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Microbiology - DNA and Gene Expression

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    Study Guide Microbiology – DNA and Gene Expression 1. DNA Structure During the 1950s, biology experienced rapid scientific growth. Researchers began to understand how genes work and how genetic information controls cell activities. These discoveries led to the development of molecular genetics , a field that studies DNA and explains how it directs the production of proteins in microorganisms and other living cells. 1.1 The Discovery of DNA Structure In 1953, James Watson and Francis Crick proposed the correct structure of DNA. They showed that deoxyribonucleic acid (DNA) is made of two long chains of nucleotides . • The two chains twist around each other to form a double helix . • The structure looks like a spiral staircase . • The two strands are held together by weak hydrogen bonds between specific nitrogenous bases. 1.2 The Building Blocks of DNA: Nucleotides Each DNA strand is made of repeating units called nucleotides . Every nucleotide has three parts : 1. A nitrogenous base 2. A phosphate group 3. A sugar molecule called deoxyribose Nitrogenous Bases There are four nitrogenous bases in DNA. These bases fall into two categories:

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    Study Guide Purines (two - ring structures) • Adenine (A) • Guanine (G) Pyrimidines (one - ring structures) • Cytosine (C) • Thymine (T) Both types of bases appear on each strand of the DNA double helix. The DNA Backbone and Base Pairing • The phosphate groups and deoxyribose sugars form the backbone of each DNA strand. • The nitrogenous bases attach to the deoxyribose sugars and face inward toward the opposite strand. • Bases pair in a specific and predictable way : o Adenine (A) always pairs with Thymine (T) o Guanine (G) always pairs with Cytosine (C) These base pairs are called complementary base pairs . This rule is known as the principle of complementary base pairing . DNA Replication Before a cell divides — either by binary fission in bacteria or mitosis in eukaryotic cells — its DNA must be copied. This process is called DNA replication . Steps of DNA Replication 1. Unzipping the DNA Specialized enzymes separate the two DNA strands by breaking the hydrogen bonds. 2. Exposing the bases As the strands separate, the bases on each strand become exposed. 3. Matching complementary bases

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    Study Guide Free nucleotides in the cell pair with the exposed bases using complementary base pairing. 4. Building new strands The enzyme DNA polymerase links the nucleotides together, forming a new DNA strand. Semiconservative Replication After replication is complete: • Each new DNA molecule contains: o One original (old) strand o One newly synthesized strand Because half of the original DNA is conserved in each new molecule, this process is called semiconservative replication . Key Takeaway DNA is a double - stranded, double - helical molecule made of nucleotides. Its structure allows it to be copied accurately and efficiently. Through complementary base pairing and semiconservative replication, DNA ensures that genetic information is faithfully passed on to new cells. 2. Protein Synthesis Protein synthesis is the process by which genetic information in DNA is used to build proteins , which function as enzymes, structural components, and cellular machinery. 2. 1. The Genetic Code • The genetic code is the sequence of nitrogenous bases in DNA. • Bases are read in groups of three called codons . • Each codon specifies one amino acid . • The sequence of codons determines the sequence of amino acids , and thus the protein.

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    Study Guide 2.2 . RNA: The Key Intermediary RNA differs from DNA in three ways: 1. Sugar: ribose (not deoxyribose) 2. Base: uracil (U) replaces thymine (T) 3. Structure: single - stranded Types of RNA • mRNA (messenger RNA): carries genetic code from DNA to ribosome • tRNA (transfer RNA): carries specific amino acids to ribosome • rRNA (ribosomal RNA): forms the structural and functional core of ribosomes 2. 3. Transcription (DNA → mRNA) Where: • Prokaryotes: cytoplasm • Eukaryotes: nucleus Steps: 1. RNA polymerase binds to DNA 2. DNA strands separate 3. Complementary RNA bases pair with DNA bases 4. mRNA strand elongates 5. At a stop signal, mRNA detaches Key point: • Uses complementary base pairing • Produces an mRNA strand made of codons

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    Study Guide Figure 1 The synthesis of mRNA using a strand of DNA as a template . 2. 4. Translation (mRNA → Protein) Where: ribosomes Steps Initiation • mRNA binds to ribosome • First tRNA (with methionine) binds to start codon (AUG) Elongation • tRNA anticodons pair with mRNA codons • Amino acids are linked by peptide bonds • Ribosome moves along mRNA Termination • Ribosome reaches a stop codon • Polypeptide chain is released Result: A completed protein (polypeptide chain)

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