Download Molecular Genetics DNA Functions Replication Molecular Genetics

Chapter 13
Lecture Outline
Genetics
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Outline

Introduction

Molecular Genetics
• Structure of DNA
• DNA Functions

Cytogenetics

Mendelian Genetics

Quantitative Traits

Extranuclear DNA

Linkage and Mapping

The Hardy-Weinberg Law
Introduction

Transposition - Movement of
a chromosome piece to
another chromosome location
• Transposable elements
(jumping genes) - Genes or
small DNA fragments that can
move to a new location
–
Can disrupt the function of a gene
or restore original function of a
gene
o
Corn showing effects of
transposable elements
Used as tool to research function
of a gene
• Discovered by Barbara McClintock in 1950’s
Molecular Genetics


Structure of DNA
Chromosomes composed two types of large
molecules: DNA and protein.
DNA molecule organized into chain of
nucleotides composed of three parts:
• Nitrogenous base
• 5-carbon sugar (deoxyribose)
• Phosphate group

Four types of DNA nucleotides, each with unique
nitrogenous base
• Two purines - Molecular structure of two linked rings
–
Adenine (A) and Guanine (G)
• Two pyrimidines - Molecular structure of a single ring
– Cytosine (C) and Thymine (T)
Molecular Genetics

Structure of DNA
Nucleotides bonded to
each other forming a ladder
twisted into a helix.
• Sides composed of
alternating sugar and
phosphate groups.
• Hydrogen bonds hold base
on one side of helix to
another base on other side =
rungs of ladder.
• Purines pair with pyrimidines.
–
G-C
–
A-T
DNA molecule
Molecular Genetics

DNA Functions
Storage of Genetic
Information
• Genetic information in DNA
molecule resides in
sequence of nucleotides.
• Gene - Segment of DNA
that directs protein
synthesis
–
Protein used by cell as
structural or storage material
or may act as an enzyme
influencing cell activities.
Portion of DNA molecule
• Genome - Sum total of DNA in an organism’s
chromosomes.
Molecular Genetics
DNA Functions

Replication (Duplication) of Information
• Occurs during S phase of cell cycle
• Strands of double helix unzip.
–
–
–
Single strands are templates for creation of new double
strands.
Nucleotides added by DNA polymerase in precise
sequence: G-C and A-T.
New DNA molecule consists of one strand from original
molecule and another built using that parental strand as
a template = semi-conservative replication.
Molecular Genetics
DNA Functions

Replication
Molecular Genetics
DNA Functions

Expression of Information
• Different subsets of genetic information read in
different cell types.
• Cell’s environment can influence set of genes
expressed.
• Expression requires two processes:
–
Transcription - Copy of gene message made from DNA
template using RNA building blocks
o RNA - Contains ribose, instead of deoxyribose
sugars; single stranded; thymine replaced by uracil
–
Translation - RNA translated to produce proteins.
o Occurs in cytoplasm
Molecular Genetics
DNA Functions

Transcription
• Three different types of RNA produced:
–
Messenger RNA (mRNA) - Translated to produce
proteins
–
Transfer RNA (tRNA) - Machinery for translation
–
Ribosomal RNA (rRNA) - Machinery for translation
• RNA synthesis
–
Nucleotides added to single stranded DNA molecule by
RNA polymerase, using complimentary base pairing.
o
Only portions of the genome transcribed.
« Remainder is noncoding DNA.
Molecular Genetics
DNA Functions

Transcription
• Promoter region at beginning of every gene
signals transcription enzymes to begin copying
gene.
• Terminator DNA sequence at end signals
transcription enzymes to fall off.
• Single-stranded RNA transcript produced.
• Nonprotein-coding DNA fundamental to control of
gene expression.
Molecular Genetics
DNA Functions

Transcription
• Chromosomes contain genes for building tRNA.
–
Acts as translator during translation
o One end binds to mRNA.
o Other end binds to specific amino acid.
o At least one tRNA for each amino acid
o Each form of tRNA has specific anticodon loop.
« Anticodon - Sequence of three amino acids that
recognize and pair with codon on mRNA
• Genes for rRNA also transcribed in nucleus.
–
Used to construct ribosomes which act as
workbenches and assist with assembly of proteins
during translation
Molecular Genetics
DNA Functions

Translation
• mRNA transcripts code for proteins.
–
Genetic code based on codons
o Codons = three nucleotides
« 64 possible combinations that code for 20 amino
acids
o Order of nucleotides on mRNA determines sequence
of amino acids during translation.
o Genetic code universal - In bacteria, protists, fungi,
plants and animals
Molecular Genetics
DNA Functions

Translation
• Anticodon of
tRNA binds to
mRNA codon.
• Start of translation
signaled by a
ribosome in
cytoplasm binding
to mRNA.
–
Codon AUG sets
reading frame.
Molecular Genetics
DNA Functions

Central Dogma of Molecular Genetics
Molecular Genetics
DNA Functions

Mutation - Change in DNA sequence
• Mutagens - Agents that alter DNA sequences
–
–
–
Ultraviolet light
Ionizing radiation
Certain chemicals
• DNA repair enzymes can often find and correct
damage.
• Somatic mutation - Occurs in body cell
• Germ-line mutation - Occurs in tissues that will
produce sex cells
–

Passed on to future generations
All genetic variability due to mutations.
Cytogenetics

Cytogenetics - Study of chromosome
behavior and structure from a genetic point
of view.

