Transcript Genetics - Mrs. Yu`s Science Classes

GENETICS
Eemin Chow
DEFINITIONS
Genetics – the study of genes and
heredity.
Genes – the genetic material on a
chromosome that contains instructions
for creating a trait.
DEFINITIONS
Allele – one of several
varieties of a gene.
Locus – the location on a
chromosome where a gene
is located.
 Every gene has a unique locus
on a particular chromosome.
Homologous chromosomes
– a pair of chromosomes
that contains the same
genetic information
DEFINITIONS
Dominant allele – the trait is expressed.
Recessive allele – the trait is hidden by the
dominant allele; will only be expressed if two
recessive alleles are inherited.
Homozygous dominant/recessive – the
inheritance of two dominant/recessive alleles.
Heterozygous – the inheritance of one
dominant and one recessive allele.
DEFINITIONS
Phenotype – the actual expression of
a gene
Genotype – the actual alleles
HISTORY
Gregor Mendel is the father of
Genetics
 Was a monk from the 19 th century
 Studied inheritance in plants
 Experimented on pea plants
 Came up with two laws of inheritance
MENDEL’S LAW OF INHERITANCE
 Law of Segregation: the separation of alleles to
individual gametes. Each chromosome pair will
separate so that each gamete will have one copy of each
chromosome.
 Law of Independent Assortment: the independent
assortment of alleles. The migration of homologues
within one pair of homologous chromosomes does not
influence the migration of homologues or other
homologous pairs.
MENDEL’S EXPERIMENT
 He crossed 2 varieties of pea
plants to form hybrids.
 P generation – parent
generation
 F1 generation – first
generation offspring
 F2 generation – second
generation offspring
 Monohybrid cross –
experiment where only one
trait is observed
 Dihybrid cross – experiment
where two traits are observed
INHERITANCE PATTERNS
Complete Dominance
 Monohybrid: traits are expressed when one allele is
dominant to a second allele,
 Dihybrid: when it involves two traits, and the
dominant alleles are expressed.
Incomplete Dominance
 the combined expression of two different alleles in
the heterozygous condition produces a blending of
the individual expressions of the two alleles
 E.g. Red (R) White (r) Pink (Rr)
INHERITANCE PATTERNS
Incomplete Dominance
the combined expression
of two different alleles in
the heterozygous
condition produces a
blending of the individual
expressions of the two
alleles
E.g. Red (R) White (r)
Pink (Rr)
INHERITANCE PATTERNS
Codominance
 both inherited alleles are completely expressed.
Multiple Alleles
 When a trait has more than 3 possible alleles
 E.g. Blood type: I A , I B, i
INHERITANCE PATTERNS
Epistasis
 when one gene affects the
phenotypic expression of a
second gene
 E.g. expression of
pigmentation: one gene
turns on/off production of
pigment, one gene controls
color/amount of pigment
INHERITANCE PATTERNS
Pleiotropy
 when a single gene has
more than one phenotypic
expression
 E.g. Sickle cell disease
Polygenic Inheritance
 the interaction of many
genes to shape a single
phenotype.
 E.g. Height, eye color
INHERITANCE PATTERNS
Link Genes
 genes that reside on the same chromosome and thus
cannot segregate independently because they are
physically connected
Sex-linked inheritance
 Inheritance of genes that reside on the sex
chromosomes, X and Y
 E.g. Hemophilia
INHERITANCE PATTERNS
 X-inactivation
 Occurs in female mammals when one of the two X chromosomes
in each cell does not uncoil into chromatin, but remains coiled
as a dark, compact body, called a Barr body
GENETIC DEFECTS
Nondisjunction - the failure of one or
more chromosome pairs or chromatids
of a single chromosome to properly
separate during meiosis or mitosis.
Polyploidy - occurs if all of the chromosomes
undergo meiotic nondisjunction and produce
gametes with twice the number of
chromosomes.
