Download DNA and Heredity

Cells
DNA and Heredity
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Nucleic acids
◦  DNA (deoxyribonucleic acid) and RNA (ribonucleic acid)
◦  Determines how cell function
"  change the DNA and you change the nature of the organism
◦  Changes of DNA allows organisms to evolve
Genetics and Heredity
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Heredity: storing the operating instructions
Heredity between cell generations determined by information stored in
the DNA protein (deoxyribonucleid acid).
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Double helix (spiral) structure, connected by bases.
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Almost all life on Earth use DNA
to encode genetic instructions
◦  Exceptions: Some viruses use
RNA (West Nile virus, SARS,
influenza)
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DNA double helix
◦  2 phosphate deoxyribose
backbones
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RNA single strand = single
backbone of ribose with bases
exposed
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The basic molecular building blocks
of DNA and RNA are the bases nucleotides.
DNA bases
On Earth, only 4 molecules are used
as bases:
Adenine and thymine make a base
pair:
Cytosine and guanine make a base
pair.
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Instead of thymine, RNA uses the nucleotide base called uracil.
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RNA is very important – carries out genetic instructions
◦  messenger RNA (mRNA)
◦  transfer RNA
(tRNA) collects amino acids,
◦  ribosomal RNA (rRNA) building proteins in ribosomes.
How is heredity encoded in DNA?
DNA determines structure and function of the cells
Operating instructions contained in the arrangement of bases (ATCG)
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Genome: complete set of genetic information that makes up an
organism.
◦  We have about 3x109 base pairs in our genome
◦  Every member of a species has the same basic genome, with some
variation between individuals.
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Gene - the instructions that represents an individual function
◦  E.g., how to build a protein
The G-value paradox
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Mycoplasma genitalium:
470 genes; smallest prokaryotic genome
microscopic parasitic organism
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Saccharomyces cerevisiae: 6144 genes; smallest eukaryotic genome
yeast
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Homo sapiens:
20,000-32,000 genes
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Common wheat:
60,000 genes
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Amoeba:
670x109 genes
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The strain of DNA that makes up a gene contains
◦  A promoter which controls the activity of the gene
◦  A coding sequence that determines what the gene produces
◦  A non-coding sequence that regulates conditions of gene expression
"  the process in which information stored in gene is converted into a form
understandable by the cell
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Chromosomes
◦  Tightly bound bundles of DNA and supporting proteins
◦  Humans have 23 pairs of chromosomes
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A chromosome contains:
◦  A single continuous piece of DNA
◦  DNA-bound proteins (serve to package
the DNA and control its functions)
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Chromosomes vary between different
organisms
◦  Eukaryotic cells (with nucleus) have large
linear chromosomes
◦  Prokaryotic cells (without nucleus) have
smaller circular chromosomes (plasmids)
Reading DNA
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A strand of DNA has a long unbroken sequence of bases
◦  e.g., ACTCATTCAAGC.
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Set of rules of how to read the sequence – break in “words”, where to
start and stop.
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Genetic code:
= set of rules for reading DNA
= the same in nearly all living organisms on Earth.
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A genetic ‘word’ consist of a chunk of three DNA base pairs
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Each word is either a:
◦  “start reading” function,
◦  “stop reading” function, or
◦  for protein building, a particular amino acid
"  Cytosine-Cytosine-Adenine = Proline
"  Guanine-Thyamine-Thyamine = Valine.
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This system of three-base-pair “words” gives enough possibilities to
code for all 20 amino acids needed by life on Earth
•  4 DNA bases (TCAG) and 3 in a row to choose from = 43 = 64
•  Much larger than the 20 amino acids used to build proteins
•  Only two bases needed to code for 16 of them (4x4=16)
•  Only 2 bases needed to code for 16 amino acids (4x4=16)
•  Earlier simpler chemistry?
•  Probably early life used a two-base language?
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The genetic code the same in nearly all living organisms on
Earth – same origin
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Variations found in the mitochondria, an organelle in eukaryote
cells that contain their own DNA
◦  Symbiotic relationship between microorganisms that lead to
gene transfer
Mitochondrion
scanning electron
microscope image
(SEM)
Non-coding DNA
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Strangely, most DNA in humans (95%) and other organisms is noncoding.
◦  Some noncoding DNA is just long sequences of repeating codes.
◦  Other noncoding DNA does not seem to be used by the organism.
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This noncoding DNA is apparently without purpose, and is often called ‘junk
DNA’.
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The pufferfish Takifugu rubripes has the same approximate genome size as
humans, but only 1/10 the junk DNA.
Is noncoding DNA purely structural?
Is it an evolutionary holdover?
Does it indicate something we don’t understand?
DNA replication process
1. 
Unzip the DNA double helix
2. 
Each strand a template, according to the base pairing rule
3. 
Two identical copies of the original DNA (going to the dividing cell)
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The two strands making up the double helix of DNA are said to be
complementary (not identical).
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DNA replication very fast.
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Three billion base sequence in human genome – in a few hours.
How is information used?
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Gene expression is the conversion of information to a form
understood by cell
◦  Transcription – process of converting a sequence of nucleotides in a
DNA section to a sequence of nucleotides in RNA
◦  Translation – process of converting a sequence of nucleotides in a
messenger RNA into a protein
translation
DNA
replication
mRNA
molecule
Protein
transcription
Ribosomes
(molecular ‘machines’)
tRNA
molecule
tRNA
molecule
Mutations – needed for evolution
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Many enzymes involved in DNA replication - errors less than one
per billion base copied.
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Mutation = change in the base sequence of an organism’s DNA
◦  attachment of the wrong base
◦  extra base in a gene
◦  a base deleted
◦  entire sequence duplicated or eliminated
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Some mutations are benign:
◦  ACC changes into ACA – code for the same amino acid = the
instructions for the same protein made by the gene
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Mutations that add or delete a base within a gene have the most
detrimental effect on protein structure
◦  no punctuation or spacing between words
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Sickle-cell disease = mutation in the gene that makes hemoglobin
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Some mutations are beneficial leading to evolution.
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Lateral gene transfer = transfer of genes from one organism to
another.
◦  Bacterial resistance to antibiotics
◦  Genetic engineering (insulin produced by bacteria that have been
inserted with human gene for insulin)
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Lateral gene transfer leads to faster appearance of a new
species than individual mutations.
In general – every cell in a living organism contains the same set of genes
as other types of cells of the same organism.
◦  Muscle/brain cells differ because they use different portions of the
full set of genes.
One cell contains the set of instructions to build an entire
organism or any type of cell.
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Storing operating instructions is essential for life to exist!
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Cloning = process by which a single cell from a living organism is used
to grow an entirely new organism with an identical set of genes.
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Extraterrestrial life may not use DNA to store information but will
very likely use a molecule with a similar function.
Summary remarks
Every cell in a living organism contains the same set of genes as other types
of cells of the same organism
!  Exceptions: muscle & brain cells use different portions of the full set of
genes
One cell contains the set of instructions to build an entire
organism or any type of cell.
! 
! 
Storing operating instructions is essential for life to exist!
Extraterrestrial life may not use DNA to store information but will very
likely use a molecule with a similar function.
Exam Friday 9/25
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All material up through 9/18.
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Study guides (sets of questions you can work with) posted on Learn.
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Two parts
◦  A: Set of short answer questions
◦  B: Essay questions
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Test time will be extended to 3:15pm for those who’d like a bit more
time.
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No material allowed (no books/notes, etc.), just need something to write
with!