Lake Tahoe Community College
Intro to Cell and Molecular Biology
Bio 110
Instructor: Sue Kloss
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Chapter 10 - Molecular Biology of the Gene
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Intro: Viruses - can not reproduce on their own so are not considered living; quasi- alive. Viruses are little more than protein
coats and DNA. They gain entry into our cells by fitting into the receptor molecules of our own cells. Once inside, they hijack
the cell’s machinery and causing the cell to reproduce many copies of itself. It does this by inserting its own DNA into the cell’s
DNA.
The study of the DNA molecule and its basis in heredity is called molecular biology.
I.
I. The Structure of Genetic Material
A. 1928- Frederick Griffith reported that chem.component of pneumonia-causing bacterium could transform a
harmless variety of bacterium and all descendents into harmful form
1. By 1930, most researchers agreed it was a specific type of molecule; focus on chromosomes which
at turn of century were known to carry heredity info
a. by 1940, scientists knew that chromosomes consisted of protein and DNA molecules
b. researchers thought it was proteins, much more diverse and complex than DNA
c. DNA seemed too simple to account for all diversity of organisms
B. DNA and RNA are polymers of nucleotides
1. DNA and RNA are nucleic acids
2. nucleotides are joined by covalent bonds btn. sugar of one nucleotide and phosphate of the next
3. sugar in DNA is deoxyribose, in RNA is ribose
4. phosphate group is the source of acid
5. nucleic acids are so named because they usually reside in the nucleus
6. nitrogenous bases are rings of nitrogen and carbon atoms with various functional groups attached
7. RNA has ribose instead of deoxyribose - it has one less oxygen atom
C. DNA is a double stranded helix - Race to find structure of DNA was huge in the science world - Franklin,
Watson and Crick set the science world on end with this discovery
1. A always pairs with T, C always pairs with G
2. bonds are hydrogen bonds, and these combos have chemical side groups that form H
3. purines always pair with pyrimidines
4. think of DNA model as a rope ladder with wooden rungs, twisted - uprights are the double helix
5. Fig. 10.3D - representations of DNA
6. structure of DNA contains the information needed to carry out all life processes
II. DNA Replication
A. DNA replication depends on specific base pairing
1. essential aspect of reproduction is that a complete set of genetic instructions is passed down from
one generation to the next
2. there must be a way to copy the instructions precisely
3. old genes could serve as template for the new genes, like a plaster cast for a face mask
4. template model for replicating DNA
5. since A always pairs with T and C with G, if you have one half of the molecule, you can figure out
the other half.
B. DNA replication: A closer look
1. DNA replication begins at specific sites on double helix called origins of replication
2. proteins that start the process attach to DNA, untwist the helix to form bubbles where replication
proceeds in both directions
3. (Fig. 10.5A) parent strands open and daughter strands (grey) elongate in each direction
4. parent strands have many origins of replication, shortening the process
5. thousands of bubbles can be present at once
6. eventually, all bubbles merge
7. 2 new daughter strands of DNA molecule
8. DNA’s sugar phosphate backbones run in opposite directions – antiparallel
9. enzymes that do the linking are called DNA polymerizes
10. DNA replication requires the cooperation of more than a dozen enzymes and other proteins
11. replication is fast and accurate- only about 1 mismatch/billion nucleotides linked
12. DNA ligase also “proofreads” - removes nucleotides that have been paired incorrectly
13. DNA polymerase and ligase also repair DNA harmed by radiation or toxic chemicals in environment
14. DNA replication ensures that all somatic cells have a full set of genetic instructions
15. Also allows DNA to continue to next generation in inheritance
III. Flow of Genetic Information from DNA to RNA to Protein
A. DNA genotype is expressed as proteins, which provide molecular basis for phenotypic traits
1. genotype is organisms genetic makeup, sequence of nucleotides
2. phenotype is expressed characteristics - organisms specific traits
3. proteins make up a major portion of the structure of our bodies.
4. enzymes catalyze all our metabolic activities.
5. DNA specifies the synthesis of proteins
6. Fig. 10.6a. DNA ______>RNA_______> proteins
transcription
translation
8. If a person lacks a particular enzyme, they can’t complete the pathway, and may lack substances they
need. The enzyme may be important in synthesis or breakdown of substances
9. inheriting a defective gene causes lack of particular enzymes.
10. Beadle and Taut 1 gene, 1 polypeptide.
IV. Flow of Genetic Information from DNA to RNA to Protein
B. Genetic Information written in codons is translated into protein sequences
1. In order to understand how transcription and translation work, we must understand how the chemical
language of DNA is transferred into the different chemical language of polypeptides or proteins
2. DNA and RNA are monomers that carry information in sequences of 4 nitrogenous bases
3. Both DNA and RNA use A,C, and G. DNA uses T, and in RNA, the 4th base is U.
4. a typical gene consists of hundreds or thousands of nucleotides; a molecule of DNA may have
thousands of genes; this figure just shows a short section of DNA nucleotides.