GENETIC DEFECTS
Point mutations - occur when a single
nucleotide in the DNA of a gene is incorrect
 Insertion, deletion, substitution
Aneuploidy - a genome with extra or missing
chromosomes
 E.g. Down Syndrome (trisomy 21, three copies of
chromosome 21), Turner Syndrome (nondisjunction
of sex chromosomes)
GENETIC DEFECTS
Chromosomal Aberrations - when
chromosome segments are changed
 Duplication - occurs when a chromosome segment is
repeated on the same chromosome.
 Inversion - occurs when chromosome segments are
rearranged in reverse orientation on the same
chromosome.
 Translocation - occurs when a segment of a
chromosome is moved to another chromosome.
MOLECULAR GENETICS
Genes are found on DNA, which consists of
polymers of nucleotides.
DNA (deoxyribose nucleic acid)
 Adenine, Thymine, Guanine, Cytosine
 Has a double helix structure
MOLECULAR GENETICS
RNA (ribose nucleic acid)
 Adenine, Uracil, Guanine, Cytosine
 mRNA – provides instructions for assembling amino
acids into a polypeptide chain; linear structure
 tRNA – delivers amino acids to a ribosome for their
addition into a polypeptide chain; “clover-leaf ” shape
structure
 rRNA – combines with proteins to form ribosomes;
globular structure
DNA REPLICATION
 Before cell division can occur, DNA has to be
copied, and the process is called DNA replication.
 DNA molecules are unzipped into two strands that serve
as templates to assemble a new strand.
 The process is a semiconservative replication because the
copied DNA molecule contains a single strand of old DNA
and a single strand of new, replicated DNA
VOCAB FROM DNA REPLICATION
 Helicase - unwinds the DNA
helix, forming a Y-shaped
replication fork.
 Single-strand binding
proteins - attach to each
strand of DNA to keep them
separate.
 Topoisomerase – break and
rejoin the double helix to
unravel twists and prevent
the formation of knots.
VOCAB FROM DNA REPLICATION
 DNA polymerase – assembles the
new DNA strand in the 3’ -> 5’
direction. A complement strand
grows in the antiparallel 5’ -> 3’
direction.
 Leading strand – the complementary
strand that is assembled in the 5’ -> 3’
direction.
 Okazaki segments – short segments of
nucleotides, which are assembled in
the 3’ -> 5’ direction, that make up the
lagging strand
VOCAB FROM DNA REPLICATION
 DNA ligase – connects okazaki
strands to make them into a single
complement strand
3’
 Primase – initiates a new
complementary strand by
beginning replication with a short
segment of RNA, called RNA
RNA Primer
primer.
Primase
5’
5’
 Every leading strand and every okazaki
segment on the lagging strand must
begin with an RNA primer.
DNA Polymerase III
LAGGING STRAND
ANIMATION
DNA Replication Animation:
 http://bcs.whfreeman.com/thelifewire/content/chp1
1/1102003.html
TELOMERES
Ends of DNA containing noncoding, repeating
segments– “junk” at the end
During replication, the telomeres can’t be
replicated.
Telomeric DNA prevents genes from being
worn away.
 Telomeres serve as a buffer– they are useless
segments that get worn away instead of genes.
PROTEIN SYNTHESIS
 Traits are the end products of metabolic processes
regulated by enzymes.
 Old definition of genes: segments of DNA that codes
for a particular enzyme (one-gene-one-enzyme
hypothesis).
 Since many genes code for polypeptides that are not enzymes,
there is a new definition.
 New definition of genes: DNA segments that code for
a particular polypeptide (one-gene-one-polypeptide
hypothesis).
PROTEIN SYNTHESIS
Protein synthesis - the process of the
production of enzymes and other proteins
from DNA.
 Transcription - RNA molecules are created by using
the DNA molecule as a template.
 RNA processing - modifies the RNA molecule with
deletions and additions
 Translation - the processed RNA molecules are used
to assemble amino acids into a polypeptide.
RNA Polymerase - the enzyme that transcribes
the DNA into RNA
TRANSCRIPTION - MRNA
 Messenger RNA (mRNA) is a single strand of RNA
that provides the template used for sequencing
amino acids into a polypeptide.
