Molecular Genetics (part 1)

1. Molecular Genetics Chapter 20 Content • The Structure of DNA • The Role of DNA in Protein Synthesis • Genes • Genetic Engineering and Medical Biotechnology

2. Lesson Objectives At the end of this lesson, you should be able to: (a) outline the relationship between DNA, genes and chromosomes (b) state the structure of DNA in terms of the bases, sugar and phosphate groups found in each of their nucleotides (c) state the rule of complementary base pairing

3. Recap

4. DNA • Deoxyribonucleic acid (DNA) is a molecule that carries genetic information • This information is important for cellular functions e.g. cell division and cell differentiation • Without DNA, cells would not be able to carry out these vital functions • Almost all cells contain DNA inside their nuclei

5. How is DNA organised in cells? • Each DNA molecule consists of 2 parallel strands twisted around each other forming a double helix • A molecule of DNA is wrapped around proteins (histone proteins) to form a single chromatin thread • During cell division, chromatin threads coil tightly into chromosomes inside the nucleus

7. How DNA is packed into a chromosome

8. Basic Unit of DNA is called a nucleotide • A nucleotide is made up of 1. Deoxyribose (a sugar) 2. A phosphate group 3. A nitrogen-containing base (ATGC) A denine T hymine G uanine C ytosine

9. 4 types of nucleotides linked to form a polynucleotide double ring structure single ring structure instead of thymine

10. Nucleotides are the building blocks of DNA • Each gene is made up of a sequence of nucleotides • The sequence of nucleotides can vary • This results in many different genes • Since there are 4 different bases and hence 4 different nucleotides, for a gene made up of n nucleotides, there are 4ndifferent combinations of nucleotides

11. Complementary base pairing between polynucleotides • Adenine (A) always binds with thymine (T) • Cytosine (C) always binds with guanine (G) A denine T hymine G uanine C ytosine

12. A DNA molecule • is made up of 2 polynucleotide chains • running in opposite directions (anti-parallel) • The bases of one chain are bonded to those of the opposite chain according to the rule of base pairing • A-T = 2 hydrogen bonds • G-C = 3 hydrogen bonds

14. The Structure of DNA

15. A DNA molecule

16. Create a DNA model worksheet

17. Lesson Objectives At the end of this lesson, you should be able to: (d) state that DNA is used to carry the genetic code, which is used to synthesize specific polypeptides (e) state that each gene is a sequence of nucleotides, as part of a DNA molecule

18. A Gene • is a small segment of DNA which controls the formation of a single protein e.g. an enzyme • Each gene stores a message that determines how an enzyme or protein should be made in the cell • The message stored by a gene is known as the genetic code

20. Structure of a gene • Each gene consists of 2 polynucleotide chains • One of the chains determines the type of protein made (called the template) • The template contains a sequence of nucleotides or bases • The polynucleotide sequence stores information as follows: three bases code for one amino acid (triplet code or codon) • The nucleotide sequence determines how an enzyme or protein should be made within a cell

21. Codons coding for different amino acids (based on codons on mRNA)

22. Structure of a gene • A single gene carries a message for the synthesis of one particular polypeptide • If a protein is made of more than one polypeptide, more than one gene will contribute to the making of this protein

24. Gene mutation • Since each protein or enzyme contributes to the development of a certain characteristic in your body, the alteration of the message stored by a gene may affect your body structure or function • For example, a gene mutation may alter the message stored by the gene that produced haemoglobin, causing sickle-cell anaemia (covered in Chapter 19)

25. How are proteins made? A cell cannot directly use the DNA template to make proteins Proteins are made through a 2-step process: 1. Transcription - the process of constructing a messenger RNA (mRNA) molecule using a DNA molecule as a template; this occurs in the nucleus; 3 bases in the mRNA make up a codon 2. Translation – the process by which the sequence of nucleotides in a mRNA molecule directs the incorporation of amino acids into a protein; this occurs in the cytoplasm DNA → mRNA → protein translation transcription

28. Difference between RNA & DNA

29. Differences between RNA and DNA DNA RNA Sugar unit = deoxyribose Sugar unit = ribose Nitrogenous bases are Adenine, Thymine, Guanine, Cytosine Ratio of A:T and C:G is 1:1 Nitrogenous bases are Adenine, Uracil, Guanine, Cytosine There is no fixed ratio between A and U, or between C and G Large insoluble molecule Small soluble molecule Permanent molecule in nucleus Temporary molecule made only when needed

31. The process of transcription and translation involves… Unzipping of the gene; one strand of gene is used as the template to make the messenger RNA (mRNA); the mRNA molecule copies the genetic code in the DNA template, following the rule of base pairing (transcription; takes place in the nucleus) mRNA leaves the nucleus and attaches to a ribosome in the cytoplasm In the cytoplasm are amino acids and transfer RNA (tRNA). tRNA have amino acids attached to one end of their structure. Each tRNA is very specific and attaches only to its own amino acid; tRNA has 3 bases at one end (called the anticodon) that binds to the complementary codons on mRNA 1. 2. 3.

32. Structure of tRNA

33. Codons coding for different amino acids (based on codons on mRNA)

34. The process of transcription and translation involves… 4. Translation starts with mRNA attaching to a ribosome. The first 2 tRNAs together with their amino acids also fit into the ribosome. They attach to codons on the mRNA according to the rule of base pairing. A peptide bond is formed between the 2 amino acids 5. Once the peptide bond is formed between the first 2 amino acids, the ribosome moves along one codon to the right of the mRNA. As the ribosome moves to this position, the 1sttRNA is released. At the same time, the 3rdtRNA and its amino acid slots into the ribosome

36. The process of transcription and translation involves… 6. Another amino acid is attached to the chain 7. The process continues as the ribosome moves along the mRNA. At the end of the mRNA is a stop codon (UGA, UAA or UAG). A stop codon does not have any tRNA with complementary codons. Eventually, the whole chain of polypeptide is produced. The ribosome leaves the mRNA 8. The ribosome may attach to the same mRNA for another round of translation

38. Transcription, Translation Worksheet

39. Breaking the Code Worksheet

40. Control of genes • Cells can control their genes. Each cell contains a complete set of genes • However, many of these genes are ‘switched off’ i.e. they do not produce proteins (these genes are not expressed) • Different cells express different genes e.g. genes for insulin production are found in liver cells and cells in the pancreas. However, liver cells do not produce insulin, thus the gene for insulin production is not expressed in liver cells, whereas the gene for insulin is expressed in the islets of Langerhans in the pancreas hence they produce insulin

41. DNA Summary Worksheet

42. ~ The End ~