Transcript Lecture 8

Chapter 8
Microbial Genetics
part A
Life in term of Biology
– Growth of organisms
• Metabolism is the sum of all chemical reactions that occur in
living organisms to maintain life.
– determined by the enzymes present in the cells
– DNA carry the information for enzymes synthesis
– Multiplication of organisms -increase number of the population
• Heredity – save the characteristics of the species
• DNA hold the information to build and maintain the
cells and pass genetic traits to offspring
How DNA carry information?
• DNA molecules in the cells exist as a double-stranded helix;
– Two molecules form the double helix
• A hydrogen bonds with T (2 hydrogen bonds)
• C hydrogen bonds with G (3 hydrogen bonds)
• Four bases A,T,C, and G – the four letters of genetic language
• The linear sequence of bases
provides the actual information
DNA - double helix structure - Chromosome
• A chromosome is an organized structure of DNA and protein that is
found in cells.
– It is a single piece of coiled DNA
– Contain DNA-bound proteins, which serve to package the DNA and control
its functions.
• In eukaryotic - linear molecules associated with histones and
various proteins that regulate genetic activity
– Different organisms have different number of chromosomes (2n)
• Human – 46, Yeast - 32, Dog – 78
• In bacteria - one chromosome which is circular structure,
associated with different proteins (no histones)
• The chromosome of E. coli, for example, contains
about 4 million base pairs and is approximately 1000
times longer than the cell.
What genes are ?
• A gene is the basic unit of heredity in a living organism.
• A gene specifies a trait of the organism - thousands of basic
biochemical processes that comprise life
• Gene - Segment of DNA (sequence of bases) that encodes a
functional product
- A protein
- Functional RNA – an RNA molecule that is not
translated into a protein
- Transfer RNA (tRNA)
- Ribosomal RNA (rRNA),
- Smal RNAs (siRNA)
Gene Structure
• Gene - contains both:
– "coding" sequences that determine what the gene does (sequences that are
transcribed into mRNA molecule)
– "non-coding" sequences that determine when the gene is active (expressed)
Gene
DNA
Promoter
Regulatory
region
Non–coding sequences
• Not all of the DNA encode genes
Coding sequence
Terminology
• Genetics: The study of what genes are, how they carry
information, how information is expressed, and how genes are
replicated.
• Chromosome: Structure containing DNA that physically carries
hereditary information; the chromosomes contain the genes
• Gene: A segment of DNA that encodes a functional product, usually
a protein
• Genome: All the genetic information in a cell (entire DNA)
• Genotype: All genes of an organism. The genetic composition of an
organism.
• Phenotype: The expression of the genes, the proteins of the cell
and the properties they confer on the organism
The Flow of Genetic Information
Parent cell
DNA
expression
recombination
Genetic information is used within a cell to
produce the proteins needed for the cell to
function.
Transcription
Genetic information can be
Insert
Fig 8.2
transferred between cells
of the same generation.
replication
Genetic information can
be transferred between
generations of cells.
New combinations
of genes
Translation
Cell metabolizes and grows
Recombinant cell
Daughter cells
The Flow of Genetic Information
Parent cell
DNA
expression
recombination
Vertical flow of genetic information
Genetic information is used within a cell to
produce the proteins needed for the cell to
function.
Genetic information can be
Insert
Fig 8.2
transferred between cells
• Replication - The DNA in a cell isof the
duplicated
same generation.
before the cell divides, so each daughter cell
receives the same genetic
information
New combinations
replication
Genetic information can
be transferred between
generations of cells.
of genes
Transcription
Replication
Translation
DNA
Cell metabolizes and grows
DNA
Recombinant cell
Daughter cells
Vertical flow of genetic information (Replication)
is the basis for biological inheritance
• The process starts with one double-stranded DNA molecule and
produces two identical copies of the molecule.