 A triplet group of three adjacent nucleotides on the mRNA,
called a codon, codes for one specific amino acid.
 There are 64 possible ways that four nucleotides can be
arranged in triplet combinations, so there are 64 possible
codons.
 However, there are only 20 amino acids, so some codons code
for the same amino acid.
TRANSCRIPTION - TRNA
 Transfer RNA (tRNA) is a short
RNA molecule that is used for
transporting amino acids to their
proper place on the mRNA
template.
 The 3' end of the tRNA attaches to an
amino acid.
 Another portion of the tRNA, specified
by a triplet combination of nucleotides,
is the anticodon.
 During translation, the anticodon of the
tRNA base pairs with the codon of the
mRNA.
 Wobble allows the anticodon of some
tRNA’s to base-pair with more than one
kind of codon.
TRANSCRIPTION - RRNA
Ribosomal RNA (rRNA) molecules are the
building blocks of ribosomes.
 The nucleolus is an assemblage of DNA actively being
transcribed into rRNA.
 Within the nucleolus, various proteins imported from
the cytoplasm are assembled with rRNA to form large
and small ribosome subunits.
 Together, the two subunits form a ribosome that
coordinates the activities of the mRNA and tRNA
during translation.
TRANSCRIPTION - RIBOSOMES
Ribosomes have three binding sites —
 one for a tRNA that carries a growing polypeptide
chain (P site,)
 and one for a second tRNA that delivers the next
amino acid that will be inserted into the growing
polypeptide chain (A site).
 And an exit site (E site)
TRANSCRIPTION - INITIATION
 Initiation - the RNA polymerase attaches to a
promoter region on the DNA and begins to unzip the
DNA into two strands.
 A promoter region for mRNA transcriptions often contains the
sequence T–A–T–A called the TATA box.
TRANSCRIPTION - ELONGATION
Elongation - occurs as the
RNA polymerase unzips
the DNA and assembles
RNA nucleotides using one
strand of the DNA as a
template.
 As in DNA replication,
elongation of the RNA
molecule occurs in the 5' → 3'
direction.
TRANSCRIPTION - TERMINATION
Termination - occurs when
the RNA polymerase reaches
a special sequence of
nucleotides that serve as a
termination point.
 In eukaryotes, the termination
region often contains the DNA
sequence AAAAAAA.
MRNA PROCESSING
 A 5' cap (–P–P–P–G-5') is added to the 5' end of the
mRNA. The 5' cap is a guanine nucleotide with two
additional phosphate groups, forming GTP.
 Capping provides stability to the mRNA and a point of
attachment for the small subunit of the ribosome.
 A poly-A tail (–A–A–A . . . A–A-3') is attached to the
3' end of the mRNA.
 The tail consists of about 200 adenine nucleotides. It provides
stability to the mRNA and also appears to control the
movement of the mRNA across the nuclear envelope.
MRNA PROCESSING
 RNA splicing removes nucleotide segments from mRNA.
 A transcribed DNA segment contains two kinds of sequences:
exons, which are sequences that express a code for a
polypeptide, and introns, intervening sequences that are
noncoding.
 Before the mRNA moves to the cytoplasm, small nuclear
ribonucleoproteins, or snRNP’s, delete the introns and splice
the exons.
 Alternative splicing allows different mRNA’s to be
generated from the same RNA transcript.
 By selectively removing different parts of an RNA transcript,
different mRNA’s can be produced, each coding for a different
protein product.
TRANSLATION
 Using the mRNA code to create the appropriate
protein.
