chapter 10&11 dna, genes and genomics. 1.the discovery of dna el: to revise dna structure and...

Chapter 10&11DNA, Genes and Genomics

1.The Discovery of DNA

EL: To revise DNA structure and learn about the discovery of DNA

Discovery of DNA

• DNA interactive interviews

• Page 377-78 of NOB

Prokaryotic DNA

• The prokaryotes usually have only one chromosome, and it bears little morphological resemblance to eukaryotic chromosomes.

• Consists of single, circular DNA molecule located in the nucleoid region of cell. Referred to as being “naked”

• Bacterial cells may also contain multiple plasmids - small circular fragment of DNA separate from the main chromosome.

Eukaryotic DNA Structure

DNA consists of two molecules that are arranged into a ladder-like structure called a Double Helix.

A molecule of DNA is made up of millions of tiny subunits called Nucleotides.

Each nucleotide consists of:1. Phosphate group2. Ribose sugar3. Nitrogenous base

Nucleotides

Phosphate

RiboseSugar

NitrogenousBase

Nucleotides

The phosphate and sugar form the backbone of the DNA molecule, whereas the bases form the “rungs”.

There are four types of nitrogenous bases.

Nucleotides

A

Adenine

T

Thymine

G

Guanine

C

Cytosine

Nucleotides

Each base will only bond with one other specific base.

Adenine (A)Thymine (T)

Cytosine (C)Guanine (G)

Form a base pair.

Form a base pair.

DNA Structure

Because of this complementary base pairing, the order of the bases in one strand determines the order of the bases in the other strand.

A

C

T

G

C

A

T

G

G

A

T

T

A

C

DNA replication

• DNA interactive animation

DNA Replication1. unfolding and unwinding of the DNA double helix at hundreds of points, known

as replication origins, along the chromosome.

2. The enzyme helicase separates the two DNA strands, separating them like opening a zipper, with the point of opening being termed the replication fork.

3. Where the DNA strands are separated, a short length of RNA binds to each DNA strand under the control of the enzyme, DNA primase. This RNA acts as a primer (see figure 11.26a page 405).

DNA Replication4. A DNA polymerase enzyme can then proceed to build new DNA strands using

each of the old strands as a template (see figure 11.26b).

5. Replication of DNA can occur only in the 5´ to 3´ direction. This is no problem with the so-called leading strand because its new complementary strand can be built continuously in the 5´ to 3´ direction. The other strand, known as the lagging strand, can be built only backwards and in short discontinuous pieces (Okasaki fragments - see figure 11.26b).

6. When finished, the RNA primers are removed, the gaps are filled by another DNA polymerase and the pieces are joined by the enzyme, DNA ligase.

DNA Replication

Watch DNAi clip

Mitochondrial DNA• Mitochondria contain

mtDNA, a double stranded circular molecule comprising:– 16 568 base pairs and

code for 37 genes:– 13 genes code for

proteins that are involved in cellular respiration

– 2 genes code for ribosomal RNA (rRNA)

– 22 genes code for transfer RNAs (tRNAs).

Activity

• Activity 7.1 – Simulation of DNA replication (handed in please)

• Ch 10 quick check qu 9-11 (pg 353), 12-14 (pg 356), 18-19 (pg 406),

• Chapter 10 review qu 2, 6, 7, 8

Reflection• After listening to some of the interviews with

scientists, do you think you would have the patience to see out a major scientific discovery like that of DNA? Why or why not?

• What learning was new today?• What learning was revision or built on what I

already know?• What did I find most challenging and what

strategies will I put in place to help me?• What percentage of the class did I spend on task

and how can I improve this if needed?

2. DNA to proteins

EL: To explore how protein synthesis occurs

PROTEIN SYNTHESIS

This is known as the GENETIC CODE.

The sequence of bases in the DNA determines the sequence of amino acids in the protein.

The most important proteins are enzymes.

An individuals characteristics are determined by their DNA.

The DNA determines which proteins are made.

THE GENETIC CODE

START AND STOP CODONS

NON-OVERLAPPING

DEGENERATEMore than one codon for each amino acid.

TRIPLET CODE3 bases in the DNA code for one amino acid in the protein. Each triplet is known as a CODON.

UNIVERSALFound in all organisms.

