1 Molecular genetics of bacteria • Emphasis: ways that bacteria differ from eukaryotes • DNA structure and function; definitions. • DNA replication • Transcription and translation • Gene regulation and regulation of metabolism • Genetic exchange among bacteria • Genetic engineering
Molecular genetics of bacteria. Emphasis: ways that bacteria differ from eukaryotes DNA structure and function; definitions. DNA replication Transcription and translation Gene regulation and regulation of metabolism Genetic exchange among bacteria Genetic engineering. DNA structure. - PowerPoint PPT Presentation
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1Molecular genetics of bacteria
• Emphasis: ways that bacteria differ from eukaryotes• DNA structure and function; definitions.• DNA replication• Transcription and translation• Gene regulation and regulation of metabolism• Genetic exchange among bacteria• Genetic engineering
2DNA structure• double helix • Sugar-phosphate
backbone• bases on inside, H
bonded• strands anti-
parallel
3
Review of directionality in DNA
4DNA: source of cellular information
• Information flow: DNARNA protein– also DNA DNA
• Segments of DNA with specific information: genes.
• Structure of DNA highly related to function– Information coded in sequence of bases– Complementary strands means that when each is
copied, two identical molecules are made.
5DNA in prokaryotes
• Most bacteria have a single, circular, molecule of DNA– Some have more than one,
some have linear DNA• Packaged w/ histone-like
proteins, coiled tightly in loops, attached to membrane– 1.6 mm of DNA in a 0.002
mm sized cell.– Area of cell containing the
DNA: nucleoid.
6Plasmids
• Plasmids are small, circular DNA molecules– Found in the cytoplasm of many bacteria– Plasmids are not essential for survival of the cell– They may exist singly or in many copies– Plasmids have a variety of functions
• Origin of DNA replication: particular site on DNA where copying of the DNA always starts.– Replication is bidirectional– In each direction, there is a replication fork.– Bacterial DNA is circular, so there is one Origin and one
terminus
• Synthesis on each DNA strand is 5’ 3’• Replication is semi-conservative
– New DNA molecules made of one old, one new strand.
8DNA replication figures
Because of requirement for 5’to 3’ synthesis, lagging strand must repeatedly top and start; needs an RNA primer each time.
9When you’re small, you need to be stingy and quick
• Look for many ways that bacteria can save energy and respond quickly to changes in environment.
• E. coli needs 30 minutes to replicate its DNA, but only 20 minutes to divide into two. How? It gets a head start.
10Methylation
• Many organisms add methyl groups (-CH3) to DNA, especially to cytosine.
• One of several reasons is self-protection from restriction endonucleases– Enzymes recognize sequences of nucleotides that occur
at random and cut the DNA– Viral DNA injected into cytoplasm of bacteria is
destroyed, protecting the bacterium.– Methylation protects bacterial DNA from cutting by its
own restriction enzymes.
11Genotype vs. Phenotype, bacterial style
• Genotype: the genetic make-up of an organism.• Phenotype: the genetic information expressed by the
organism.• Eukaryotes: difference is often due to masking of
recessive alleles by dominant ones• In bacteria, which are generally monoploid,
phenotype is determined by which genes are being expressed at the present time in response to environmental conditions.
• As mRNA is made, it is ready to use.• Info from more than one gene is typically found on
one mRNA molecule.• Simpler process than in eukaryotes
– no introns to remove– no cap or poly-A tail– no nuclear membrane to transport through
• Transcription is expensive: each NTP leaves behind 2 Pi; like spending 2 ATP for every base used.
20The Genetic Code
• Four bases taken how many at a time? Need to code for 20 different amino acids.– Each base = 1 amino acid: only 4– Every 2 bases = 1 a.a.: 16 combinations, 4 short.– Every 3 bases: 64 combinations, enough.
• Every 3 bases of RNA nucleotides: codon– Each codon is complementary to 3 bases in one strand of
DNA
21Properties of the Genetic Code
• Code is unambiguous: 1 codon always specifies only 1 amino acid.
• Code is degenerate: although unambiguous, an amino acid can be coded for by more than one codon.
• Punctuated: certain codons specify “start” and “stop”.• Universal: by viruses, both prokaryotic domains, and
eukaryotes (except for some protozoa, mitochondria).• Ordered: similar codons specify the same amino acid; see