 Occurs in the cytoplasm/on the rough ER
 Sequence of 3 nucleotides codes for a particular
amino acid = codon
 64 different codons
 Begins with the start codon – AUG
 Codes for methionine (Met)
TRANSLATION
Ribosome moves along
mRNA in a 5’->3’
direction catalyzing the
translation of the mRNA
into protein
 breaks bond between tRNA
and amino acid
 creates a new peptide bond
to link it to polypeptide
chain
TRANSLATION
Ribosome is released when a stop codon is
reached
 UAA, UAG, UGA = stop codons (don’t code for any
tRNA anticodons)
 A release factor binds to the mRNA instead
 Ribosome breaks apart, mRNA and protein are
released
SUMMARY OF PROTEIN SYNTHESIS
 Translation/Transcription
Animation:
 http://wwwclass.unl.edu/biochem/gp2/m_biolo
gy/animation/gene/gene_a1.html
 Transcription Animation:
 http://www.stolaf.edu/people/giann
ini/flashanimat/molgenetics/transcr
iption.swf
 Transcription Animation:
 http://wwwclass.unl.edu/biochem/gp2/m_biolo
gy/animation/gene/gene_a2.html
 Translation Animation:
 http://www class.unl.edu/biochem/gp2/m_bi
ology/animation/gene/gene_a3.h
tml
MUTATIONS
 Mutation – any sequence of nucleotides in a DNA
molecule that does not exactly match the original
DNA molecule from which it was copied.
 Point mutation is a single nucleotide error
 A substitution occurs when the DNA sequence contains an
incorrect nucleotide in place of the correct nucleotide.
 A deletion occurs when a nucleotide is omitted from the
nucleotide sequence.
 An insertion occurs when a nucleotide is added to the
nucleotide sequence.
MUTATIONS - FRAMESHIFT
A frameshift mutation
occurs as a result of a
nucleotide deletion or
insertion. Such mutations
cause all subsequent
nucleotides to be displaced
one position.
MUTATIONS – SILENT
A silent mutation occurs when the new codon
still codes for the same amino acid.
 This occurs most often when the nucleotide
substitution results in a change of the last of the
three nucleotides in a codon
MUTATIONS – MISSENSE
A missense mutation
occurs when the new
codon codes for a new
amino acid.
 The effect can be minor, or
it may result in the
production of protein that
is unable to fold into its
proper three-dimensional
shape and, therefore, is
unable to carry out its
normal function.
MUTATIONS - NONSENSE
A nonsense mutation occurs when the new
codon codes for a stop codon.
MUTATIONS
Mutagens - radiation or chemicals that cause
mutations
Carcinogens - mutagens that activate
uncontrolled cell growth
REPAIR
 Some mechanisms can repair
replication errors
 Proofreading of a newly attached base
to the growing replicate strand is
carried out by DNA polymerase. DNA
polymerase checks to make sure that
each newly added nucleotide correctly
base pairs with the template strand. If
it does not, the nucleotide is removed
and replaced with the correct
nucleotide.
 Mismatch repair enzymes repair errors
that escape the proofreading ability of
DNA polymerase.
 Excision repair enzymes remove
nucleotides damaged by mutagens.
The enzymes identify which of the two
strands of the DNA contain a damaged
nucleotide and then use the
complementary strand as a template to
repair the error.
DNA ORGANIZATION
 In eukaryotes, DNA is packaged with proteins to form a matrix
called chromatin.
 The DNA is coiled around bundles of eight or nine histone
proteins to form DNA-histone complexes called nucleosomes.
 During cell division, DNA is compactly organized into
chromosomes.
 When the cell is not dividing, the DNA is
 arranged as either of two types of chromatin, as follows.
 Euchromatin describes regions where the DNA is loosely bound to
nucleosomes. DNA in these regions is actively being transcribed.
 Heterochromatin represents areas where the nucleosomes are more
tightly compacted and where DNA is inactive.
REGULATION OF GENE EXPRESSION
Gene expression = transcribing and
translating the gene
Regulation allows an organism to selectively
transcribe and translate only the genes it
needs to.
Genes expressed depend on
 the type of cell
 the particular needs of the cell at that time.
REGULATION OF GENE EXPRESSION
 Operon - a unit of DNA that controls the
transcription of a gene.
 The promoter region is a sequence of DNA to which the
RNA polymerase attaches to begin transcription.
 The operator region can block the action of the RNA
polymerase if this region is occupied by a repressor
protein.