PROTEIN SYNTHESIS

TRANSCRIPTION

AMINO ACID ACTIVATION

TRANSLATION

Protein Synthesis Summary

TRANSCRIPTION

T A C G G C T A A C A T A C A A T C

U

A

GC

G

G

G

C

C

A

UA

A

A

A

UU

UU

GU

HELICASE unwinds and unzips the relevant part of the DNA helix.

RNA POLYMERASE attaches to the DNA.

One DNA strand acts as the template (SENSE STRAND), the other is redundant (ANTISENSE STRAND).

As RNA POLYMERASE moves along the sense strand, ribonucleotides are assembled in a precise order due to complementary base pairing (A <-> T/U ; G <-> C).

U

A

GC

G

G

G

C

C

A

UA

A

A

A

UU

UU

GU

T A C G G C T A A C A T A C A A T C

HELICASE unwinds and unzips the relevant part of the DNA helix.

RNA POLYMERASE attaches to the DNA.

One DNA strand acts as the template (SENSE STRAND), the other is redundant (ANTISENSE STRAND).

As RNA POLYMERASE moves along the sense strand, ribonucleotides are assembled in a precise order due to complementary base pairing (A <-> T/U ; G <-> C).

T A C G G C T A A C A T A C A A T CU

A

GC

G

G

G

C

C

A

UA

A

A

A

UU

UU

GU

HELICASE unwinds and unzips the relevant part of the DNA helix.

RNA POLYMERASE attaches to the DNA.

One DNA strand acts as the template (SENSE STRAND), the other is redundant (ANTISENSE STRAND).

As RNA POLYMERASE moves along the sense strand, ribonucleotides are assembled in a precise order due to complementary base pairing (A <-> T/U ; G <-> C).

T A C G G C T A A C A T A C A A T CU

A

GC

GG

G

C

C

A

UA

A

A

A

UU

UU

GU

HELICASE unwinds and unzips the relevant part of the DNA helix.

RNA POLYMERASE attaches to the DNA.

One DNA strand acts as the template (SENSE STRAND), the other is redundant (ANTISENSE STRAND).

As RNA POLYMERASE moves along the sense strand, ribonucleotides are assembled in a precise order due to complementary base pairing (A <-> T/U ; G <-> C).

T A C G G C T A A C A T A C A A T CU

A

G

CGG

G

C

C

A

UA

A

A

A

UU

UU

GU

HELICASE unwinds and unzips the relevant part of the DNA helix.

RNA POLYMERASE attaches to the DNA.

One DNA strand acts as the template (SENSE STRAND), the other is redundant (ANTISENSE STRAND).

As RNA POLYMERASE moves along the sense strand, ribonucleotides are assembled in a precise order due to complementary base pairing (A <-> T/U ; G <-> C).

T A C G G C T A A C A T A C A A T CU

A

G

CGG

G

C

C

A

UA

A

A

A

UU

UU

GU

HELICASE unwinds and unzips the relevant part of the DNA helix.

RNA POLYMERASE attaches to the DNA.

One DNA strand acts as the template (SENSE STRAND), the other is redundant (ANTISENSE STRAND).

As RNA POLYMERASE moves along the sense strand, ribonucleotides are assembled in a precise order due to complementary base pairing (A <-> T/U ; G <-> C).

T A C G G C T A A C A T A C A A T CU

A

G

CGG

G

C

C

A

UA

A

A

A

UU

UU

GU

HELICASE unwinds and unzips the relevant part of the DNA helix.

RNA POLYMERASE attaches to the DNA.

One DNA strand acts as the template (SENSE STRAND), the other is redundant (ANTISENSE STRAND).

As RNA POLYMERASE moves along the sense strand, ribonucleotides are assembled in a precise order due to complementary base pairing (A <-> T/U ; G <-> C).

T A C G G C T A A C A T A C A A T CU A

G

CGG

G

C

C

A

UA

A

A

A

UU

UU

GU

HELICASE unwinds and unzips the relevant part of the DNA helix.

RNA POLYMERASE attaches to the DNA.

One DNA strand acts as the template (SENSE STRAND), the other is redundant (ANTISENSE STRAND).

As RNA POLYMERASE moves along the sense strand, ribonucleotides are assembled in a precise order due to complementary base pairing (A <-> T/U ; G <-> C).

T A C G G C T A A C A T A C A A T CU A

G

CGG

G

C

C

A

UA

A

A

A

UU

U

U

GU

HELICASE unwinds and unzips the relevant part of the DNA helix.

RNA POLYMERASE attaches to the DNA.

