1. Molecular Biology of the Gene
Chapter 10
Molecular Biology of the Gene

2. Gene: a discrete unit of hereditary information consisting of a specific nucleotide sequence in DNA (or RNA, in some viruses). Most of the genes of a eukaryote are located in its chromosomal DNA; a few are carried by the DNA of mitochondria and chloroplasts.
Gene: portion of a chromosome responsible for the expression of a character
Bacteriaphages (or phages for short): viruses that infect bacterial cells

3. 10.2 DNA & RNA are Polymers of Nucleotides
DNA and RNA are nucleic acids
One of the strands of DNA is a DNA polynucleotide, a nucleotide polymer (chain)
A Nucleotide is composed
- Nitrogenous Base
- 5-Carbon Sugar
- Phosphate Group
The nucleotides are joined to one another by a sugar-phosphate backbone
Each type of DNA nucleotide has a different Nitrogen-containing base
- Adenine (A)
- Cytosine (C)
- Thymine (T)
- Guanine (G)
RNA (Ribonucleic Acid)
- uses sugar ribose (instead of deoxyribose in DNA)
- has a Nitrogenous base Uracil (U) instead of Thymine

5. 10.4

6. DNA Replication

7. 10.5 DNA Replication proceeds in 2 directions at many sites simultaneously
DNA replication begins at the origins of replication where
DNA unwinds at the origin to produce a “bubble”
Replication proceeds in both directions from the origin and
Replication ends when products from the bubbles merge with each other
DNA replication occurs in the 5’ to 3’ direction
Replication is continuous on the 3’ to 5’ template
Replication is discontinuous on the 5’ to 3’ template, forming short segments

8. 10.5 DNA Replication proceeds in 2 directions at many sites simultaneously
2 key proteins are involved in DNA replication
1. DNA Ligase joins small fragments into a continuous chain
2. DNA Polymerase
Adds nucleotides to a growing chain
Proofreads and corrects improper base pairings
DNA polymerases and DNA Ligase also repair DNA damaged by radiation and toxic chemicals
DNA replication ensures that all the somatic cells (all cells except for sex cells) in a multicellular organism carry the same genetic information

9. 10.5 B

10. The flow of Genetic info from DNA to RNA to Protein

11. 10.6 The DNA genotype is expressed as proteins, which provide the molecular basis for genotypic traits
DNA specifies traits by dictating protein synthesis
The molecular chain of command is from
DNA in nucleus to RNA and
RNA in cytoplasm to protein
Transcription: synthesis of RNA under the direction of DNA
Translation: synthesis of proteins under the direction of RNA

13. 10.7 Genetic info written in codons is translated into amino acid sequences
The sequence of nucleotides in DNA provides a code for constructing a protein
Protein construction requires a conversion of a nucleotide sequence to an amino acid sequence
Transcription rewrites the DNA code into RNA, using the same nucleotide ‘language’
The flow of info from gene to protein is based on a Triplet Code: the genetic instructions for the amino acid sequence of a polypeptide chain are written in DNA and RNA as a series of non-overlapping 3-base “words” called codons
Translation involves switching from the nucleotide “language” to the amino acid “language”
Each amino acid is specified by a codon
64 codons are possible
Some amino acids have more than one possible codon

14. 10.7

15. 10.8 The genetic code dictates how codons are translated into amino acids
Characteristics of the Genetic Code
3 nucleotides specify one amino acid
61 codons correspond to amino acids
AUG codes for methionine and signals the start of transcription. This is one of the 61 codons above
The genetic code is
Redundant, with more than one codon for some amino acid
Unambiguous, in that any codon for one amino acid does not code for any other amino acid
Nearly universal, the genetic code is shared by organisms from the simplest bacteria to the most complex plants and animals
Without punctuation in that codons are adjacent to each other, with no gaps in between

16. 10.9 Transcription produces genetic messages in the form of DNA
Over view of Transcription:
An RNA molecule is transcribed from a DNA template by a process that resembles the synthesis of a DNA strand during DNA replication
RNA nucleotides are linked by the transcription enzyme RNA Polymerase
Specific sequences of nucleotides along the DNA mark where transcription begins and ends
The “start transcribing” signal is a nucleotide sequence called a promotor
Transcription begins with initiation as the RNA Polymerase attaches to promotor
During 2nd phase, elongation, the RNA grows longer
As the RNA reels away, the DNA strands rejoin
Finally the 3rd phase, termination, the RNA polymerase reaches a sequence of bases in the DNA template called a Terminator, which signals the end of the gene
The polymerase molecule now detaches from the RNA molecule and the gene