REGULATION OF GENE EXPRESSION
 The structural genes contain DNA sequences that
code for several related enzymes that direct the
production of some particular end product.
 A regulatory gene, lying outside the operon region,
produces repressor proteins, substances that occupy
the operator region and block the action of RNA
polymerase. Other regulatory genes produce activator
proteins that assist the attachment of RNA
polymerase to the promoter region.
LAC OPERON
 Inducible operon - operon is usually “OFF” but can be
stimulated/activated
 The lac operon in E. coli controls the breakdown of
lactose. A regulatory gene produces an active
repressor that binds to the operator region.
LAC OPERON
 When the operator region is occupied by the
repressor, RNA polymerase is unable to transcribe
several structural genes that code for enzymes that
control the uptake and subsequent breakdown of
lactose.
 When lactose is available, however, some of the
lactose (in a converted form) combines with the
repressor to make it inactive.
 When the repressor is inactivated, RNA polymerase is
able to transcribe the genes that code for the enzymes
that break down lactose.
LAC OPERON
ANIMATION
Lac Operon Animation:
http://bcs.whfreeman.com/thelifewire/con
tent/chp13/1302001.html
POSITIVE GENE REGULATION
In the lac operon there are other molecules
to further stimulate transcription.
Lactose will only be digested for energy
when there isn’t much glucose around
When glucose levels are low, level of cAMP
molecule builds up
POSITIVE GENE REGULATION
 CAP – a regulatory protein that
binds to cAMP
 CAP is inactive unless cAMP
binds to it
 If there isn’t much glucose 
high levels of cAMP
 CAP and cAMP bind  CAP
can bind to the promoter 
stimulates RNA Polymerase
to bind
POSITIVE GENE REGULATION
 When glucose levels rise again, cAMP levels will
drop  no longer bound to CAP
 CAP can’t bind to promoter  transcription
slows down
 The lac operon is controlled on 2 levels:
 Presence of lactose determines if transcription can occur
 CAP in the active form determines how fast transcription
occurs
TRP OPERON
Repressible Operon - Operon that is usually
“ON” but can be inhibited
The trp operon in E. coli produces enzymes for
the synthesis of the amino acid tryptophan.
 A regulatory gene produces an inactive repressor
that does not bind to the operator.
 As a result, the RNA polymerase proceeds to
transcribe the structural genes necessary to produce
enzymes that synthesize tryptophan.
TRP OPERON
 When tryptophan is available to E. coli from the surrounding
environment, the bacterium no longer needs to manufacture its
own tryptophan.
 In this case, rising levels of tryptophan induce some tryptophan to
react with the inactive repressor and make it active.
 Here tryptophan is acting as a corepressor.
 The active repressor now binds to the operator region, which, in
turn, prevents the transcription of the structural genes.
ANIMATION
Trp Operon Animation:
http://bcs.whfreeman.com/thelifewire/conte
nt/chp13/1302002.html
MECHANISMS OF GENE EXPRESSION IN
EUKARYOTES
 Regulatory proteins, repressors and activators,
operate similarly to those in prokaryotes, influencing
how readily RNA polymerase will attach to a
promoter region.
 Nucleosome packing influences whether a section of
DNA will be transcribed.
 DNA segments are tightly packed by methylation (addition of
methyl groups) of histones, making transcription more
difficult.
 In contrast, acetylation (addition of acetyl groups) of histones
allows uncoiling and transcription of specific DNA regions.
MECHANISMS OF GENE EXPRESSION IN
EUKARYOTES
 RNA interference occurs when short interfering RNAs
(siRNAs) block mRNA transcription or translation or
degrade existing mRNA.
 Under certain conditions, an RNA molecule will fold back and
base-pair with itself, forming dsRNA. An enzyme then cuts the
dsRNA into short pieces (siRNAs), which then base-pair to
complementary DNA regions—those regions that made the
original RNA molecule—preventing further transcription of
that gene.
 The siRNAs also inactivate mRNA already produced by base pairing with it.
 In other cases, siRNAs combine with enzymes to degrade
existing mRNAs with complementary sequences.