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

2DNA structure

• double helix

• Sugar-phosphate backbone

• A-T, C-G base pairs

• 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.

6DNA replication

• 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 of each DNA strand is 5’ 3’

• Replication is semi-conservative– New DNA molecules made of one old, one new strand.

7DNA replication figures

Because of requirement for 5’to 3’ synthesis, lagging strand must repeatedly top and start; needs an RNA primer each time.

8When 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.

9Genotype 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.

10About RNA

genetics.gsk.com/graphics/ dna-big.gif http://www.fhi-berlin.mpg.de/th/JG/RNA.jpghttp://www.santafe.edu/images/rna.gif

1) DNA is double stranded, but RNA is single stranded.

However, RNA can base-pair with itself to create double stranded regions.

DNA

RNA

tRNA

11About RNA-2

www.layevangelism.com/.../ deoxyribose.htm http://www.rothamsted.bbsrc.ac.uk/notebook/courses/guide/images/uracil.gif

2) RNA contains ribose instead of deoxyribose

3) RNA contains uracil instead of thymine.

123 kinds of RNA

http://www.cu.lu/labext/rcms/cppe/traducti/tjpeg/trna.jpeg;

Tobin and Duschek, Asking About Life; http://www.tokyo-ed.ac.jp/genet/mutation/nort.gif

mRNA: a copy of the gene; is translated to make protein.

tRNA: smallest RNA, does actual decoding.

rRNA: 3 sizes that, along with proteins, make up a ribosome.

tRNArRNA

13Some enzymes of molecular biology

• DNA polymerases– Synthesize DNA; operate in 5’→3’ direction only

• RNA polymerases– Synthesize RNA by “copying” DNA

• DNA ligase– Enzyme that connects adjacent 3’-OH and 5’ phosphate

• Exonuclease– Enzyme that cuts bases from the end of a strand

• Endonuclease– Makes internal cuts in a DNA or RNA molecule– Restriction endonuclease: cuts at a specific internal

sequence.

14Methylation

• 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.