B3.3 Genetics ANSWERS Worksheet Two Molecular Genetics 1 1. Nucleotide drawn and labelled showing deoxyribose sugar, nitrogenous base and phosphate group. A section of DNA drawn showing the nucleotides running anti-parallel. 2. Nucleotides are asymmetrical. This is based on the numbering of the carbon atoms. The 5’carbon joins to the phosphate group while the 3’ carbon joins to the next nucleotide. When they are joined together to form DNA, the DNA has a direction. For base pairing to occur on the opposite side of the DNA, the direction runs in the opposite direction giving an antiparallel structure, i.e. 3’ 5’ 5’ 3’ 3. Enzymes e.g. helicase etc Structural e.g. hair Hormones e.g. insulin Immune system e.g. antibodies 4. This refers to the flow of information from DNA to mRNA to protein. 5. Proteins primary structure is the sequence of amino acids. This is then folded into either pleated sheet or an alpha helix shape due to hydrogen bonds to form the secondary structure. The tertiary structure occurs when attraction between the alpha helix and the pleated sheet structures forms a globular shape. The quaternary structure occurs if more than one amino acid chain is present. 6. To make an identical copy of the DNA for cell division. 7. Diagram shows DNA replication occuring with the following features: helicase, replication fork, DNA ligase, DNA polymerase, leading strand, lagging strand, okazaki fragments. 8. a) The replication fork is the point where the DNA is unzipped by helicase to enable free nucleotides to be attached to the parent strand by the enzyme DNA polymerase to make identical copies. b) Nucleotides can only be attached to the template strand in the 5’ to 3’ direction. Therefore on the lagging strand the nucleotides are added in small sections, called Okazaki fragments, as the DNA is unzipped and joined together later by DNA ligase. c) As DNA runs antiparallel the leading strand runs from the 5’ direction to the 3’ direction meaning nucleotides can be added continuously. The lagging strand is the opposite (3’ to 5’) so the nucleotides form Okazaki fragments. 9. Because the process creates two identical strands of DNA each consisting of one of the original strands and one newly synthesised strand. 10. Both DNA and RNA consist of repeating units of nucleotides with the bases A, G and C. However, DNA has T and RNA has U. DNA nucleotides have a deoxyribose sugar and RNA has a ribose sugar. RNA is shorter and has a variety of forms and functions e.g. tRNA, mRNA and rRNA. DNA is double stranded (double helix) and more stable than RNA. 11. A codon is a series of three nucleotides on mRNA that is complementary to a triplet on the coding strand of DNA. Whereas an anticodon is complementary to the codon and is found on the tRNA. 12. Transcription is the process of copying the DNA code by making mRNA. This is done by the DNA unwinding and unzipping. The enzyme RNA polymerase attaches free RNA nucleotides by complementary base pairing to the coding strand of DNA to produce mRNA. This then leaves the nucleus for translation to occur. 13. Both processes involve making a copy of the DNA code in the nucleus. Transcription makes a copy of the code by producing mRNA with RNA nucleotides. Whereas DNA replication uses DNA nucleotides to produce an identical copy. DNA replication uses both sides of the DNA, whereas transcription only uses the coding strand. The enzymes are also different; DNA replication uses helicase, DNA polymerase and DNA ligase, whereas transcription uses RNA polymerase. 14. Degeneracy refers to the fact that there is more than one codon for each amino acid. This is an advantage because if there is a mutation (a change in the sequence of bases) then there is a chance that there would be no effect to the sequence of amino acids in the protein. 15. Translation is the process where a protein is formed from mRNA. mRNA moves into the cytoplasm where tRNA brings the complementary amino acid. These are joined with peptide bonds to form a polypeptide chain and eventually a protein. 16. Enzyme Process(es) Role Helicase DNA replication Unwinds and unzips the DNA DNA polymerase DNA replication Attaches DNA nucleotides to DNA RNA polymerase Transcription Attaches RNA nucleotides to DNA DNA ligase DNA replication Joins Okazaki fragments together 17. Anticodon A sequence of three nucleotides in a transfer RNA (tRNA), that codes for an amino acid. Central Dogma The transcription of DNA to RNA and then translated into a protein. Coding Strand The strand of DNA that has the complementary sequence to mRNA. Codon Sequence of three mRNA nucleotides coding for one amino acid. Complementary The specific pairing of base A with T and base C with G in base pairing double-stranded DNA. Disulphide Bond between two sulfur-containing amino acids in a folded Bridge protein chain. DNA Ligase An enzyme involved in the DNA synthesis and repair. Its function is to join the fragments of DNA together, e.g. Okazaki fragments. DNA An enzyme that adds bases to a replicating DNA strand. Polymerase Duplication A part of the chromosome is present in two or more copies. Helicase An enzyme that unwinds and unzips the DNA helix. Lagging Strand The strand of DNA that is synthesised discontinuously during replication (because DNA synthesis can proceed only in the 5´ to 3´ direction). Leading Strand The strand of DNA that is synthesised continuously during replication. mRNA (messenger RNA) RNA species that contains the information to specify the amino acid sequence of proteins and that is translated on the ribosome. Nucleotide A building block of DNA or RNA, consisting of one nitrogenous base, one phosphate molecule, and one sugar molecule (deoxyribose in DNA, ribose in RNA). Okazaki An Okazaki fragment is a relatively short fragment of DNA Fragment created on the lagging strand during DNA replication. Protein The formation of a specific protein. Individual amino acids are Synthesis joined together in a specific sequence, determined by an mRNA molecule. Purines One of two families of nitrogenous bases found in nucleotides. Two members are adenine and guanine. Pyrimidines One of two families of nitrogenous bases found in nucleotides. Three members are cytosine, thymine and uracil. Replication The replication fork is a structure that forms within the Fork nucleus during DNA replication. It is created by helicases, which break the hydrogen bonds holding the two DNA strands together. RNA An enzyme which helps synthesize mRNA by transcribing the Polymerase nucleotide sequence in DNA. Template The strand of the DNA double helix that is transcribed by Strand complementarily base pairing to make an mRNA. Transcription The synthesis of mRNA copy from a sequence of DNA (a gene); the first step in gene expression. Translation The synthesis of a protein using mRNA code. Triplet A sequence of three nucleotides on the template DNA strand. Each triplet represents the code for a particular amino acid. tRNA A type of RNA molecule that carries a specific amino acid and matches it to its corresponding codon on an mRNA during translation.
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