How is genetic information stored in DNA, what is the genome, and what causes organisms to vary?
The structure of DNA as a double helix of nucleotides with complementary base pairing, the genome as the entire genetic material of an organism, the value of sequencing the human genome, and the causes of genetic and environmental variation including mutation.
A focused answer to the OCR Gateway GCSE Biology A topic B5 on DNA, the genome and variation, covering the double helix and complementary base pairing, genes and chromosomes, the genome and the value of the Human Genome Project, and the genetic and environmental causes of variation including mutation.
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What this dot point is asking
OCR wants you to describe DNA as a double helix of nucleotides with complementary base pairing, define the genome, explain why sequencing the human genome is useful, and describe the genetic and environmental causes of variation, including mutation.
The structure of DNA
Each nucleotide is made of a sugar, a phosphate group and one of four bases: adenine (A), thymine (T), cytosine (C) and guanine (G). The sugar and phosphate groups link together to form the two backbones of the helix, and the bases point inwards and join the strands together.
The bases pair in a fixed way, called complementary base pairing: A always pairs with T, and C always pairs with G. Because the pairing is fixed, the order of bases on one strand fixes the order on the other strand. This is what lets DNA be copied accurately before a cell divides.
Genes, chromosomes and the genome
A gene is a short section of DNA that codes for a particular sequence of amino acids, and so for a particular protein. Proteins determine the organism's characteristics. Genes are arranged along chromosomes, the long DNA molecules found in the nucleus; humans have 23 pairs.
The value of sequencing the human genome
Knowing the order of bases in the human genome has several uses that OCR expects you to be able to state:
- Tracing human migration and the history of populations, by comparing the genomes of people from different parts of the world.
- Understanding inherited disorders, by finding which genes (and which faulty alleles) cause them.
- Searching for genes linked to disease, so people at higher risk can be identified and treatments can be targeted (the basis of personalised medicine).
Variation and its causes
Genetic variation arises because individuals inherit different combinations of alleles (versions of genes). The main sources are the shuffling of alleles in meiosis, the random fusion of gametes at fertilisation, and mutation (see below). Characteristics controlled mainly by genes include blood group and eye colour.
Environmental variation is caused by the conditions an organism lives in. A plant grown in the shade grows tall and spindly; a person who eats more gains mass; scars and a suntan are purely environmental. Many characteristics, such as height and body mass, are affected by both genes and environment.
A mutation is a random change to the DNA base sequence. Mutations happen continuously and are usually harmless, but they create new alleles, which is the ultimate source of all genetic variation. Some mutations change a protein and so a characteristic; rarely a mutation is beneficial and may spread by natural selection.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20184 marksDNA is described as a polymer made of nucleotides. Describe the structure of DNA, and state what is meant by complementary base pairing.Show worked answer →
A B5 structured question on DNA structure.
Structure: DNA is a polymer made of two strands twisted into a double helix. Each strand is made of repeating units called nucleotides. Each nucleotide is made of a sugar, a phosphate group and one of four bases (A, T, C and G). The sugar and phosphate form the backbone, and the bases point inwards, joining the two strands like rungs of a ladder.
Complementary base pairing: the bases always pair in a fixed way, A with T and C with G. Because the pairing is fixed, the sequence of bases on one strand determines the sequence on the other. Markers reward double helix, nucleotide (sugar, phosphate, base), the four bases, and the A-T and C-G pairing.
OCR 20214 marksTwo identical twins are raised in different countries. Explain why they may still differ in body mass, using the ideas of genetic and environmental variation.Show worked answer →
A B5 application question on the causes of variation.
Identical twins have the same genes (the same genotype), so any genetic factors affecting body mass are the same in both. The differences must come from the environment: differences in diet (how much and what they eat) and in activity levels (exercise) affect body mass. This is environmental variation.
Reward the idea that the twins are genetically identical, that body mass is affected by both genes and environment, and that the environmental factors (diet, exercise) explain the difference here. A good answer notes that many characteristics, such as height and mass, are influenced by both genes and environment together.
Related dot points
- The structure of DNA as a double helix of two strands made of nucleotides, complementary base pairing (A-T, C-G), the gene as a section of DNA coding for a sequence of amino acids, and an overview of protein synthesis using mRNA and ribosomes.
A focused answer to the OCR Gateway GCSE Biology A topic B1 on DNA and protein synthesis, covering the double helix, nucleotides, complementary base pairing, the gene as a code for a protein, and the overview of transcription and translation using mRNA and ribosomes.
- Protein synthesis as transcription of a gene into mRNA and translation at the ribosome, the role of the triplet code and amino acids, how the order of bases determines the protein made, and how mutations can change a protein and its function.
A focused answer to the OCR Gateway GCSE Biology A topic B5 on protein synthesis, covering transcription of a gene into mRNA, translation at the ribosome, the triplet code and amino acids, how the base order determines the protein made, and how mutations can change a protein and its function.
- Alleles, dominant and recessive, genotype and phenotype, homozygous and heterozygous, monohybrid crosses and Punnett squares, the inheritance of sex by the X and Y chromosomes, and single-gene inherited disorders such as cystic fibrosis and polydactyly.
A focused answer to the OCR Gateway GCSE Biology A topic B5 on genetic inheritance, covering alleles, dominant and recessive, genotype and phenotype, homozygous and heterozygous, monohybrid crosses with Punnett squares, the inheritance of sex, and single-gene inherited disorders such as cystic fibrosis and polydactyly.
- Sexual and asexual reproduction, the formation of gametes by meiosis, how meiosis halves the chromosome number and produces genetically varied cells, and the advantages and disadvantages of each type of reproduction.
A focused answer to the OCR Gateway GCSE Biology A topic B5 on reproduction and meiosis, covering sexual and asexual reproduction, how meiosis halves the chromosome number and produces genetically varied gametes, fertilisation, and the advantages and disadvantages of each type of reproduction.
- Variation and natural selection, Darwin's theory of evolution by natural selection, the development of antibiotic-resistant bacteria, the evidence for evolution from fossils and from resistant bacteria, extinction, and the formation of new species by isolation.
A focused answer to the OCR Gateway GCSE Biology A topic B5 on natural selection and evolution, covering variation and selection, Darwin's theory of evolution by natural selection, antibiotic-resistant bacteria, the evidence from fossils, extinction, and the formation of new species by isolation.