Inheritance cont.

Dihybrid crosses – two genesThe crosses discussed so far relate to one trait

(e.g., flower colour).

Mendel’s second Principle of Inheritance can be illustrated by considering crosses involving two genes that affect two traits.

We can cross pure-breeding strains of flies that differ for two traits: eye colour and body colour and then conduct a dihybrid cross (‘di’ meaning two).

Brown body, yellow eyes—independent assortment The two traits in this example are eye colour

and body colour in Drosophila.

The eye-colour gene in this example is the ‘yellow eye’ gene. It is a different gene from the white eye-colour gene previously discussed and is located on a different chromosome.

The yellow-eye gene is autosomal; the alleles are Y and y.

The wild-type red-eye phenotype (genotypes YY and Yy) is dominant and yellow (genotype yy) is recessive.

Brown body, yellow eyes—independent assortment The second trait is body colour and the gene in

this example is called ‘brown body’.

It is an autosomal gene with two alleles B and b.

The wild-type green body colour phenotype (genotypes BB and Bb) is dominant and brown body (genotype bb) is recessive.

The two genes yellow eye and brown body are on different chromosomes.

F generation

First we set up a cross between a pure-breeding red eye, green body (YY;BB) strain and a yellow eye, brown body (yy;bb) strain.

The homozygous YY;BB strain will produce only Y;B gametes and the homozygous yy;bb strain will produce only y;b gametes.

These gametes fuse to produce F 1 progeny, which are heterozygous for both genes (Yy;Bb).

Following meiosis, a gamete will end up with any of four possible combinations of alleles: Y;B, y;b, y;B or Y;b.

During gamete formation by the F1 fl y, the chance of a sperm or egg cell containing a Y allele is 1/2 (as ½ of the gametes contain a y allele).

The chance that the same gamete will contain a B allele is also ½ .

Therefore the chance of the gamete being Y;B is 1/ 2 × 1/2 = 1/4.

The probability of each of the other three possible gametic combinations is also 1/ 4.

This is because the segregation of alleles of a gene on one chromosome in meiosis is independent of the segregation of alleles of a gene on another chromosome.

F2 generation The heterozygotes generated in the F 1 can be

crossed together (a dihybrid cross) to produce an F 2 generation.

The genotypes and phenotypes of the F progeny are shown in the Punnett square.

The expected ratio of phenotypes in the F 2 generation is –

9 red eye, green body : 3 red eye, brown body : 3 yellow eye, green body : 1 yellow eye, brown body.

If these crosses were actually performed, the phenotypic ratio in the F 2 generation would be close to the 9 : 3 : 3 : 1 ratio.

Dihybrid cross In summary, a phenotypic ratio approximating 9 : 3

: 3 : 1 will be observed in the F 2 generation of a dihybrid cross if the following five conditions apply:

the two genes control two distinct traits

there are two alleles for each of the genes

for each trait one phenotype is dominant

both genes are on autosomes

the two genes assort independently.

In this example, independent assortment occurs because the two genes are on different chromosomes. However, we will see that independent assortment can occur via another mechanism.

For the cross AaBb x AaBbGametes

¼ AB ¼ Ab ¼ Ab ¼ ab

¼ AB AABB AABb AaBB AaBb

¼ Ab AABb AAbb AaBb Aabb

¼ Ab AaBB AaBb aaBB aaBb

¼ ab AaBb Aabb aaBb aabb

The resulting phenotypes show a 9:3:3:1 ratio

Testcrosses and phenotypic ratios If an individual has a dominant phenotype, we

can find out how many genes control the phenotype by carrying out a testcross.

A testcross is a particular type of backcross.

A backcross is a cross between the F 1 heterozygote and either one of the pure- breeding (homozygous) parental strains.

A testcross involves crossing the F 1 heterozygote with the parental strain showing the recessive phenotype(s).

Testcrosses are also used to determine whether an individual of dominant phenotype is homozygous or heterozygous.

An example:Another example can be seen

in your textbook on pg 225.

Also see page 227 for ratios.

So far…. Transmission of heritable characteristics:

Genes as units of inheritance Eukaryote chromosomes, alleles, prokaryote

chromosomes, plasmids

Cell reproduction: Cell cycle, DNA replication, apoptosis, binary fission,

gamete production, inputs and outputs of meiosis. Variation: genotype, phenotype, continuous &

discontinue Patterns of inheritance:

Monohybrid cross – dominance, recessive, codominance

Dihybrid cross Pedigree charts

DNA – the universal molecule of life Genome – total genetic material in a cell

Only 25% of DNA in our cells is involved in coding for biological molecules.

Most of these molecules are proteins but DNA also codes for the production of some RNA molecules.