17. 10.10Eukaryotic RNA is processed before leaving the nucleus as mRNA
Messenger RNA (mRNA)
Encodes amino acid sequences
Conveys genetic messages from DNA to the translation machinery of the cell, which in
Prokaryotes occurs in the same place that mRNA
Eukaryotes, mRNA must exit the nucleus via nuclear pores to enter the cytoplasm
Eukaryotic mRNA has
Introns, interrupting sequences that separate
Exons, the coding regions
Eukaryotic mRNA undergoes processing before leaving the nucleus
RNA splicing removes introns and joins exons to produce a continuous coding sequence
A cap and tail of extra nucleotides are added to the ends of the mRNA to
Facilitate the export of the mRNA from the nucleus
Protect the mRNA from attack by cellular enzymes
Help ribosomes bind to mRNA

18. 10.11 Transfer RNA molecules serve as interpreters during translation
Transfer RNA (tRNA) molecules function as a language interpreter
Converting the genetic message of RNA
Into the language of proteins
Transfer RNA molecules perform this task by
Picking up the appropriate amino acid and
Using a special triplet of bases, called an anticodon, to recognize the appropriate codons in the mRNA

19. 10.12 Ribosomes build Polypeptides
Translation occurs on the surface of the ribosome
Ribosomes coordinate the functioning of mRNA and tRNA and, ultimately, the synthesis of polypeptides
Ribosomes have 2 subunits: small and large
Each subunit is composed of ribosomal RNAs and proteins
Ribosomal subunits come together during translation
Ribosomes have binding sites for mRNA and tRNAs

20. 10.13 An initiation codon marks the start of an mRNA message
Translation can be divided into the same 3 phases as Transcription
Initiation
Elongation
Termination
Initiation brings together
mRNA
A TRNA bearing the first amino acid and
The 2 subunits of a ribosome
Initiation establishes where translation will begin

21. 10.13 An initiation codon marks the start of an mRNA message
Initiation occurs in 2 steps
An mRNA molecule binds to a small ribosomal subunit and the first tRNA binds to mRNA at the start codon
The start codon reads AUG and codes for Methionine
The 1st tRNA has the anticodon UAC
A large ribosomal subunit joins the small subunit, allowing the ribosome to function
The 1st tRNA occupies the P Site, which will hold the growing peptide chain
The A Site is available to receive the next tRNA

22. 10.14 Elongation adds amino acids to the polypeptide chain until a stop codon terminates translation
Once initiation is completed, amino acids are added one by one to the 1st amino acid
Elongation is the addition of amino acids to the polypeptide chain

23. 10.14 Each cycle of elongation has 3 steps:
Codon Recognition: the anticodon of an incoming tRNA molecule; carrying its amino acid, pairs with the mRNA codon in the A site of the ribosome
Peptide Bond Formation: the new amino acid is joined to the chain
Translocation: tRNA is released from the P site and the ribosome makes tRNA from the A site into the P site
Elongation continues until the termination stage of translation when:
- the ribosome reaches a stop codon

24. 10.15 REVIEW: Flow of genetic info in cell is DNA RNA Protein
Transcription is the synthesis of RNA from a DNA template.
In Eukaryotic cells:
Transcription occurs in the nucleus
The mRNA travels from nucleus to cytoplasm
Translation = 4 steps. It occurs in the Cytoplasm
Amino acid attachment
Initiation of polypeptide synthesis
Elongation
Termination

25. 10.6 Mutations can change the meaning of jeans
A mutation is any change in the nucleotide sequence of DNA
A mutation can involve
Large chromosomal regions or
Just a single nucleotide pair
Mutations within a gene can be divided into 2 categories
Base Substitutions involve the replacement of one nucleotide w/ another. Base substitutions may:
Have no effect at all, producing a silent mutation
Change the amino acid coding producing a Missense mutation, which produces a different amino acid
Lead to a base substitution that produces an improved protein that enhances the success of the mutant organism and its descendent or
Charge an amino acid into a stop codon, producing a nonsense mutation

26. 10.6 Mutations can change the meaning of jeans
2. Mutations can result in deletions or insertions that may
Alter the reading frame (triplet grouping) of the mRNA, so that nucleotides are grouped into different codons
Lead to significant changes in amino acid sequence downstream of the mutation and
Produce a non-functional polypeptide