Changes in Chromosome Structure
• Inversion - Chromosomal piece breaks and
reinserts in opposite orientation.
–
Inverted regions not rearranged by meiosis and
inherited in blocks.
• Translocation - Chromosomal piece breaks off
and attaches to another chromosome.
• Inversion and translocation important in
speciation.
Cytogenetics

Changes in Chromosome Number
• Mistakes during chromosome pairing and separation
can result in gametes carrying extra or missing
chromosomes.
–
Aneuploid - Carries one or more extra chromosome(s),
or is missing one or more chromosome(s)
–
Polyploid - Has at least one complete extra set of
chromosomes
o Meiosis fails to halve chromosome number, resulting in
2n gametes.
« Fusion of gametes results in polyploid.
o Often larger or have higher yield
« Cotton, potato, peanuts, wheat, oats, strawberry,
sugar cane
Mendelian Genetics

Gregor Mendel crossed tall and short pea
plants (1860’s).
• Parental generation (P)
–
All offspring were tall.
• First filial generation (F1) - Offspring of parental
generation
–
Crossing offspring yielded ratio of three tall individuals to
one short individual.
• Second filial generation (F2) - Offspring of F1 plants
Mendelian Genetics
Two generations of offspring
Mendelian Genetics

Law of unit characters
• Factors (alleles), which always occur in pairs, control
the inheritance of various characteristics.
–

Genes are always at the same position (locus) on
homologous chromosomes.
Law of dominance
• For any given pair of alleles, one (dominant) may
mask the expression of the other (recessive).


Phenotype - Organism’s physical appearance
Genotype - Genetic information responsible for
contributing to phenotype
• Homozygous - Both alleles identical.
• Heterozygous - Alleles are contrasting.
Mendelian Genetics

Start with cross
between two truebreeding parents
differing for a trait.
• Produces F1 generation

Monohybrid cross - F1
plants intercrossed to
produce F2 generation.
• Results in 1:2:1
genotypic ratio, and 3:1
phenotypic ratio
Monohybrid cross
Mendelian Genetics

Dihybrid cross - Start with parents differing in
two traits.
• Law of independent assortment
–
Factors (genes) controlling two or more traits segregate
independently of each other.
o Linked genes - Genes on same chromosome
« Do not segregate independently
o Unlinked genes - Genes on different chromosomes
• F1 generation composed of dihybrids.
–
Produces 4 kinds of gametes
o Punnett square used to determine genotypes of
zygotes.
–
Dihybrid cross produces 9:3:3:1 phenotypic ratio.
Mendelian Genetics
Dihybrid cross
Mendelian Genetics

Backcross - A cross between a hybrid and one
of its parents
• Can be used to test inheritance theory
• Expect phenotypic ratio of 1:1.

Testcross - Cross between a plant having a
dominant phenotype with a homozygous
recessive plant
• Will determine whether plant with dominant
phenotype is homozygous or heterozygous

Incomplete dominance (absence of dominance)
• Heterozygote is intermediate in phenotype to the two
homozygotes.
Mendelian Genetics

Interaction Among Genes - More than one gene
controls phenotype.
• Responsible for production of proteins that are
components of biochemical pathways

How Genotype Controls Phenotype
• Dominant allele codes for protein that effectively
catalyzes reaction, producing phenotype.
• Recessive allele represents a mutant form.
–
Cannot catalyze reaction and does not produce functional
product
Quantitative Traits

Quantitative traits exhibit range of phenotypes
rather than discrete phenotypes as studied by
Mendel.
• Include traits like fruit yield and days to flowering
• Under identical environments phenotypes differ due to
genetic differences.
• Genetically identical plants produce different
phenotypes under different environments.
• Molecular geneticists identify chromosomal fragments,
quantitative trait loci (QTL’s), associated with
quantitative traits.
–
QTL’s contain genes that influence trait and behave like
Mendelian genes.
Extranuclear DNA

Entranuclear DNA - In mitochondria and
chloroplasts
• Endosymbiont hypothesis
–
–
Mitochondria and chloroplasts were free-living bacteria.
o
Established a symbiotic relationship with cells of
organisms that evolved into plants
o
DNA in mitochondria and chloroplasts similar to
bacteria DNA.
Sperm rarely carry mitochondria and chloroplasts, thus
passed to next generation only by female = maternal
inheritance.
Linkage and Mapping

Linked genes - Genes together on a chromosome
• Closer genes are to one another, more likely to be
inherited together
• Each gene has a specific location (locus) on a
chromosome.
• Crossing-over more likely between two genes located
far apart on chromosome than between two genes
located closer together.
–
–
–
Recombinant types - Offspring in which crossing over has
occurred
Crossing over frequency used to construct genetic map of
chromosomes.
o 1 map unit = 1% crossing over between pair of genes
DNA sequence information used to explore gene function in
other species.
The Hardy-Weinberg Law

Hardy-Weinberg law - Proportions of
dominant alleles to recessive alleles in a
large, random mating population will remain
same from generation to generation in the
absence of forces that change those
proportions.
• Forces that can change proportions of dominant
to recessive alleles:
–
Small populations - Random loss of alleles can occur if
individuals do not mate as often.
–
Selection - Most significant cause of exception to H-W
Review

Introduction

Molecular Genetics
• Structure of DNA
• DNA Functions

Cytogenetics

Mendelian Genetics

Quantitative Traits

Extranuclear DNA

Linkage and Mapping

The Hardy-Weinberg Law