Replication
1 DNA molecule
DNA polymerase
2 identical DNA molecules
• Each strand of the original double-stranded
DNA molecule serves as template for the
production of the complementary strand
• Because each daughter double-stranded DNA
molecule contains one original and one new strand, the replication
process is called semi conservative
DNA
• Polymer of nucleotides:
Adenine, Thymine, Cytosine,
and Guanine
• "Backbone" is deoxyribosephosphate
• Strands are held together by
hydrogen bonds between
A-T and C-G
• Strands are antiparallel
5’…..AAGCTTA…. 3’
3’…..TTCGAAT…. 5’
Figure 8.3b
Figure 8.4
DNA replication
5’ end
5’ end
3’ end
3’ end
3’ end
3’ end
5’ end
5’ end
Enzyme – DNA polymerase – add nucleotides in 5’ 3’ direction
DNA replication
Figure 8.6
DNA replication
• DNA replication begins when enzyme helicase unwinds a segment
of the DNA and breaks the hydrogen bonds between the two
complementary strands of DNA.
• Replication fork - the junction where the double-stranded DNA
splits apart into 2 single strands
• DNA is copied by enzyme DNA polymerase in the 5  3 direction
– Leading strand synthesized continuously
– Lagging strand synthesized discontinuously
• Initiated by an RNA primer
• Okazaki fragments
• RNA primers are removed and Okazaki fragments joined by a DNA
polymerase and DNA ligase
• DNA polymerase makes mistakes with frequency 10-9 bases mutation
Bacterial DNA replication
• Bacterial chromosome is circular
• Replication is initiated at a particular sequence in a genome - origin
of replication.
• Forms two replication forks
• DNA replication proceed in two directions – Bidirectional
Figure 8.6b
Figure 8.7
The Flow of Genetic Information
Parent cell
DNA
Flow of genetic information within a cell –
when
a gene is expressed:replication
recombination
• DNA is transcribed to produce RNA
expression
Genetic information is used within a cell to
produce the proteins needed for the cell to
function.
Genetic information can
Genetic information can be
Insert
Fig 8.2
(mRNA,
rRNA, tRNA, siRNA)
be transferred between
transferred between cells
generations of cells.
of the same generation.
• mRNA is then translated into proteins.
Transcription
New combinations
of genes
Transcription
Translation
Cell metabolizes and grows
Translation
DNA
RNA
Double stranded
Single stranded
Recombinant cell
Protein
Daughter cells
Chain of amino
acids
1. Transcription
RNA polymerase
DNA
RNA
mRNA
tRNA
rRNA
siRNA
• DNA transcription is a process that involves the transcribing of
genetic information from DNA to RNA
– DNA gene sequence is a template for synthesis of RNA
• RNA polymerase - enzyme responsible for the transcription of
DNA
• Only genes are transcribed
Transcription
GENE sequence
DNA
RNA
Promoter sequence

coding sequence 
terminator sequence
AUG……………………………
• Transcription begins when RNA polymerase binds
to the specific DNA sequence of the gene promoter
• Transcription proceeds in the 5  3 direction of
RNA sequence
• Complementary base are A-U (UTP) and G-C
• Only one of the DNA strands is transcribed
DNA 3’….. AATTACGACCCAATTGAGGC …. 5’ antisense strand
RNA
5’AUGUGGGUUAACUCCG….. 3’
DNA 5’…. TTAATGTGGGTTAACTCCG……3’ sense strand
• Transcription stops when it reaches the terminator
sequence
Eukaryotic mRNA
• Transcription - in the nucleus
• Coding sequence of eukaryotic genes consist of:
–
Exons – code for amino acids order in proteins
– Introns – no coding sequence
• mRNA is synthesize as a precursor (exons + introns) and undergo
splicing (processing, introns are deleted and exons are connected)
– One mRNA – more then one protein
– Variability
• One gene → more than one mRNA→
more than one protein
Nucleus
Bacterial mRNA
• Bacterial genes don’t have introns
• Bacterial mRNA does not undergo splicing
• One bacterial mRNA could carry sequences for more then one
protein (usually with related metabolic functions)
• One promoter - One mRNA
Promoter sequence

coding sequence 
terminator sequence
DNA
mRNA
AUG……..….AUG…………..AUG………..
protein1
protein2
protein3
(enzyme1)
(enzyme2)
(enzyme3)
• DNA – gene with specific sequence of bases (DNA letters)
A, T, G, C
• RNA – complementary to the DNA with bases (RNA letters)
A, U, G, C
Translation
Transcription
Gene expression and genetic language
• Protein – sequence of amino acids ( only 20 amino acids)
•
Genetic code
• –Variability 4,
42=16, 43=64
• Codon - three-base segments of mRNA that specify amino
acids.
