Chapter 9: Microbial Genetics: Central Dogma

Front 1

Genetics

Back 1

The science of heredity, how traits are passed from generation to generation

Front 2

Genome

Back 2

A cells genetic information (usually DNA)

Front 3

Chromosomes

Back 3

Structures that contain DNA

Front 4

Genes

Back 4

segments of DNA (generally) that code for functional products

Front 5

Genotype

Back 5

the genes of an organism (potential properties)

Front 6

Phenotype

Back 6

actual expressed properties (phenotype is the manifestation of genotype)

Front 7

DNA and chromosomes (Figure 9.3)

Back 7

Bacterial chromosome
Length of the E. coli chromosome = 1.7 millimeters
Length of the E. coli cell = 2 micrometers
DNA is supercoiled (topoisomerase: DNA gyrase)

Front 8

DNA (Figure 9.4)

Back 8

Polymer of nucleotides: adenine, thymine, cytosine, guanine
"Backbone" is deoxyribose-phosphate
Strands held together by hydrogen bonds between AT and CG
Strands are directional, complementary and antiparallel

Front 9

DNA replication

Back 9

One "parental" double stranded DNA molecule to two identical "daughter" molecules (semi-conservative)
How
- DNA unwinds and strands are separated (replication fork)
- Free nucleotides are matched to exposed bases
- DNA polymerase joins nucleotides
- New and original strands rewind

Front 10

Addition of nucleotides

Back 10

DNA polymerase only adds nucleotides to the 3' end of DNA
Synthesis requires energy (gained from hydrolysis of phosphate bonds)

Front 11

Events at the replication fork (Figure 9.6d)

Back 11

Helicase separates the double-stranded molecule and the replication fork forms
Synthesis of the leading strand proceeds continuously (DNA polymerase III)
Synthesis of the lagging strand is discontinuous (~1000bp) at a time (Okazaki fragments)
- Primase synthesizes RNA primers
- DNA polymerase III adds nucleotides onto the 3' end of the RNA primers
- DNA polymerase I replaces the RNA primers with deoxynucleotides
- DNA ligase seals the gaps between adjacent fragments

Front 12

Replication of bacterial DNA (Figure 9.6)

Back 12

Bidirectional replication (two replication forks)
Following replication
- Each origin binds to opposite poles of the membrane
- Each daughter cell receives one DNA molecule

Front 13

The Central Dogma (Figure 9.9)

Back 13

Information in DNA is transcribed into RNA
Information in RNA is translated into proteins

Front 14

Transcription (Figure 9.12)

Back 14

DNA is transcribed to make RNA (mRNA, tRNA, and rRNA)
RNA is like DNA except the sugar (ribose not deoxyribose) and uses uracil (U) instead of T
Begins when RNA polymerase binds to the promoter sequence
Proceeds in the 5'-3' direction
Stops when RNA polymerase reaches the terminator sequence
Makes an RNA copy of the DNA sequence for a gene or genes

Front 15

Translation (Figure 9.13)

Back 15

mRNA is translated in codons (3 nucleotides)
Each codon specifies an amino acid in the protein
The genetic code is degenerate, that is more than one codon codes for one amino acid
Translation occurs on ribosomes (organelles composed of protein + rRNA)
Starts (AUG) and stops (UAA, UAG, UGA)
tRNAs used to translate codons into amino acid sequence

Front 16

Steps of Translation

Back 16

1) The first codon ATG is recognized by the tRNA carrying formyl methionine. This tRNA moves to the P site and the second tRNA moves into the A site
2) An enzyme catalyzes the formation of a peptide bond between amino acids 1 and 2 on tRNA 2
3)The empty tRNA 1 is released from via the E site. The mRNA moves along one codon so that tRNA 2 moves from the A site to the P site
4) The third tRNA then enters the A site using its anticodon to recognize the codon
5) A peptide bond is formed between the growing peptide chain and amino acids 3 on tRNA 3
6) The empty tRNA 2 leaves the ribosome through the E site. tRNA 3 with the peptide attached moves the P site. Codon 4 on the mRNA moves into the A site and is recognized by the anticodon on tRNA 4
7) Amino acid 4 is added to the peptide chain on tRNA 4. The process is repeated for each new amino acid added until a stop codon moves into the A site

Front 17

When a stop codon enters the A site

Back 17

- Signals an enzyme to cleave the polypeptide from the final tRNA
- Components are then released for another round of translation

Front 18

The protein assembly line (Figure 9.17)

Back 18

Bacterial cells can translate mRNA molecules into proteins as they are being transcribed
Allows rapid response of protein production in response to changing conditions

Front 19

Summary of the central dogma (Figure 9.15)

Back 19

Double stranded RNA is transcribed to single stranded RNA
Codons on the RNA are translated in protein sequence