Only about 1.5% of our DNA actually codes for the production of functioning protein molecules.

The rest is either never transcribed or never translated

Page 243

Human genome

Mitochondrial Genome

Nuclear Genome

RNA genes

Poly peptide genes

Genes 25%

Non coding DNA

Coding DNA 1.5%

Regulatory sequence

introns

Non coding DNAA few hypotheses for their existence:

Just junk Remnant from our past From past infections May protect DNA from mutations

Genes revisited….Basic unit of heredity

More than 30, 000 genes in the human genome

Each chromosome carries many hundred genes

The position of a gene on a chromosome is known as the gene locus (plural – loci)

Gene structure

Upstream region

Exon Exon ExonIntron IntronDown

stream region

Promotes & regulates

the activity of the gene

Coding regionContains the DNA which is transcribed to

pre-mRNA

Contains DNA

sequence which stops transcriptio

n

Types of Genes1. Structural Genes

These genes express structure and/or functional proteins

2. Regulatory Genes

These genes are short nucleotide sequences that express proteins that control the activity of structural genes by feedback mechanisms.

DNA StructureDNA is a complex molecule composed of basic

structural units called nucleotides.

Each nucleotide consists of three components: Deoxyribose Phosphate Nitrogenous base

Deoxyribose5-carbon sugar molecule

Forms part of the backbone of the DNA molecule

PhosphateAn inorganic molecule: PO4 -3

Provide the bond or link between neighboring nucleotides

Nitrogenous BaseTwo main types:

Purines Pyrimidines

Bonds between bases form the links between the two strands of the double helix

Purines always bind to pyrimidines according to specific rules: A – T G - C

PurinesDouble ringed structures

Two of them: Adenine Guanine

A G

PyrimidinesSingle ringed structures

Cytosine Thymine C

T

NucleotideA single nucleotide is composed of a

phosphate, a deoxyribose sugar and a base

A chain of nucleotides form a single DNA strandNulceotides are held by covalent bonds

between the phosphate and sugar molecules

Double strand DNA Is formed from two strands held together by

weak hydrogen bonds between the bases.

Form a double helix structure

DNA ReplicationDNA replication is semi-conservative: each new

molecule consists of one original strand and one new strand

DNA replication is carried out by enzymes.

It is important to know which ones do what job!

The enzymes, helicase and gyrase catalyses the unwinding and unzipping of the parent molecule leaving the bases exposed to act as templates

DNA ReplicationThe enzyme RNA polymerase builds a short

RNA primer sequence using the initial section of DNA as a template

Then the enzymes DNA polymerase extends this primer by adding nucleotides to the growing chain, in a 5’ to 3’ direction

One strand is built as a continuous length (leading strand) while the other strand is built up backwards in short fragments (Okazaki fragments) that are joined together by DNA ligase.

The two strands in DNA are antiparallel

Gene Expression The expression of genetic information is one of

the fundamental activities of all cells. The central role of DNA is to determine what proteins the cell makes.

Instructions stored in DNA are transcribed (copied) and processed into various RNA molecules.

These RNA molecules have specific roles in how the information is translated and expressed as a polypeptide/protein product

Gene ActionInvolves two processes

TranscriptionWhen a gene becomes active it first makes a mobile copy of the coded instruction that it contains.

TranslationIs the process whereby the instructions are decoded

The mobile gene code has to leave the nucleus and move to the cytoplasm.

In the cytoplasm the instruction is decoded. Ribosomes in the cytoplasm make the proteins.

All cellular life forms have about 60 proteins in common!Most of these proteins are involved in

translation as ribosomal proteins and tRNA enzymes.

A couple are involved in transcription.

A few are involved in DNA replication and repair.

This makes sense: all life forms share the same molecular processes.

The DNA codeGenes determine the production of protein

molecules.

The sequence of bases in a gene determines the amino acids which are used in building the protein molecule.

The genetic code is “written” in sequences of 3 bases – called a DNA triplet. Eg. A DNA triplet containing the bases TTA will

code for one amino acid, a DNA triplet containing AGC codes for a different amino acid.

A gene which

codes for a protein

mRNA ribosomes

protein

tRNA & amino acids

transc

ripti

on

Travels to transl

ati

on

TranscriptionOccurs in the nucleus

When the time comes for a particular gene to be expressed, the relevant segment of the appropriate chromosomes unwinds.

The hydrogen bonds between the two nucleotide chains break and the bases become exposed.

Messenger RNA (mRNA) then forms the length of one of the nucleotide chains (the template strand).

Transcription mRNA nucleotide will pair with the bases on the DNA

template according to the base pair rule: A with U T with A C with G G with C

A promoter region of DNA at the start of the gene starts the process.

