are found in the cytoplasm.
• Gene diversity is the differences in DNA or differences in base sequences of DNA.
• A homologous pair of chromosomes are two chromosomes that carry the same genes and have the same gene loci, but different alleles.
• An allele is a different version of a gene that codes for a different amino acid.
• Adenine binds to thymine and guanine binds to cytosine in DNA, but in RNA adenine binds to uracil.
• Haploid cell has one copy of each chromosome (half of DNA), diploid cell has two copies of each chromosome.
• The genome is the entire set of genetic material in an organism, including all the genes and the DNA.
• An exon is a base sequence coding for a polypeptide.