6. Under the dark blue DNA strand, you see pink RNA that complements DNA; this strand represents
the results of transcription
7. the language of DNA has been rewritten in the language of RNA; how do you know? You see the
base U instead of the base T
8. but the “language” is still in nucleic acids; RNA was synthesized using the DNA as template
9. The purple chain is written in the “new language” of polypeptides - the result of translation
10. polypeptides are made up of monomers we call amino acids
11. there are 20 amino acids that makeup the molecules of living things.
12. the sequence of nucleotides in RNA dictates the sequence of amino acids that will be linked to form the
polypeptide
13. remember that the RNA is only a messenger – it’s the DNA that determines the sequence, ultimately
14. there are only 4 nucleotide bases, and 20 amino acids; the 4 nucleotides if translated directly could only
account for 4 of the amino acids – so what is the trick?
15. there are brackets around each 3 nucleotides. Each set of three is called a codon
C. Genetic Code is the code of life
1. sometimes, two different codons can code for the same amino acid e.g. – UUU and UUC both code for
the same amino acid called phenylalanine
2. You can see that many of the codons specify for the same amino acid as other codons. Because of this,
the genetic code is said to be redundant.
3. It is not, however, ambiguous. Each codon only ever specifies for one possible amino acid.
4. So the code is said to be redundant but not ambiguous
5. Notice that AUG is the start codon, to start translation of an RNA molecule; UAA, UAG and UGA = stop
6. Translate the following mRNA sequence into a polypeptide sequence: AUGCAUCACAGAGGUUAG
7. there are no gaps in the code, and almost all organisms share the code – bacteria can translate human
DNA and humans can translate bacterial DNA.
D. Transcription produces genetic messages in the form of RNA
1. transcription is the transfer of information from DNA to RNA and it occurs in the cell nucleus
2. RNA transcription is very similar to DNA synthesis
3. area of DNA to be transcribed unwinds and separates
4. only 1 DNA strand serves as a template for transcription to RNA
5. RNA follows the same base pairing rules, except U is substituted for the T in DNA
6. RNA nucleotides are linked by an enzyme called RNA polymerase
7. Fig. 10.9b. There are 3 phases to transcription
a. Initiation
b. Elongation
c. Termination
E. Eukaryotic RNA is “processed” before it leaves the nucleus
1. mRNA (messenger RNA) is the type of RNA that encodes amino acid sequences
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mRNA takes info from the gene (a sequence of DNA) to the place where translation occurs
In prokaryotic cells, transcription and translation occur in the same place – no nucleus
in eukaryotic cells, the mRNA must leave the nucleus for translation to occur
before leaving the nucleus, mRNA is processed, or modified in several ways
In addition, parts of the DNA that are noncoding, called introns, are removed
coding regions of a gene are called exons
both introns and exons are transcribed from DNA into mRNA, but then the introns are removed and the
exons are spliced together to make an mRNA with a continuous coding sequence
9. to translate the mRNA into polypeptide sequences, we need:
F. Transfer RNA Molecules Serve as Interpreters During Translation
G. Ribosomes build Polypeptides
H. An Initiation Codon Marks the Start of an mRNA Message
1. Translation has the same 3 phases as transcription
a. initiation
b. elongation
c. termination
2. polypeptide initiation
3. Initiation
4. There are 2 steps in initiation
a. mRNA binds to a specific codon
b. large ribosomal subunit binds to the small one
I. Elongation
1. amino acids are added one at a time till translation is complete
2. Each amino acid that is added onto the growing chain does so in 3 steps
a. codon recognition
b. peptide bond formation
c. translocation .
3. stop codon reaches the A site
J. Mutations can change the meaning of genes
Chapter Homework Questions/Lesson Objectives
Due Wednesday Wk 9
1. Compare and contrast the structure of DNA and RNA.
2. Explain how the structure of DNA facilitates its replication.
3. Describe the process of DNA replication.
Due Monday Wk 10
4. Draw the locations, structures, substances and processes involved in Transcription and translation.
5. Explain the “languages” of DNA and RNA that are used to produce polypeptides. Describe how DNA actually provides the
amino acid sequence designated by a gene to form a particular polypeptide.
6. Explain how RNA is produced.
7. Explain how eukaryotic RNA is processed before leaving the nucleus.
8. Explain how tRNA functions in the process of translation.
9. Describe the structure and function of ribosomes.
10. Explain how translation begins.
11. Dscribe the step by step process by which amino acids are added to a growing polypeptide chain.
12. Describe the major types of mutations and their consequences.