The enzyme RNA polymerase is needed.

Sequences of unnecessary bases (introns) are removed from the mRNA strands so that only coding bases (exons) remain.

Within genes, bases are found in sets of three called triplets. When mRNA forms, triplets are transcribed to codons.

mRNA leaves the nucleus through pores in the nuclear membrane.

Transcription

A T C G T G

T A G C A C

DNA coding strand

DNA template strand

A U C G U GmRNA RNA polymerase uses the

DNA template strand to produce mRNA that is a transcript of the DNA coding strand.

Remember in RNA:

U substitutes for T

A triplet base code

DNA Replication

Transcription

mRNA codons

TranslationtRNA - anticodons

Polypeptides assembled on ribosomes - rRNA

A minimum combination of 3 bases, a triplet base pair or codon, is needed to code for one amino acid. Codons or triplet codes form the basis of the genetic code.DNA stays in the nucleus, and another molecule, acting as a messenger carries instructions from the nucleus to the cytoplasm.

Players & Places in Translation

Agents Analogy

DNA in the nucleus Master plan with complete set of instructions

mRNA(messenger RNA)

Working copy of one instruction

ribosomes Construction site

tRNA (Transfer RNA)

Carriers of raw material

Amino acids Raw material

Protein chain(Polypeptide)

End product

Translation Occurs in the cytoplasm on the surface ribosomes.

Ribosomes attach to mRNA that has left the nucleus.

Transfer RNA (tRNA) molecules in the cytoplasm bring their specific amino acids to the mRNA for polypeptide synthesis.

Each tRNA molecule has an amino acid binding site and an anticodon.

Each successive codon on the mRNA is paired with an anticodon of a tRNA

The amino acid attached to the tRNA is then released.

Peptide bonds form between the amino acids.

A polypeptide is the end result.

An overview of Gene Structure

Regulatory region

START

STOP

Promoter

region

Terminator region

Coding region

DNA sequence that will be transcribed from the template strand

5

3

3

5

DNA is a double stranded moleculeSections of the DNA act like traffic lights for enzymes.Promoter region of nitrogen bases says start/on.Terminator region of nitrogen bases that say stop/off.It is like walking into a room and switching lights on and off.

One gene can code for more than one protein!The human genome has about 30 000 genes but our

proteome (the total number of different proteins) is much larger approximately 60 000 proteins.

How can this occur?

Many genes can produce more than one protein because the mRNA transcript contains different combinations of exons. This process is called alternative splicing.

Alternative Splicing

Exon 1 Exon 2 Exon 3 Exon 4

Exon 1 Intron Exon 2 Intron Exon 3 Intron Exon 4

Exon 1 Exon 2 Exon 4

Possible mRNA’s using different combinations of exons

Exon 2 Exon 3 Exon 4

Result

When each mRNA is translated, a different protein is produced

Alternative splicing means that the number of outputs from the genetic instructions (genes) in a genome is far greater than the number of genes. A gene can regulate in different ways to produce more than one protein.

At different stages of development it can produce different proteins.

• Intron retention

• Exon juggling

In different types of tissue a gene can produce different types of protein.

The complexity of mammals is not in the number of genes they have but in the processes by which genes can be regulated.

Gene RegulationEach cell contains an entire organism’s

genome.

All cells of an organism have the same genome but can have different phenotypes. For example cells in the eye have the gene for producing fingernail protein (keratin) but this gene is not expressed.

How do genes get switched “on” or “off”?

Why have cells evolved complex mechanisms to regulate their genes?Cells conserve energy and materials by

blocking unneeded gene expression.If a substrate is absent in the

environment why produce the enzyme for that substrate!

Repressor molecules keep the cell from wasting energy by not making mRNA and enzyme molecules that have no use.

The cell can control its metabolism – resources are used only when there is a metabolic need and can be redirected to other metabolic pathways.

Gene Regulation in Prokaryotes Bacteria have groups of genes that are controlled together and are

turned on/off as required

One example, the Lac operon is a set of genes

in bacteria used for lactose metabolism

Some bacteria use the lactose as an energy

source

Bacteria produce the enzymes to break

down lactose to glucose and galactose only

when lactose is present.

Bacteruim

Lactose

The Lac Operon3 catalytic proteins with specific functional roles in the metabolism of lactose

Repressor protein expressed by Lac regulatory gene

ß galactosidasepermease Trans

acetylase

When Lactose is absent

Repressor protein sits on the start/promoter region.

RNA polymerase not happy

that its place is occupied Genes are switched off

Repressor protein

When Lactose is present

Lactose binds to repressor and can’t block promoter site because of change in shape.

Genes are switched onRNA

polymerase

ß galactosidase

permease

Trans acetylase