Chap. 4 —
Genetics and Cellular
Function
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1. Critically read Chapter 4 up to page 129 right before
4.3 “DNA Replication and the Cell Cycle” section
2. Comprehend Terminology (those in bold in the textbook)
3. Study-Figure questions, Think About It questions, and Before You Go On (section-ending) questions
4. Do end-of-chapter question s:
– Testing Your Recall— 2, 4, 5, 6, 7, 18
–
True or False– 1, 2, 4-7
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•
General
– DNA– deoxyribonucleic acid
– Most human cells have 46 molecules of DNA
– A uniform diameter of 2 nm and the average length @ 2in.
•
Molecular level—
– Nucleic acids (DNA + RNA) are polymers of
__________________________
– A nucleotide consists of (1) ________ + (2) ________ +
(3) ___________________
– DNA is a double helix (@ spiral staircase)
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Fig. 4.1 a +b and 4.2

A nucleotide consists of three components
Adenine
NH
2
See next slide
HC
N
C
C
N
H
C
N
N
CH
O
HO P O CH
2
O
OH
H
H H
H
Phosphate
OH H
Deoxyribose
(a)
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Five nitrogenous bases:
Purines
(b)
NH
2 N
CH
C C
N NH
C
N C
H
Adenine (A)
O
N
HN
C C
C N
C
CH
NH
NH
2
Guanine (G)
Pyrimidines
HC
H
C C
NH
2
N
N
H
C
O
Cytosine (C)
O
CH
3
HC
C
O
C
NH
N
H
C
O
Thymine (T) T: Only in DNA
C
O
HN
C
N
H
CH
CH
Uracil (U) U: Only in RNA
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“Twisted ladder”
Space-filling model 7

• DNA = a double helix molecule; a spiral staircase; a soft rubber ladder that you can twist
•
Details:
–
Each sidepiece is a backbone-composed of phosphate groups alternating with the sugar deoxyribose.
–
Step-like connections-between the backbones are pairs of nitrogenous bases.
–
The arrangement of these nitrogenous bases–
How? (Next slide)
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Law of complementary base pairing :
•Base pairs (2 kinds):
– A-T and C-G
•Nitrogenous bases form hydrogen bonds
Segment of DNA
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• Carry instructions of genes for protein synthesis
•
A gene
– a segment of DNA that codes for one polypeptide (or closely related proteins)
– Genes determine the characteristics of a species and each individual
•
Genome - all the genes of one person
– humans have estimated 25,000-35,000 genes (2% of
DNA)
– The other 98% of DNA is noncoding – either “junk” or organizational DNA
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1. What would be the base sequence of the DNA strand across from
ATTGACTCG?
2. If a DNA molecule were known to be 20% adenine, predict its percentage of cytosine.
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1. Chroma tin — filamentous material making up 46 chromo somes (DNA and proteins) in the interphase nucleus
– Chromatin appears like “beads on a string” packed together (Fig. 4.2 a-f)
– The beaded string is divided into segments called nucleosomes ( consist of histones and linker DNA)
2. In dividing cells , DNA coils and supercoils itself to form chromosomes (can be seen with light microscope) . Fig. 4.5
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2 nm 1
DNA double helix
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Core particle
Linker DNA
Nucleosome
11 nm 2 DNA winds around core particles
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30 nm 3
Nucleosomes
Fold into zigzag fiber
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300 nm 4 fiber is thrown into irregular loops
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In dividing cells only
700 nm 5 looped chromatin coils further into a chromatid
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Chromatids Centromere
700 nm 6 Chromosome at the midpoint
(metaphase) of cell division
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(a)
Kinetochore
Centromere
Sister chromatids
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1. RNA-- much smaller than DNA (fewer bases)
A. messenger RNA (mRNA) has over 10,000 bases
B. ribosomal RNA (rRNA)
C. transfer RNA (tRNA), smallest, has 70 - 90 bases
( Fig. 4.8
)
– Are these bases ( of RNAs ) paired or unpaired?
2. Only one nucleotide chain (not a double helix)
– ribose replaces deoxyribose as the sugar
– uracil replaces thymine as a nitrogenous base
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• DNA directs the synthesis of proteins by means of its smaller cousins, the RNAs
• Essential function of RNA--
– interpret DNA code
– direct protein synthesis in the cytoplasm
•
(Location) RNA works mainly in the cytoplasm while DNA remains safely behind in the nucleus
•
Table 4.1 is an excellent summary
(Comparison of DNA/RNA)
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What are four nitrogenous bases found in RNA?
a) U, G, C, T; b) A, G, C, T c) A, U, G, C; d) A, T, G, C
In RNA, when does the secondary structure called a hairpin form?
a) When hydrophilic residues act with water b) When complementary base pairing between ribonucleotides on the same strand creates a stem-and-loop structure c) When complementary base pairing forms a double helix
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•
DNA codes for the synthesis of all cell proteins
– including enzymes that direct the synthesis of nonproteins
– For example,
Testosterone production
• Different cells synthesize different proteins
– Why?
–
Due to differing gene activation
• See Fig. 4.13 (next slide)
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= LH
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• DNA contains a genetic code that specifies which proteins a cell can make; protein synthesis as: DNA
 mRNA
 protein
1. Transcription (DNA
 mRNA ); What? Details?
– messenger RNA (mRNA) is formed next to an activated gene
– mRNA migrates to cytoplasm
2. Translation (mRNA
 protein ) (Fig. 4.7) What?
How?
– mRNA code is “read” by ribosomes
– transfer RNA (tRNA) delivers the amino acids to the ribosome
– Ribosomes assemble amino acids in the order . . .
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• Def.
-- System that enables the 4 nucleotides (A,T,G,C) to code for the 20 amino acids
•
Base triplet : ( of DNA ) Fig. 4.10
–
Def.– A sequence of 3 nucleotides that stand for 1 amino acid
– found on DNA molecule (ex. TAC codes for AUG in mRNA)
• Codon : (genetic code is expressed in terms of codons)
–
Def.--
“mirror-image” sequence of nucleotides found in mRNA (ex. AUG is the codon of mRNA, code for methionine, an amino acid ) (Table 4.2)
– 64 possible codons (4 3 )
• often 2-3 codons represent the same amino acid
• start codon = AUG
• 3 stop codons = UAG, UGA, UAA
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Seven base triplets
A
B
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i.
P (peptidyl) site— ii.
A (acceptor) site— iii. E (exit) site--
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• Watch a video-- An animation: protein synthesis, when available
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