Chapters 10 & 11

Mendel, DNA & Genes

Gregor Mendel

n     Austrian monk, gardener, scientist

n     First acknowledged to study heredity – the passing on of characteristics from parents to offspring

n     Traits – characteristics that are inherited

n     Father of genetics – the branch of biology that studies heredity

The Peas

n     Mendel chose garden peas

n     They reproduce sexually, so they have both male and female sex cells, called gametes

n     Pollen and egg unite in a process called fertilization

n     Fertilization results in a fertilized cell, called a zygote, that develops into a seed

 

The Peas Mate

n     Pollination is the transfer of pollen grains from a male reproductive organ to a female productive organ

n     Both male and female organs are close together in the same pea flower

n    As a result, peas normally self-pollinate

n    This is what Mendel wanted in most cases

n     Mendel also removed the male organ and dusted pollen on flower of another plant

n    Called cross-pollination

Mendel’s Peas

n     Mendel’s peas had been self-pollinating for a long time

n     This meant that the tall ones had been tall for a long time and the short ones had been short for generations

n    Called purebreds

Mendel’s Monohybrid Crosses

n     Mendel performed cross-pollination with a tall pea plant (6 foot purebred) and a short pea plant (2 foot purebred) – these are the parental generation (P₁)

n     Hybrid – offspring of parents that have different forms of a trait

n     All of the offspring grew as tall as the tall parent – first filial generation (F₁)

n    The “short trait” seemed to disappear

Monohybrid Crosses

n     Mendel let his F1 plants self pollinate

n    Second filial generation (F₂)

n     He counted over 1000 plants

n    About 75% were tall

n    About 25% were short

n    Ratio of 3:1

Monohybrid Crosses

n     Mendel did these same types of crosses for 7 traits

n    Seed shape:  round vs. wrinkled

n    Seed color: yellow vs. green

n    Flower color: purple vs. white

n    Flower position: axial vs. terminal

n    Pod color: green vs. yellow

n    Pod shape: inflated vs. constricted

n    Plant height: tall vs. short

n    back

The Rule of Unit Factors

n     Mendel concluded that each organism has two factors that control each of its traits

n     We now know these factors are genes and that they are located on chromosomes

n     Alternate forms of genes are called alleles

n     Each of Mendel’s traits had two forms of genes – two alleles

n    One comes from mother, one from father

The Rule of Dominance

n      Some alleles are dominant over recessive alleles

n     Dominant alleles cover recessive alleles

n      In genetics, capital letters are used to express dominant alleles and lower case letters are used to express recessive alleles

n     Ex:  “T” for tall allele, “t” for short allele

n      So what will a “TT” plant look like?

n     A Tt plant?

n     A tt plant?

n      Earlobes, Forelock, Dimples, Straight thumb, Bent Pinky, Mid-digit hair

The Law of Segregation

n     Mendel’s first law of heredity

n     Every individual has two alleles of each gene and when gametes are produced, each gamete receives one of these alleles

n     So how did this work in Mendel’s F₂ generation ?

Phenotypes

n     Two organisms can look alike on the outside, but have different allele combinations

n     Phenotype is the way an organism looks and behaves

n    ex: yellow seeds can be TT or Tt

 

Genotypes

n     The allele combination an organism contains is known as its genotype

n    You can’t always see this because of dominiance

n    TT and Tt are different genotypes

n     An organism is homozygous for a trait if the two alleles for the trait are the same

n    Ex: TT or tt

n     An organism is heterozygous for a trait if the two alleles for the trait are different

n    Ex: Tt

Mendel’s Dihybrid Crosses

n    Mendel crossed peas that differed from each other in two traits

n    He crossed plants that were homozygous for round yellow seeds (RRYY) with plants that were homozogous for green wrinkled seeds (rryy)

n    F₁ generation

n   All plants produced round, yellow seeds

n    What was dominant?

The Second Generation (F₂)

n     Let F₁ plants self-pollinate (were heterozygous for two traits)

n     He got all four combinations in a certain ratio

n    Round, yellow – 9

n    Round, green – 3

n    Wrinkled, yellow – 3

n    Wrinkled, green - 1

The Law of Independent Assortment

n     Mendel’s second law of heredity

n     Genes for different traits are inherited independently of each other

n    Inheritance of one trait has no influence on another trait

n     Instead of a ratio of 9:3:3:1, what would the dihybrid cross have looked like?

Punnett Squares

n     1905, Reginald Punnet, English biologist created a shorthand way of finding EXPECTED proportions of possible genotypes in the offspring of a cross

n     Monohybrid crosses

n    See overhead

n     Dihybrid crosses

n    See overhead

Probability

n    Reality is rarely like a Punnett square

n    When you toss a coin, what’s the likelihood it will be heads?

n    You toss 20 heads in a row, what’s the likelihood the next will be heads?

