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DNA and Molecular Genetics

DNA and Molecular Genetics. Chapter 3. Introduction. Until now we have talked about genes simply as a functional part of the chromosome Need to consider how genes actually work in the cell (called expression of a gene)

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DNA and Molecular Genetics

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  1. DNA and Molecular Genetics Chapter 3

  2. Introduction • Until now we have talked about genes simply as a functional part of the chromosome • Need to consider how genes actually work in the cell (called expression of a gene) • To understand how genes express, need to understand both their chemical composition and biochemical function

  3. DNA and RNA Structure and Function • Review: • DNA found mostly in chromosomes in nucleus • When cell is undergoing mitosis, chromosomes are short and thick • Rest of the time, chromosomes (and DNA) are long and thin [>5 ft in each cell!) • DNA exists in the form of a double-stranded helix • Helix – spiral staircase or twisted ladder shaped-structure • Double strand structure allows “easy” replication (making more) of the long, complicated DNA

  4. Fig. 03-01 Fig. 03-01DNA structure and location

  5. DNA Structure and Replication • DNA stands for deoxyribonucleic acid • All nucleic acids formed by bonding together of nucleotides (type of molecule) • Nuleotides – formed by bonding of three smaller molecules 1. Phosphate 2. Sugar (= deoxyribose molecule) 3. Nitrogen-containing base • Note a base can “take up” (soak up) protons (positively charged particles = H+) • Acids “give off” protons (H+) • When phosphate and N-containing base bond together  give off (H+) protons = the acid in DNA

  6. DNA Structure and Function (con’t)Nucleotides Fig. 03-02 • Four nucleotides that make up DNA are • Adenine (A) • Thymine (T) • Cytosine (C) • Guanine (G) • Nucleotides are joined together in a specific way, with phosphates forming the backbone of the DNA strand and bases projecting to the side

  7. DNA Structure and Function (con’t)Double strands and base pairing • DNA has two strands of nucleotides; this makes DNA a double helix • Weak hydrogen bonds between the bases hold the strands together • Different #’s of bonds causes only certain bases to bond together (called complementary base pairing) • Complementary base pairing • Adenine (A) with thymine (T) • Guanine (G) with cytosine (C) • Could be vice-versa (T-A, C-G) Fig. 03-03

  8. Replication of DNA • Occurs as part of chromosome duplication • Requires four steps • H-bonds between two strands of DNA break as enzymes unwind and “unzip” the DNA molecule • New nucleotides (always present in the nucleus) fit into place beside each old (parental) strand by complementary base pairing • New nucleotides become joined by enzyme called DNA polymerase (forms DNA polymer (molecule) • End up with two complete DNA molecules, identical to each other and to the original molecule • Each new DNA is partly old (parental strand) and partly new (daughter strand)

  9. Gene Expression • Gene expression is the making of specific proteins (in ribosomes in cytoplasm and on rough ER) from specific nucleotide sequences (in DNA of genes in nucleus) • Need a way to get information from nucleus to ribosomes  done with RNA • Actually three types of RNA that all help to read the DNA code and produce proteins

  10. Structure of RNA • RNA (ribonucleic acid) made up of nucleotides containing the sugar ribose • Four nucleotides making up RNA have 3 of the same bases as DNA (A,C,G) and one different, uracil (U) instead of thymine (T) • RNA is single-stranded Fig. 03-05 Structure of RNA

  11. Types of RNA Note: all types produced in nucleus by DNA according to DNA nucleotide sequence in specific gene • Messenger RNA (mRNA) – carries genetic information from DNA to ribosomes where protein synthesis occurs • Ribosomal RNA (rRNA) – combines with certain proteins to form ribosomes • Transfer RNA (tRNA) – transfers amino acids “floating around” in cytoplasm, brings them to ribosomes in certain order specified by mRNA, bonds them together to form proteins

  12. Structure and Function of Proteins • Made up of subunits called amino acids (20 different AA’s) • Specific sequence of amino acids dictates specific protein A (shortened) protein Another (shortened) protein • Proteins can be structural (muscles) or enzymes = catalyze (speed up) chemical reactions

  13. Structure and Function of Proteins (con’t) • Reactions in cells form metabolic (chemical) pathways EA EB EC ED A  B  C  D  E • Letters represent molecules, notations over arrows are enzymes. • For example EA catalyzes reaction converting chemical A to chemical B, EB catalyzes catalyzes B to C, etc • By DNA producing certain enzymes, can “turn on” certain chemical pathways in cell as needed  can form and maintain entire organism!

  14. Back to Gene Expression • Requires two steps • Transcription – making mRNA from specific portion of DNA (gene) • Translation – mRNA goes out into cytoplasm to ribosome, directs tRNAs to bring certain amino acids to ribosome, rRNA joins them together in certain sequence = a specific protein!

  15. Transcription • Occurs in nucleus • Nucleotides in DNA are complementarily matched to form mRNA, substituting U for T • mRNA then goes out of nucleus to ribosome for translation

  16. Translation • Occurs in cytoplasm • Synthesis of polypeptide (many amino acids bonded together) under direction of mRNA • mRNA tells rRNA which amino acid to go get from cytoplasm • rRNA and protein in ribosome binds amino acids together in sequence directed by mRNA

  17. Overview of Transcription and Translation Transcription Translation

  18. Genetic Mutations • Defined as a permanent change in the sequence of nucleotides in DNA • Effect on protein activity (construction and/or function) may range from no effect to complete inactivity

  19. Effect of Mutations • Some genetic disorders already talked about in class are due to mutations • Example: Phenylketonuria due to defect in gene expression for EA below; Albinism due to defect in gene expression for EB below • Other genetic disorders due to gene defects include hemophilia B, Cystic fibrosis, and androgen insensitivity

  20. Androgen Insensitivity • Androgens are hormones needed by males (e.g. testosterone) to show secondary sex characteristics (broad shoulders, extra body hair, deeper voice, etc) • In androgen insensitivity, a mutated gene prevents proper formation of androgen receptors on cells • Results in cells not responding to androgens at puberty --> individual will instead develop some female secondary sexual characteristics (breasts, wider hips, etc.) • A problem is realized when the person does not start to menstruate and seeks medical assistance  both X and Y chromosomes found in cells and person found lacking in internal sexual organs of a female. A genetic male with androgen insensitivity

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