Higher Biology DNA Structure & Organisation Past Paper PDF

ANSWERS TO PUPIL NOTES Higher Biology Unit 1: DNA and the Genome Key Area 1.1 The Structure of DNA (a) The Structure of DNA 1 Name the molecules in a DNA nucleotide and identify them in a diagram. 2 The...

ANSWERS TO PUPIL NOTES Higher Biology Unit 1: DNA and the Genome Key Area 1.1 The Structure of DNA (a) The Structure of DNA 1 Name the molecules in a DNA nucleotide and identify them in a diagram. 2 The base sequence of DNA forms the genetic code 3 Give the full names of the 4 DNA bases. 4 Describe the base pairing rule for DNA bases. 5 Describe the role of hydrogen bonds in the DNA structure. 6 State the name of the coiled structure adopted by DNA. 7 Identify the positions of 3’ and 5’ carbons on a DNA nucleotide. 8 Identify the positions of 3’ and 5’ ends on a DNA strand. 9 Describe how 2 strands of DNA align themselves to each other. (b) The Organisation of DNA 1 Describe the key similarities and differences between prokaryotes and eukaryotes. 2 Describe structure of circular chromosomes and identify the location and types of cells where they are found. 3 Describe structure of a plasmid and name the types of cells where they are found. 4 Describe the organisation of DNA in linear chromosomes wrapped around histone proteins, found in nucleus of eukaryotes. 1  Glossary Hydrogen Antiparallel Prokaryotic Sugar-phosphate backbone Nucleotide Linear Deoxyribose DNA Plasmid Eukaryotic Mitochondrion Base Phosphate Double helix Chloroplast Histone Circular Word Definition Mitochondrion Cell organelle where aerobic respiration takes place which contains circular chromosomes. Prokaryotic Cell that has no nucleus, an example is bacteria. Eukaryotic Cell with a nucleus, examples are plant, animal & fungal. Plasmid Ring of genetic material found in prokaryotes and some eukaryotic yeast cells. Phosphate Component of a DNA nucleotide that attaches to the deoxyribose sugar to make the ‘backbone’ Nucleotide Component of DNA consisting of a deoxyribose sugar, a phosphate group and a base. Sugar-phosphate backbone What the DNA backbone is composed of. DNA A molecule that holds the genetic code in living organisms. Base There are 4 types of these: Adenine, thymine, guanine & cytosine. Chloroplasts Organelle found in plant cells only where photosynthesis takes place and which has circular chromosomes. Antiparallel DNA is described as this because one strand runs 5’ to 3’ and the other runs 3’ to 5’. Linear The type of chromosome found in eukaryotic nuclei. Deoxyribose Sugar that is a component of a DNA nucleotide. Double helix Three-dimensional shape of a DNA molecule. Hydrogen Weak bond joining complementary base pairs in DNA. Histone The type of protein that linear chromosomes are wrapped around. Circular The type of chromosome found in prokaryotic cells & within chloroplasts & mitochondria. 2  Notes DNA (deoxyribonucleic acid) is a complex molecule present in all living cells. DNA stores genetic information in its sequence of bases which determines the organisms genotype and the structure of its proteins. A molecule of DNA consists of two strands each made up of repeating units called nucleotides. Phosphate Base Deoxyribose sugar A nucleotide is made up of a molecule of deoxyribose sugar joined to a phosphate group and an organic base. The shape of the deoxyribose sugar in a nucleotide is determined by the arrangement of carbon atoms in the molecule. Each carbon atom is numbered. This can help us describe the arrangement of a DNA molecule: Note 3C = 3’ (3 prime) carbon atom 5C = 5’ (5 prime) carbon atom To produce a strand of DNA, a strong chemical bond forms between the phosphate group of one nucleotide and carbon 3 on the deoxyribose sugar of another nucleotide. This forms the molecule’s sugar-phosphate backbone. 3  There are four different types of nucleotide in a DNA molecule. They differ from each other because they have a different base. The base attaches to carbon 1. Two strands of nucleotides become joined together by hydrogen bonds forming between their bases. Hydrogen bonds are weak so the strands can easily become separated. Each base can only join up with one other type of base: – Adenine joins with thymine – Guanine joins with cytosine This is called the base-pairing rule. DNA takes the form of a double helix. 4  DNA is made up of two antiparallel strands. This means that the strands run in opposite directions to each other. Using the numbered carbon atoms on the deoxyribose sugar allows us to show the antiparallel structure, with the deoxyribose and phosphate at 3’ and 5’ ends of each strand respectively. Key Area 1.1b The Organisation of DNA Cell types DNA is present in the cells of every living thing. However, the DNA is organised differently in different types of organism. We can divide cells into two groups based on how they organise their DNA – eukaryotes and prokaryotes. 5 Prokaryotes: Bacteria are prokaryotes. They do not have a membrane- bound nucleus and their DNA is free in the cytoplasm. Bacteria have a single circular chromosome in the centre of the cell that holds all the genes needed for that bacterium. Bacteria also have extra circles of DNA called plasmids. These plasmids contain additional genes, such as for antibiotic resistance, which may increase a bacterium’s chance of survival. Eukaryotes: Animals, plants and fungi are eukaryotes. They have a membrane-bound nucleus and their chromosomes are linear. The DNA found in the linear chromosomes within the nucleus is tightly coiled and packaged around special proteins called histones. In eukaryotes DNA can also be found in mitochondria and chloroplasts where it forms circular chromosomes. This DNA is used to make proteins essential to the functioning of the organelle. Yeast are eukaryotic cells, but they contain plasmids, just like prokaryotes. 6  1.1 Past Paper Questions 1 2 7  3 4 Name the type of bond which links the base of one nucleotide to the base of another. (1) Hydrogen bond 5 Name the substance with which DNA is packaged in eukaryotes. (1) Histone 6 Describe how DNA is organised in prokaryotes. (2) Plasmids (1) Circular chromosomes (1) 8  7 State one location, other than the nucleus, where DNA is found in eukaryotic cells. (1) Mitochondria or chloroplasts 9