 

Genetic Variation

n    Crossing over during meiosis provides variability

n    How many different kinds of gametes can a pea plant produce?

n    Each cell has 7 pairs of chromosomes

n   Each can line up at the equator in two different ways and separate by segregation

n   2ⁿ = 2⁷ = 128 possible combinations without crossing over

n    When you include the egg, 128 x 128 = 16,384 different combinations of offspring

Genetic Variation

n     In humans, how many possible combinations are there in a single sperm or egg?

n     2²³ = 8,388,608 combinations

n     How many possible combinations with fertilization

ⁿ       8,388,608 x 8,388,608 = 7.04 x 10¹³ (over 70 trillion)

Genetic Recombination

n    With crossing over, additional variation is added providing an almost endless amount of variation possible

n    This reassortment of chromosomes and the genetic information they carry, either by crossing over or by independent segregation of homologous chromosomes is called genetic recombination

n    Variation is the raw material that forms the basis for evolution to act on

Nondisjunction

n     Go back to anaphase I

n     The failure of homologous chromosomes to separate properly during meiosis is called nondisjunction

n     In one type of nondisjunction, two kinds of gametes result

n    One with an extra chromosome (trisomy)

n   Trisomy 21 – Down syndrome

n    One missing a chromosome (monosomy, usually don’t survive)

n   One exception – Turner syndrome, female is XO

Nondisjunction

n     The other type of nondisjunction involves a total lack of separation of homologous chromosomes

n    Gamete inherits a complete diploid set

n    Zygote has polyploidy 

n     Not uncommon in plants – often larger and healthier

n    chrysanthemum (tetraploid, 4n)

n    Wheat (hexaploid, 6n)

n    Apples (3n)

n     There are even chemicals that help plant breeders do this artificially

Gene Linkage

n     Genes that are close together on a chromosome are often inherited together

n    Crossing over rarely works for just one gene

n    These genes are said to be linked

n     So chromosomes, not genes follow Mendel’s independent assortment

Chromosome Mapping

n     Crossing over occurs

n     Geneticists use the frequency of crossing over to map the relative position of genes on a chromosome

n    Genes that are further apart are more likely to have crossing over occur

 

Chromosome Mapping

n     Suppose there are 4 genes on a chromosome – A, B, C, D

n     Frequencies of recombination as follows:

n    Between A & B: 50% (50 map units)

n    Between A & D: 10% (10 map units)

n    Between B & C: 5% (5 map units)

n    Between C & D: 35% (35 map units)

n     These give a relative distance between genes

n     A  -10 units- D  -35units-  C  -5 units- B (whole thing is 50 units)

 

What is DNA?

n    DNA contains the complete instructions for manufacturing all the proteins for an organism

n    DNA achieves its control by determining the structure of proteins

n    The moral - DNA codes for PROTEINS

n    Let’s start with an animated tour

Alfred Hershey & Martha Chase

n     For a long time, many scientists believed protein was the genetic material

n    Protein is more complex than DNA

n     1952, Hershey & Chase experimented with radioactively labeled viruses (bacteriophages) made only of protein & DNA

n    DNA was labeled with                                             one isotope

n    Protein with a different                                      isotope

 

 

Hershey & Chase

n     Bactriophages inject genetic material into bacteria, causing the bacteria to create more bacteriophages

n     Hershey & Chase “followed” the radioactively labeled viruses

n     Discovered that DNA was the genetic material being injected into the bacteria

Structure of Nucleotides

n    DNA is a polymer of nucleotides

n    Nucleotides have 3 parts

n    A simple sugar  (deoxyribose in DNA)

n    A phosphate group

n    A nitrogenous base

n    4 possible nitrogenous bases in DNA

n    Adenine (A)

n    Thymine (T)

n    Guanine (G)

n    Cytosine (C)

Watson & Crick

n     In 1953, published a paper proposing that DNA is made of two chains of nucleotides held together by nitrogenous bases

n   Hydrogen bonds  hold strands together (weak)

n   A only bonds with T, C only bonds with G

n     DNA is a double helix

n     Also correctly described now DNA replicates

n     Won the Nobel Prize

DNA- Common Thread

n    DNA is the same among ALL organisms with DNA

n    The differences come with the different sequences of the four bases

n    Like different words meaning different things, even though the letters are the same

n    The more alike the sequences, the more related he organisms are

 

Replication of DNA

n    Without replication, new cells (after mitosis) would only have half the DNA of their parents

n    All organisms undergo DNA replicaton

How DNA Replicates

n      Due to complementary base pairing, “knowing” the sequence of one strand helps you predict what the other will look like

n      Replication begins when an enzyme breaks the hydrogen bonds between bases

n    Called “unzipping” the DNA

n      Cells have stockpiles of nucleotides and these bond with the newly exposed bases

n    Done by a set of enzymes called DNA polymerases

 

