Unit 1: DNA and the Genome. Sub-Topic (1.3) Gene Expression

Transcription

1 Unit 1: DNA and the Genome Sub-Topic (1.3) Gene Expression

2 Unit 1: DNA and the Genome Sub-Topic (1.3) Gene Expression On completion of this subtopic I will be able to State the meanings of the terms genotype, phenotype and allele. State that the phenotype of an organism is determined by the proteins produced as a result of gene expression. This is affected by environmental factors acting inside and outside the cell. Know that only a fraction of the genes inside a cell are expressed. State that gene expression is controlled by the regulation of transcription and translation. State the differences between DNA and RNA. Describe the process of transcription. State that RNA polymerase is the enzyme responsible for transcription. It unwinds and opens up (unzips) the DNA strand to bring about the synthesis of an mrna molecule. State that introns are non-coding regions within a gene and play no part in coding for a polypeptide. State exons are coding regions within a gene that have a part to play in coding for a polypeptide. Know that during RNA splicing introns (non-coding regions) within the primary mrna transcript are removed and exons (coding regions) are joined together to form the mature transcript. Know that translation of mrna into a protein takes place at the ribosome. Describe the process of translation. Describe the structure and function of mrna, trna and rrna. Different mrna molecules are produced from the same primary transcript depending on which exons are included in the mature mrna pranscript. State that many proteins can come from one gene as a result of alternative mrna splicing. Know that proteins can be modified after the translation process by cutting and combining polypeptide chains or by adding phosphate or carbohydrate groups. Know that proteins have a three dimensional shape; peptide bonds link amino acids together in the polypeptide chain which then folds as a result of hydrogen bonds forming between amino acids. Cross connections may also form between amino acids in one or more chains to form the protein s final structure. Know that the final three dimensional structure of the proteins allows it to carry out its function. Duncanrig Secondary LS 2016 page 1 of 17

3 Prior Learning Unit 1.4 DNA and the production of proteins Each section of DNA which codes for a protein is known as a gene. Proteins are built from subunits called amino acids. A polypeptide is a chain of amino acids linked together. The sequence of bases on the DNA molecule encodes information for the sequence of amino acids in proteins. The sequence of the bases on the DNA molecule therefore determines the function of the proteins they code for. Messenger RNA (mrna) is a single stranded molecule which carries a complementary copy of the code from the DNA, in the nucleus, to a ribosome. The ribosome is the site of protein synthesis. Proteins are assembled from amino acids at the ribosome. 1. Gene Expression Genes are DNA sequences that code for particular proteins. The instructions contained within the DNA molecule must be interpreted and translated into proteins that carry out actions in the cell. Although every individual will have genes for the same proteins, there may be different forms of the gene, these are called alleles. In humans, since you have two copies of each chromosome you will have two copies of each gene. The copies may not be in the same form. The phenotype of an organism is determined by the proteins produced as a result of the genes expressed. This is influenced by environmental factors acting inside and outside the cell. Only a fraction of the genes in a cell are expressed. State the meaning of the word phenotype: State the meaning of the word genotype: Duncanrig Secondary LS 2016 page 2 of 17

4 Gene Expression through protein synthesis A protein s exact molecular structure, shape and ability to carry out its function depends on the sequence of its amino acids. The order of the amino acids is determined by the sequence of bases on the DNA molecule. Therefore, the information for the manufacture of a protein has to be carried from the nucleus to the ribosome where proteins are synthesised. This is carried out by a nucleic acid called ribonucleic acid (RNA). RNA is a single strand of RNA nucleotides. They are similar to DNA nucleotides but the sugar is a ribose sugar rather than a deoxyribose sugar and one of the bases is different- Uracil replaces thymine. Complete the diagram of the RNA nucleotide below: - Bases: Complete the table below to summarise the differences between a DNA and an RNA molecule: DNA RNA Found in Number of strands Sugar in nucleotide Adenine paired to Duncanrig Secondary LS 2016 page 3 of 17

