ch. 12 & 13
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Ch. 12 & 13. DNA: what is it Where in cell cycle does DNA get replicated? Mutation What is transcription & translation? Define introns and exons What regulates gene expression?. 12.3 Structure of DNA. DNA is comprised of subunits called nucleotides each DNA nucleotide has three parts - PowerPoint PPT PresentationTRANSCRIPT
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Ch. 12 & 13Ch. 12 & 13
• DNA: what is itDNA: what is it
• Where in cell cycle does DNA get Where in cell cycle does DNA get replicated?replicated?
• MutationMutation
• What is transcription & What is transcription & translation?translation?
• Define introns and exonsDefine introns and exons
• What regulates gene expression?What regulates gene expression?
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12.3 Structure of DNA
DNA is comprised of subunits called nucleotides
each DNA nucleotide has three parts
a central dexoyribose sugara phosphate groupan organic base
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Nucleotides differ with regards to their bases
large bases (purines) with double-ring structure: either adenine (A) or guanine (G)
small bases (pyrimidines) with single rings either cytosine (C) or thymine (T)
DNA always has = amnts of purines & pyrimidines
Chargaff’s rule: the amount of A = the amount of T the amount of C = the amount of G
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Figure 12.3 The 4 nucleotide
subunits that make up DNA
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12.3 Discovering the Structure of DNA
structure of DNA is a double helix two strands of DNA bind together by
their bases because a purine of one strand binds
to a pyrimidine on the other strand to form a base pair, the molecule keeps a constant thickness
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Figure 12.4 The DNA double helix
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12.4 How the DNA Molecule Copies Itself
The two strands of DNA that form the double helix DNA molecule are complementary to each other each chain is essentially a mirror
image of the other this makes it possible for DNA to copy
itself in preparation for cell division
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The process of DNA replication involves several enzymes
helicase unzips the DNA to expose the templatesthis creates a replication fork
DNA polymerase
adds the correct complementary nucleotide to the growing daughter strandbut can only add to an existing strand or primer
DNA ligase
seals fragments of DNA together
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Figure 12.7 How nucleotides are added in DNA replication
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12.4 How the DNA Molecule Copies Itself At the replication fork, a primer must first be
added to give a place for DNA polymerase to start
from one template, DNA polymerase adds nucleotides in a continuous fashion; this new daughter strand is called the leading strand
this second daughter strand is assembled in segments, each one beginning with a primer
the segments will be joined together to form the lagging strand
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Figure 12.8 Building the leading and lagging strands
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12.4 How the DNA Molecule Copies Itself
Before the newly formed DNA molecules wind back into the double helix shape, the primers must be removed and the DNA fragments sealed together
DNA ligase joins the ends of the fragments of DNA to form continuous strands
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Figure 12.9 Figure 12.9 How DNA How DNA replication replication worksworks
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12.4 How the DNA Molecule Copies Itself
Because so much DNA is being replicated in the many cells of the body, there is a potential for errors to occur DNA repair involves comparing the
daughter strand to the parent DNA template to check for mistakes
the proofreading is not perfect because mutations are still possible, although rare
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12.5 Mutation
2 general ways to alter the genetic message encoded in DNA mutation
results from errors in replication can involve changes, additions, or
deletions to nucleotides recombination
causes change in the position of all or part of a gene
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Mutations can alter the genetic message and affect protein synthesis
the effect of a mutation depends on the identity of the cell where it occurs
mutations in germ-line cells passed to future generations
important for evolutionary change
mutations in somatic cells
not passed to future generations but passed to all other somatic cells derived from it
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Some mutations alter the sequence of DNA nucleotides
base substitution changes identity of base(s) insertion adds a base(s) deletion removes a base(s)
frame-shift mutation: insertion or deletion throws the reading frame of the gene message out of register, extremely detrimental because the final
protein intended by the message may be altered or not made
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Figure 12.11 Base Substitution
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12.5 Mutation
Some mutations affect how a genetic message is organized Transposition: when individual genes
move from one place in the genome to another
sometimes entire regions of chromosomes may change their relative location or undergo duplication
this is called chromosomal rearrangement
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All evolutionary change begins with mutation
mutation and recombination provide the raw materials for evolution
Chemicals that causes mutation, called mutagens, appear to be linked to cancer for example, chemicals in cigarette
smoke cause cancer
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Ch. 13
So how do these genes work? What is transcription? Translation?
