Download Chapter 2 - The Structure of Genes

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CHAPTER 2 – THE STRUCTURE OF GENES AND GENOMES
Questions to be addressed in this chapter:
1. What is the genetic material?
2. What is the structure of DNA?
3. How are genes organized?
4. How is DNA packaged into the cell
Terminology (see also Glossary pages 655-681 and web site
http://helios.bto.ed.ac.uk/bto/glossary/)
Virulent – a disease producing organism
Gene - the fundamental unit of heredity (in molecular terms, a transcribed segment of DNA
and the region which enables transcription)
genetic symbol: a symbol designating a gene which usually relates to the characteristic
controlled by that gene
intron – (intervening sequence) a segment of the gene that is initially transcribed, but is not in
the final mRNA product
exon – a segment of the gene that is translated into the protein
genome – the complement of genetic material in the chromosomes
haploid – a cell or individual having one chromosome set
diploid – a cell or individual having two chromosome sets
plasmid – autonomously replicating extrachromosomal DNA
organelle – a subcellular structure (e.g. chloroplast, vacuole, mitochondrion)
centromere – part of chromosome to which the spindle fibres attach via the kinetochore
spindle fibres – microtubule fibres that move the chromosomes during cell division
kinetochore – a set of proteins that bind to the centromere and to which the spindle fibres
attach
heterochromatin – densely staining, highly condensed regions of the chromosome
nucleosome – 8 histone molecules wrapped in DNA
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1. What is the genetic material (Pages 30-31)?
Protein, RNA, DNA…?
Frederick Griffith, 1928: What causes transformation from avirulent to virulent?
Box 2-1
Hypothesis: a heritable substance is present in the bacterium Pneumococcus that confers
virulence
Experiment: transform avirulent cells with an extract from dead virulent cells
Result: avirulent cells become virulent
Conclusion:
Oswald Avery, 1943: What compounds when eliminated eliminate transformation?
Box 2-1
Hypothesis: an isolated compound can confer virulence
Experiment: systematically eliminate each compound
Result: Dnase eliminates transforming capacity
Conclusion:
2. DNA STRUCTURE (Pages 26-29): James Watson and Francis Crick, 1953
Box 2-2
Question: what is the structure of DNA?
Experimental data:
1) DNA
= nucleotides = A, C, G, T (Figure 2.3)
= phosphate + deoxyribose sugar + base
2) Erwin Chargaff - ratio of nucleotides constant
C+T=A+G
A = T and C = G
3) Rosalind Franklin, Maurice Wilkins (Box 2-2)
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x-ray diffraction
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molecule is long and helical
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consists of two parallel parts running length of molecule
Conclusion: DNA is a double helix (Watson and Crick; Figure 2-4, Figure 2.5)
1) nucleotides linked by phosphodiester bonds
= sugar-phosphate backbone
2) backbones are linked through hydrogen bonds forming between bases (A+T, G+C)
3) backbones coil to form a helix
Question: how is DNA replicated (Page 32, Chapter 4)
Structure suggests mechanism
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hydrogen bonds between backbones
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H-bonds between specific bases
During replication:
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H-bonds break
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exposed bases form H-bonds with new bases, providing a template
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new molecule = one parent + one new
3. THE ORGANIZATION OF GENETIC MATERIAL (pages 32-48)
What is a gene? (pages 32-33)
GENE = A region of chromosomal DNA that can be transcribed into functional RNA at the
correct time and place during development.
Gene = region of DNA that is transcribed into RNA + associated regulatory regions
In prokaryotes: Initiation + coding + termination (Figure 2-6)
In eukaryotes, gene includes introns (transcribed, but not part of functional gene product) and
exons (transcribed, and remain in functional gene product)
-gene number, exon number, and gene size varies widely between species (Figure 2.7, 2.8,
Table 2.1)
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Genome: the entire complement of genetic material within the chromosomes
Genome size varies considerably between different organisms (Figure 2.10, Table 2-2)
Factors influencing genome size?
Viral genomes (Figure 2-11):
Virus: a non-living particle composed of a protein coat and a nucleic acid core
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incapable of self-replicating – requires entry into host cell
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genome may be DNA (single or double stranded) or RNA (single or double stranded)
Prokaryotic genomes (Figure 2-12):
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most genes are within a single, circular, double-stranded DNA chromosome
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genes occur in high density
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often a set of functionally related genes is transcribed as a single RNA molecule and
regulated by a single regulatory region
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genes + single regulatory region = operon
Eukaryotic genomes (Figure 2-13):
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most genes are within linear chromosomes within the nucleus
haploid (n) – one set of chromosomes per nucleus
diploid (2n) – two sets of chromosomes per nucleus
Plasmid (Figure 2-11): autonomously replicating extrachromosomal DNA
-contains non-essential genes (e.g. toxins, antibiotics, cell fusion)
Organellar DNA: DNA in mitochondria or chloroplasts
-genes important for organellar function
Characteristics of Eukaryotic Chromosomes (pages 41-44)
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size (Table 2-4)
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centromere position (telocentric, metacentric, acrocentric)
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positions of nucleolar organizers (rRNA genes) (Figure 2.16)
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chromomere patterns
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heterochromatin patterns (Figures 2.18, 2.19)
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banding patterns (Figures 2.18c)
4. Chromosome Packaging (pages 46-48)
Nucleosome (Figures 2.22, 2.26) = DNA wrapped around an octamer of proteins called
histones
Octamer = 2(H2A+H2B+H3+ H4)
Solenoid (Figures 2.22, 2.26) = coil of histones stabilized by histone H1
Supercoils (Figures 2.23, 2.24) = coil of solenoids held in place by scaffold proteins