How DNA Replicates

n     Each DNA strand has 2 ends, labeled the 5’ and and the 3’ end.

n   Replication proceeds in the 5’Ô 3’ direction

n     Each new strand formed is a complement  of one of the parents strands

n   Half old, Half new - called semiconservative replication

n     Genetic continuity is maintained

 

Genes & Proteins

n    The information in DNA is put to work through the production of proteins

n    Some proteins become important structures

n    Other proteins (enzymes) control chemical reactions

n    Remember that proteins are polymers of amino acids

n    The sequence of nucleotides                      determines the string of amino                          acids and the protein

RNA

n     RNA differs from DNA in 3 ways:

n   RNA is single stranded (DNA is double)

n   The sugar in RNA is ribose (DNA is deoxyribose)

n   Instead of thymine, RNA contains the base uracil

n    3 types of RNA

n   Messenger RNA (mRNA)

n   rRNA

n   tRNA

Transcription

n    In the nucleus (if cell is eukaryotic), enzymes make an RNA copy of a portion of a DNA strand

n   This process is called transcription

n    Begins as enzymes unzip the molecule of DNA in region of gene being transcribed

n    RNA polymerases pair free nucleotides with their complementary strands

n    The mRNA strand breaks away

RNA Processing

n    Genes usually contain long non-coding sequences of bases, called introns

n    Regions that contain coding sequences are called exons

n    Enzymes in the nucleus cut out the intron segments from the mRNA strand and then paste it back together

Genetic Code

n    A code is needed to convert the language of mRNA (4 letters) into the language of proteins (20 amino acids)

n   How big do the words have to be??????

n    A codon is a group of 3 nitrogenous bases in mRNA that codes for an amino acid

n   1st one found was UUU - coding for phenalynine

n   64 possible codons, so some amino acids correspond to multiple codons

Genetic Code

n    Some codons don’t code for an amino acid - they code for “instructions”

n   UAG - codon for “stop”

n   AUG is the “start” codon as well as coding for methionine

n    The code is universal

n   Used in ALL organisms the same

Translation

n    Translation is the process of converting the information in a sequence of nitrogenous bases in mRNA into a sequence of amino acids in a protein

n    Takes place at the ribosomes in the cytoplasm

Transfer RNA

n     Transfer RNA (tRNA) molecules bring the amino acids (which are in the cytoplasm) to the ribosome

n   Each tRNA molecule attaches to only one type of amino acid

n     There is a sequence of 3 nucleotides on the opposite side of the tRNA molecule from the amino-acid attachment site

n   It is complementary to a specific codon, thus it is called an anti-codon

 

Mutations

n    Any change in the DNA sequence is called a mutation

n    Can be caused by errors in replication, transcription, cell division, or by external agents

n    Most mutations are detrimental - create disfunctional proteins, etc.

n    Occasionally, they may have positive effects

Mutations

n    Mutations in reproductive cells are passed on to offspring

n   Happen to a sperm or egg

n   All of offspring’s cells have this trait

n   Most embryos do not survive

n    Mutations in body cells effect only that individual

n   Usually only one cell or a few cells at first

n  Passed on if continue to divide

Point Mutations

n    A change in a single base pair in DNA

n    Will change one amino acid

n   Affects depend on which amino acid is changed

n    Example

n   THE DOG BIT THE CAT

n   THE DOG BIT THE CAR

 

Frameshift Mutation

n    A mutation in which a single base is added or deleted from the DNA

n    Every codon after the mutation would be different

n    Example

n   THE DOG BIT THE CAT

n   THE DOB ITT HEC AT

Chromosomal Alterations

n    Structural changes in chromosomes

n   Parts of chromosomes break off

n   Parts of chromosomes switch places

n    Especially common in plants

n    Often happens in meiosis

n   Most zygotes fail to grow

n    Non-disjunction is an example

 

Chromosomal Mutations

n     4 kinds

n     Deletion:  part of a chromosome is left out

n     Insertion: part of a chromatid breaks off and attaches to sister chromatid (duplication of segment)

n     Inversion: part of a chromosome breaks off and reattaches backwards

n     Translocation: part of a chromosme breaks off and attaches to non-homologous chromosome

Causes of Mutations

n    Spontaneous mutations occur as a mistake during DNA replication (probably - we don’t know for certain)

n    Mutagens are environmental agents that can cause a change in DNA

n   X-rays, UV light, nuclear radiation

n   Dioxins, asbestos, benzene, formaldehyde

Repairing DNA

n     Cells have repair mechanisms for fixing DNA

n     These enzymes “proofread” new DNA strands during replication and replace incorrect nucleotides with correct ones

n   Work well, but not perfect

n   The more exposure to a mutagen, the more likely the mutation will not be fixed