5 2. The Genetic Code As we have learned, the order of the amino acids in a protein is determined by the sequence of bases on the DNA molecule. The sequence of the bases along the strand represents a sequence of codewords. This is called the genetic code. Each group of three bases on the DNA molecule, or triplet, is called a codon. These three letter codes are designated by the code letters of the bases e.g. CGC, AAT depending on the order of the nucleotides. Each amino acid is coded for by one or more triplets of bases. The triplet code allows for 64 combinations which is more than enough to code for 20 amino acids. Gene expression is controlled by the regulation of transcription and translation. 3. Transcription Transcription is the first step in gene expression. In this process a molecule of mrna is formed using the DNA as a template. This occurs in the nucleus. An enzyme called RNA polymerase attaches to a sequence of DNA called the promoter. It moves along the DNA unwinding the double helix and opens up (unzips) the DNA molecule breaking the hydrogen bonds holding the base pairs together. Free RNA nucleotides align with the complementary base pairs on the DNA moving from 3 to 5. RNA nucleotides are held in place by hydrogen bonds while strong covalent bonds form between the phosphate of one nucleotide and the 3 carbon of the adjacent nucleotide. The molecule lengthens until a terminator sequence of nucleotides is reached on the DNA strand and the RNA polymerase enzyme is released. As a result of the base pairing rule, the mrna strand gets a nucleotide sequence complementary to one of the two DNA strands. The mrna strand becomes separated from the DNA template. This is known as the primary transcript of mrna. Duncanrig Secondary LS 2016 page 4 of 17

6 Modification of the primary transcript Eukaryotic organisms have long stretches of DNA that exist within a gene that do not play a part in coding for a polypeptide chain. These non-coding regions are called introns. These are located between coding regions called exons. This means that the region in the primary transcript responsible for coding for the polypeptide is fragmented. RNA splicing The primary transcript needs to be modified in a process called RNA splicing. The introns are cut out of the primary transcript and the exons are spliced together to form a mature mrna strand with a continuous sequence of nucleotides. The mature mrna passes out of the nucleus and enters the cytoplasm. Once in the cytoplasm ribosomes and another RNA molecule called transfer RNA (trna) work together to translate mrna into a polypeptide chain. Duncanrig Secondary LS 2016 page 5 of 17

7 RNA splicing Mature mrna transcript Complete the diagram below by: 1. labelling the template strand of DNA. 2. adding the missing complementary base-pairs on the DNA strand. 3. labelling the mrna strand and adding the complementary base pairs. 4. labelling the sugar phosphate bonds between adjacent nucleotides on the mrna molecule. A T C C A G T A T C C A T C C G G T T C A A Duncanrig Secondary LS 2016 page 6 of 17

8 Answer the questions that follow: 1. How many bases in the genetic code are needed to code for one amino acid? 2. Complete the diagram to show the mrna strand that would be transcribed from the section of DNA in the diagram below. ATA TCG CGA CCT TGA DNA strand mrna stand 3. Name the enzyme that would direct this process. 4. State the difference between an exon and an intron. 5. Which of these is removed during modification of the primary transcript of mrna? 6. State the name of the process in which this occurs. Ribosomes Translation of mrna into a protein takes place at the ribosome Duncanrig Secondary LS 2016 page 7 of 17

9 Ribosomes are small spherical structures found in all cells. They can be found free in the cytoplasm or attached to endoplasmic reticulum. They are formed from a type of RNA called ribosomal RNA (rrna) and protein. They contain enzymes required for protein synthesis. Ribosomes allow mrna and transfer RNA (trna) to come together during a process called translation. 4. Translation Translation is the process that translates the mrna into a polypeptide. The ribosome reads the information carried by the mrna molecule as three letter codewords. Each group of three bases on the mrna (or triplet) is called a codon. Remember! The information contained in the triplet of bases on the mrna molecule is complementary to a triplet of bases on the original DNA molecule. Transfer RNA (trna) Transfer RNA (trna) is a type of RNA found in the cell s cytoplasm. It is made up of a single strand of nucleotides that folds due to base pairing to form a site for attachment for a specific amino acid at one end and a triplet anticodon at the other. amino acid attachment site anticodon The triplet anticodons on the trna are complementary to codons on the mrna strand. Each anticodon corresponds to a specific amino acid. The specific amino acid is carried at the attachment site of the trna molecule to the ribosome. Duncanrig Secondary LS 2016 page 8 of 17

10 Start and Stop codons Before translation can begin the ribosome must bind with the mrna molecule at the mrna binding site. A start codon at the beginning of an mrna molecule signals where translation is to begin. A trna molecule carrying the specific amino acid for the start codon becomes attached to the binding site within the ribosome by hydrogen bonds between the anticodon and the start codon. After the start codon there are further codons on the mrna that are particular for the polypeptide to be synthesised. trna molecules carry specific amino acids for each of these codons to the ribosome one at a time. As they enter the ribosome the codons on the mrna strand form hydrogen bonds with the anticodon of a trna molecule carrying the specific amino acid for that codon. When two amino acids are aligned next to each other they become joined by a peptide bond to synthesise a growing polypeptide chain. As the ribosome moves along to the next codon the trna molecule is released from the ribosome to be reused and the next trna molecule carrying its specific amino acid moves into the binding site. This process is repeated many times allowing the mrna to be translated into a complete polypeptide chain. Eventually a triplet sequence called a stop codon is reached within the mrna that signals termination of translation. A release factor frees the polypeptide from the ribosome. Duncanrig Secondary LS 2016 page 9 of 17

