1. Molecular Biology Fifth Edition
Lecture PowerPoint to accompany
Molecular Biology Fifth Edition
Robert F. Weaver
Chapter 1
A Brief History
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. A Brief History What is molecular biology?
The attempt to understand biological phenomena in molecular terms
The study of gene structure and function at the molecular level
Molecular biology is a melding of aspects of genetics and biochemistry

3. 1.1 Transmission Genetics
Transmission genetics deals with the transmission of traits from parental organisms to their offspring
The chemical composition of genes was not known until 1944
Gene - genetic units
Phenotype - observable characteristics

4. Mendel’s Laws of Inheritance
A gene can exist in different forms called alleles
One allele can be dominant over the other, recessive, allele
The first filial generation (F1) contains offspring of the original parents
If each parent carries two copies of a gene, the parents are diploid for that gene

5. Mendel’s Laws of Inheritance
Homoozygotes have two copies of the same allele
Heterozygotes have one copy of each allele
Parents in 1st mating are homozygotes, having 2 copies of one allele
Sex cells, or gametes, are haploid, containing only 1 copy of each gene
Heterozygotes produce gametes having either allele
Homozygotes produce gametes having only one allele

6. Summary Genes can exist in several different forms or alleles
One allele can be dominant over the other, so heterozygotes having two different alleles of one gene will generally exhibit the characteristic dictated by the dominant allele
The recessive allele is not lost; it can still exert its influence when paired with another recessive allele in a homozygote

7. The Chromosome Theory of Inheritance
Chromosomes are discrete physical entities that carry genes
Thomas Hunt Morgan used the fruit fly, Drosophila melanogaster, to study genetics
Autosomes occur in pairs in a given individual (not the X or the Y chromosome)
Sex chromosomes are identified as X and Y
Females have two X chromosomes
Males have one X and one Y chromosome

8. Location of Genes on a Chromosome
Every gene has its place, or locus, on a chromosome
Genotype is the combination of alleles found in an organism
Phenotype is the visible expression of the genotype
Wild-type phenotype is the most common or generally accepted standard
Mutant alleles are usually recessive

9. Genetic Recombination and Mapping
In early experiments genes on separate chromosomes behaved independently
Genes on the same chromosome behaved as if they were linked
This genetic linkage is not absolute
Offspring show new combinations of alleles not seen in the parents when recombination occurs

10. Recombination
During meiosis, gamete formation, crossing over can occur resulting in the exchange of genes between the two homologous chromosomes
The result of the crossing-over event produces a new combination of alleles
This process is called recombination

11. Genetic Mapping
Morgan proposed that the farther apart two genes are on a chromosome, the more likely they are to recombine
If two loci recombine with a frequency of 1%, they are said to be separated by a map distance of one centimorgan (named for Morgan)
This mapping observation applies both to prokaryotes and to eukaryotes

12. Physical Evidence for Recombination
Microscopic examination of the maize chromosome provided direct physical observation of recombination using easily identifiable features of one chromosome
Similar observations were made in Drosophila
Recombination was detected both physically and genetically in both animals and plants

13. Summary
The chromosome theory of inheritance holds that genes are arranged in linear fashion on chromosomes
Certain traits tend to be inherited together when the genes for those traits are on the same chromosome
Recombination between two homologous chromosomes during meiosis can scramble the parental alleles to yield nonparental combinations
The farther apart two genes are on a chromosome the more likely it is that recombination will occur

14. 1.2 Molecular Genetics
The Discovery of DNA: The general structure of nucleic acids was discovered by the end of the 19th century
Long polymers or chains of nucleotides
Nucleotides are linked by sugars through phosphate groups
Composition of Genes: DNA? RNA? Protein? In 1944, Avery and his colleagues demonstrated that genes are composed of DNA

15. The Relationship between Genes and Proteins
Experiments have shown that a defective gene gives a defective or absent enzyme
This lead to the proposal that one gene is responsible for making one enzyme
Proposal not quite correct for 3 reasons:
One enzyme may be composed of several polypeptides, each gene codes for only one polypeptide
Many genes code for non-enzyme proteins
End products of some genes are not polypeptides (i.e. tRNA, rRNA)

16. Activities of Genes Genes perform three major roles
Replicated faithfully
Direct the production of RNAs and proteins
Accumulate mutations thereby allowing for evolution

17. Replication
Franklin and Wilkins produced x-ray diffraction data on DNA, Watson and Crick proposed that DNA is double helix
Two DNA strands wound around each other
Strands are complementary – if you know the sequence of one strand, you automatically know the sequence of the other strand
Semiconservative replication keeps one strand of the parental double helix conserved in each of the daughter double helices

18. Genes Direct the Production of Polypeptides
Gene expression is the process by which a gene product is made
Two steps are required
1. Transcription: DNA is transcribed into RNA
2. Translation: the mRNA is read or translated to assemble a protein
Codon: a sequence of 3 nucleic acid bases that code for one amino acid within the mRNA

19. Genes Accumulate Mutations
Genes change in several ways
Change one base to another
Deletions of one base up to a large segment
Insertions of one base up to a large segment
The more drastic the change, the more likely it is that the gene or genes involved will be totally inactivated

20. Summary All cellular genes are made of DNA arranged in a double helix
This structure explains how genes replicate, carry information and collect mutations
The sequence of nucleotides in a gene is a genetic code that carries the information for making an RNA
A change in the sequence of bases constitutes and mutation, which can change the sequence of amino acids in the genes polypeptide product

21. 1.3 The Three Domains of Life
Current research theories support the division of living organisms into three domains
Bacteria
Eukaryota
Archaea
Like bacteria as they are organisms without nuclei
More similar to eukaryotes in the context of their molecular biology

22. Archaea Archaea live in the most inhospitable regions of the earth
Thermophiles tolerate extremely high temperatures
Halophiles tolerate very high salt concentrations
Methanogens produce methane as a by-product of metabolism