• All organisms have the same codons to specify the particular
amino acid
Genetic code
• The genetic code is degenerate;
– most amino acids are coded for by more than
one codon.
•
Of the 64 codons:
– 61 are sense codons
(which code for amino acids),
– 3 are nonsense codons
(which do not code for amino acids)
are stop signals for translation.
• The start codon, AUG, codes for methionine.
Protein Met - Phe -Ser - Arg……Val
mRNA AUGUUUUCCAGG…GUGUGA
Start
Stop
One codon: Met, Trp.
Two codons: Asn, Asp, Cys, Gln, Glu, His, Lys, Phe, Tyr,
Three codons: Ile, STOP ("nonsense").
Four codons: Ala, Gly, Pro, Thr, Val.
Five codons: none.
Six codons: Arg, Leu, Ser.
Figure 8.9
Translation
• Translation is the process in which the information in the nucleotide
base sequence of mRNA (codons of mRNA) is converted to the
order of amino acid sequence of a protein.
• Transfer RNA (tRNA) - a small RNA, each containing about 80
nucleotides.
– A tRNA molecules has two functional sites:
• Recognize a specific codon (anticodon sequence)
– For each sense codon these is a tRNA with complementary antisense codon
• Binds to a specific amino acid (at 3’ end)
– Transport the required amino acid to the ribosomes
• The site of translation is the ribosome.
Figure 8.2
•
•
•
•
mRNA
Ribosomal subunits
Met-tRNA
Amino acid
Figure 8.9, step 1
Translation
1. Initiation of translation – AUG codon – Met tRNA
2. Elongation – protein synthesis
3. Termination – Stop codon
Translation
• In Eukaryotic cells:
– Transcription – nucleus
– Translation – EPR
• In Bacterial cells there is no nucleus
– The transcription and translation processes go simultaneously.
mRNA
AUG……..UAA… AUG…….…UAA AUG.……UAA..
protein1
(enzyme1)
protein2
(enzyme2)
protein3
(enzyme3)
Gene expression and cell energy
• Genes, through transcription and translation, direct the synthesis of
proteins, many of which serve as enzymes
• The very enzymes used for cellular metabolism.
• Protein synthesis requires a tremendous expenditure of energy
• The regulation of protein synthesis is important to the cell’s energy
economy.
The cell conserves energy by making only those proteins
needed at a particular time
• There are 2 classes of genes
– Structural genes
• genes that code for any protein or RNA molecules that are required for normal enzymatic or structural
functions in the cell
– Regulatory genes
• genes that code for protein and RNA molecules whose function is to regulate the expression of other
genes
The Operon model of gene expression
• GENE sequence
Operon
DNA
Promoter Operator
Coding sequence
(regulatory region)
non–coding sequences
Bacteria
mRNA
AUG……..Stop….AUG…..Stop…..AUG……Stop
protein1
protein2
protein3
(enzyme1)
(enzyme2)
(enzyme3)
• In bacteria - a group of coordinately regulated structural genes with
related metabolic functions is organized as a unit - Operon
– Promoter (sequence), operator (sequence) and structural genes ( sequence)
are called an operon.
• The promoter and operator are sites that control structural gene transcription.
• Structural genes are expressed as a single messenger RNA.
The Operon model of gene expression
• Promoter is the site to which RNA binds
• Expression of structural genes is regulated by a regulatory protein - repressor
protein
• In the operon model a regulatory gene codes for the repressor protein
• The repressor protein acts by binding to a site on the DNA.
• The site on the DNA to which the repressor protein binds is called an Operator.
• Activity of the repressor protein depends on the presence or absence of an Effector
substance.
Lac
Tryptophan
Figure 8.14.1
Learning objectives
• Define genetics, genome, chromosome, gene, genetic
code, genotype, phenotype, and genomics.
• Describe how DNA serves as genetic information.
• Describe the process of DNA replication.
• Describe protein synthesis, including transcription,
RNA processing, and translation.
• Describe the operon model of gene expression