11 One gene, many proteins One gene can produce many proteins as a result of the way in which RNA splicing occurs. Different mrna molecules are produced from the same primary transcript depending on which exons are included in the mature mrna transcript. This results in several different mrna molecules each with a different sequence of base triplets coding for a different polypeptide. Duncanrig Secondary LS 2016 page 10 of 17

12 Overview There are 3 forms of RNA involved in protein synthesis: Messenger RNA (mrna) Transfer RNA (trna) Ribosomal RNA (rrna) Each type of RNA molecule has its own form and function: Complete the table below using the class resources: Abbreviation Full name Where it is found in the cell Function mrna trna rrna Complete the diagram showing translation by labelling the parts A-G Duncanrig Secondary LS 2016 page 11 of 17

13 Complete the diagram by naming the stages shown in protein synthesis: DNA code section = gene Primary mrna transcript Excised Introns + Mature mrna transcript Polypeptide chain amino acid amino acid amino acid amino acid amino acid Duncanrig Secondary LS 2016 page 12 of 17

14 5. Proteins Proteins are chains of amino acids held together by peptide bonds. The chain is called a polypeptide. During protein synthesis amino acids are linked together in a specific order which is determined by the order of the bases on the DNA molecule. amino acids amino acids become joined together by peptide bonds in a particular genetically determined sequence hydrogen bonds form between certain amino acids peptide chain coiled peptide chain folded peptide chains become folded together into a spherical shape which incorporate another chemical peptide chains become arranged in long parallel strands peptide chains become folded together into a spherical shape Duncanrig Secondary LS 2016 page 13 of 17

15 An enormous number of different proteins are found in living cells. A human has over 10,000 different proteins. Proteins can be fibrous, globular or conjugated (contain a non-protein chemical) and each has structural properties that suit their role in the organisms body. Use the class resources to complete the table below. Protein Function Post-Translational Modifications After translation is complete proteins may be modified by cutting and combining polypeptide chains or by adding phosphate groups and carbohydrate groups to the polypeptide chain. The hormone insulin consists of two polypeptide chains that originated as one chain. Answer the questions that follow: 1. State the function of a ribosome. 2. Describe the structure of a trna molecule. 3 Name the type of bond that forms between the mrna molecule and trna molecule within the ribosome. Duncanrig Secondary LS 2016 page 14 of 17

16 4 Name the type of bond that forms between amino acids during protein synthesis. 5. Explain how different functional mrna molecules can be produced from the same primary transcript sequence. 6. State 3 ways in which a polypeptide chain may be modified following translation. Duncanrig Secondary LS 2016 page 15 of 17

17 1.3 Gene Expression How well do you rate your knowledge and understanding? Complete: Column 1 before your Unit assessment Column 2 before your Prelim Column 3 before your May exam State the meanings of the terms genotype, phenotype and allele. State that the phenotype of an organism is determined by the proteins produced as a result of gene expression. This is affected by environmental factors acting inside and outside the cell. Know that only a fraction of the genes inside a cell are expressed. State that gene expression is controlled by the regulation of transcription and translation. State the differences between DNA and RNA. Describe the process of transcription. State that RNA polymerase is the enzyme responsible for transcription. It unwinds and opens (unzips) up the DNA strand to bring about the synthesis of an mrna molecule. State that introns are non-coding regions within a gene and play no part in coding for a polypeptide. State exons are coding regions within a gene that have a part to play in coding for a polypeptide. Know that during RNA splicing introns (non-coding regions) within the primary mrna transcript are removed and exons (coding regions) are joined together to form the mature transcript. Know that translation of mrna into a protein takes place at the ribosome. Describe the process of translation. Describe the structure and function of mrna, trna and rrna. Different mrna molecules are produced from the same primary transcript depending on which exons are included in the mature RNA transcript. State that many proteins can come from one gene as a result of alternative mrna splicing. Know that proteins can be modified after the translation process by cutting and combining polypeptide chains or by adding phosphate or carbohydrate groups. Know that proteins have a three dimensional shape- peptide bonds link amino acids together in the polypeptide chain which then folds as a result of hydrogen bonds forming between amino acids. Cross connections may also form between amino acids in one or more chains to form the protein s final structure. Know that the final three dimensional structure of the proteins allows it Duncanrig to carry out Secondary its function. LS 2016 page 16 of 17