One DNA strand acts as the template (SENSE STRAND), the other is redundant (ANTISENSE STRAND).

As RNA POLYMERASE moves along the sense strand, ribonucleotides are assembled in a precise order due to complementary base pairing (A <-> T/U ; G <-> C).

T A C G G C T A A C A T A C A A T CU A

G

CGG

G

C

C

A

UA

A

A

A

U

UU

U

GU

HELICASE unwinds and unzips the relevant part of the DNA helix.

RNA POLYMERASE attaches to the DNA.

One DNA strand acts as the template (SENSE STRAND), the other is redundant (ANTISENSE STRAND).

As RNA POLYMERASE moves along the sense strand, ribonucleotides are assembled in a precise order due to complementary base pairing (A <-> T/U ; G <-> C).

T A C G G C T A A C A T A C A A T CU A

G

CG GG

C

C

A

UA

A

A

A

U

UU

U

GU

HELICASE unwinds and unzips the relevant part of the DNA helix.

RNA POLYMERASE attaches to the DNA.

One DNA strand acts as the template (SENSE STRAND), the other is redundant (ANTISENSE STRAND).

As RNA POLYMERASE moves along the sense strand, ribonucleotides are assembled in a precise order due to complementary base pairing (A <-> T/U ; G <-> C).

T A C G G C T A A C A T A C A A T CU A

G

CG GG

C

C

A

UA

A

A

A

UUU

U

GU

HELICASE unwinds and unzips the relevant part of the DNA helix.

RNA POLYMERASE attaches to the DNA.

One DNA strand acts as the template (SENSE STRAND), the other is redundant (ANTISENSE STRAND).

As RNA POLYMERASE moves along the sense strand, ribonucleotides are assembled in a precise order due to complementary base pairing (A <-> T/U ; G <-> C).

T A C G G C T A A C A T A C A A T CU A

G

CG GG

C

C

A

U

A

A

A

A

UUU

U

GU

HELICASE unwinds and unzips the relevant part of the DNA helix.

RNA POLYMERASE attaches to the DNA.

One DNA strand acts as the template (SENSE STRAND), the other is redundant (ANTISENSE STRAND).

As RNA POLYMERASE moves along the sense strand, ribonucleotides are assembled in a precise order due to complementary base pairing (A <-> T/U ; G <-> C).

T A C G G C T A A C A T A C A A T CU A

G

CG GG

C

C

A

U

A

A

A

A

UUU U

GU

HELICASE unwinds and unzips the relevant part of the DNA helix.

RNA POLYMERASE attaches to the DNA.

One DNA strand acts as the template (SENSE STRAND), the other is redundant (ANTISENSE STRAND).

As RNA POLYMERASE moves along the sense strand, ribonucleotides are assembled in a precise order due to complementary base pairing (A <-> T/U ; G <-> C).

T A C G G C T A A C A T A C A A T CU A GCG GG

C

C

A

U

A

A

A

A

UUU U

GU

HELICASE unwinds and unzips the relevant part of the DNA helix.

RNA POLYMERASE attaches to the DNA.

One DNA strand acts as the template (SENSE STRAND), the other is redundant (ANTISENSE STRAND).

As RNA POLYMERASE moves along the sense strand, ribonucleotides are assembled in a precise order due to complementary base pairing (A <-> T/U ; G <-> C).

T A C G G C T A A C A T A C A A T CU A GCG GG

C

C

A

U

A

A

A

A

UUU U

G

U

HELICASE unwinds and unzips the relevant part of the DNA helix.

RNA POLYMERASE attaches to the DNA.

One DNA strand acts as the template (SENSE STRAND), the other is redundant (ANTISENSE STRAND).

As RNA POLYMERASE moves along the sense strand, ribonucleotides are assembled in a precise order due to complementary base pairing (A <-> T/U ; G <-> C).

T A C G G C T A A C A T A C A A T CU A GCG GG

C

C

A

UA

A

A

A

UUU U

G

U

HELICASE unwinds and unzips the relevant part of the DNA helix.

RNA POLYMERASE attaches to the DNA.

One DNA strand acts as the template (SENSE STRAND), the other is redundant (ANTISENSE STRAND).

As RNA POLYMERASE moves along the sense strand, ribonucleotides are assembled in a precise order due to complementary base pairing (A <-> T/U ; G <-> C).