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13.1 Transcription The information contained in DNA is stored
in blocks called genes the genes code for proteins the proteins determine what a cell will be like
The DNA stores this information safely in the nucleus where it never leaves instructions are copied from the DNA into
messages comprised of RNA these messages are sent out into the cell to
direct the assembly of proteins
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DNA RNA protein The path of information is often referred to
as the central dogma
gene expression; takes place in two stages
Transcription: a messenger RNA (mRNA) is made from a gene within the DNA
Translation: mRNA is used to direct the production of a protein
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RNA is the same as DNA except the sugars in RNA have an extra oxygen and T is replaced by another pyrimidine
called uracil (U) The cell uses three kinds of RNA
messenger RNA (mRNA), ribosomal RNA (rRNA) transfer RNA (tRNA)
Types of RNA
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Transcription is how is RNA made
A protein called RNA polymerase produces the mRNA copy of DNA during transcription. 1st: it binds to one strand of the DNA at a
site called the promoter & then moves down the DNA molecule and assembles a complementary copy of RNA
transcription ends when the RNA polymerase reaches a certain nucleotide sequence that signals it stop
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Figure 13.1 Transcription
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13.2 Translation
To correctly read a gene, the mRNA is “read” in three-nucleotide units called codons
each codon corresponds to a particular amino acid
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Figure 13.2 The genetic code (RNA codons)
There are 64 different codons in the genetic code.
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13.2 Translation
Translation occurs in ribosomes, which are the protein-making factories of the cell ribosome= proteins + ribosomal RNA
(rRNA) ribosomes are comprised of two subunits
small subunit large subunit
mRNA binds to the small subunit
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13.2 Translation
The large RNA subunit has three binding sites for transfer RNA (tRNA) located these binding sites are called the A, P,
and E sites it is the tRNA molecules that bring
amino acids to the ribosome to use in making proteins
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Figure 13.3 A ribosome is composed of two subunits
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The structure of a tRNA molecule is important to its function
One end: holds an AA attachment Other end: holds a three-nucleotide
sequence this three-nucleotide sequence is called the
anticodon and is complementary to 1 of the 64 codons of the genetic code
activating enzymes match the amino acids with their proper tRNAs
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Figure 13.4 The structure of tRNA.
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13.2 Translation
Once the ribosomal unit is assembled the mRNA threads through the ribosome three nucleotides at a time
a new tRNA holding an amino acid to be added enters the ribosome at the A site
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previous tRNA in the A site shifts to the P site
At the P site peptide bonds from between the incoming amino acid and the growing chain of amino acids
The now empty tRNA in the P site eventually shifts to the E site where it is released
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13.2 Translation
Translation continues until a “stop” codon is encountered that signals the end of the protein
The ribosome then falls apart and the newly made protein is released into the cell
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Figure 13.6 Ribosomes guide the translation process
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13.3 Architecture of the Gene
The coding portions of the DNA nucleotide sequence are interrupted by non-coding sections of DNA the coding portions: exons the non-coding portions: introns
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13.3 Architecture of the Gene When a eukaryotic cell first
transcribes a gene, it produces a primary RNA transcript of the entire gene the primary transcript is then processed
in the nucleus enzyme-RNA complexes cut out the introns
and join together the exons to form a shorter mRNA transcript
the sequences of the introns (90% of typical human gene) are not translated
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What is alternative splicing?
In humans, genes may be spliced together in different ways by using different combinations of the
same exons, different proteins can be created
this is termed alternative splicing the 25,000 genes of the human
genome appear to encode as many as 120,000 different mRNAs
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