T A C G G C T A A C A T A C A A T CU A GCG GG

C

C AUA

A

A

A

UUU U

G

U

HELICASE unwinds and unzips the relevant part of the DNA helix.

RNA POLYMERASE attaches to the DNA.

One DNA strand acts as the template (SENSE STRAND), the other is redundant (ANTISENSE STRAND).

As RNA POLYMERASE moves along the sense strand, ribonucleotides are assembled in a precise order due to complementary base pairing (A <-> T/U ; G <-> C).

T A C G G C T A A C A T A C A A T C

C

A

A

U A GCG GGC AUAA UUU U GU

HELICASE unwinds and unzips the relevant part of the DNA helix.

RNA POLYMERASE attaches to the DNA.

One DNA strand acts as the template (SENSE STRAND), the other is redundant (ANTISENSE STRAND).

As RNA POLYMERASE moves along the sense strand, ribonucleotides are assembled in a precise order due to complementary base pairing (A <-> T/U ; G <-> C).

U

A

G

CG

G

GC

AU

A

A

UUU

U

GU

Fully formed mRNA peels off the DNA and leaves the nucleus via a nuclear pore.

The DNA rewinds.

U

A

G

CG

G

GC

AU

A

A

UUU

U

GU

Fully formed mRNA peels off the DNA and leaves the nucleus via a nuclear pore.

The DNA rewinds.

U

A

G

CG

G

GC

AU

A

A

UUU

U

GU

U

A

G

CG

G

GC

AU

A

A

UUU

U

GU

AMINO ACID ACTIVATION

U A C

ACTIVATION OCCURS WHENTHE tRNA COMBINES WITH A SPECIFIC AMINO ACID

THE ANTICODON DETERMINES WHICH SPECIFIC AMINO ACID IS

ATTACHED

TRANSLATION

A U G C C G A U U G U A U G U U A G

U A CG G C

U A A

CA

U

AC A

A ribosome binds to the mRNA near the START CODON.

A U G C C G A U U G U A U G U U A G

U A C

G G C

CA

U

AC A

tRNA with the complementary ANTICODON (UAC) binds to the start codon (AUG) held in place by the large subunit of the ribosome. It brings with it the amino acid methione.

U A A

A U G C C G A U U G U A U G U U A G

U A C

G G C

CA

U

AC A

The ribosome now slides along the mRNA to “read” the next codon.

U A A

U A C

A U G C C G A U U G U A U G U U A G

G G C

CA

U

AC A

A second tRNA now bind to this codon, bringing a second amino acid with it.

U A A

U A C

A U G C C G A U U G U A U G U U A G

G G C

CA

U

AC A

A peptide bond is formed between the two amino acids.

U A A

UA

C

A U G C C G A U U G U A U G U U A G

G G C

CA

U

AC A

The tRNA which carried the first amino acid is released but leaves its amino acid behind as a DIPEPTIDE.

U A A

UA

C

A U G C C G A U U G U A U G U U A G

G G C

U A A

CA

U

AC A

The ribosome now slides along the mRNA to “read” the next codon.

UA

C

A U G C C G A U U G U A U G U U A G

G G C U A A

CA

U

AC A

One by one each codon is read as the ribosome moves along the mRNA.

UA

C

A U G C C G A U U G U A U G U U A G

G G C U A A

CA

U

AC A

Each time the growing polypeptide is linked to the amino acid on the incoming tRNA.

GG

C

A U G C C G A U U G U A U G U U A G

U A A

UA

CC

AU

AC A

GG

C

A U G C C G A U U G U A U G U U A G

U A A

UA

C

C A UA

C A

GG

C

A U G C C G A U U G U A U G U U A G

U A A

UA

C

C A UA

C A

GG

C

A U G C C G A U U G U A U G U U A G

UA

C

C A U

UA A

AC A

GG

C

A U G C C G A U U G U A U G U U A G

UA

C

C A U

UA A

A C A

GG

C

A U G C C G A U U G U A U G U U A G

UA

C

C A U

UA A

A C A

GG

C

A U G C C G A U U G U A U G U U A G

UA

C

UA A

A C A

CA U

GG

C

A U G C C G A U U G U A U G U U A G

UA

C

UA A

A C A

CA U

STOP !

The polypeptide is complete when the ribosome reaches the STOP codon.

GG

C

A U G C C G A U U G U A U G U U A G

UA

C

UA A C

A U A C A

The polypeptide is released.

GG

C

A U G C C G A U U G U A U G U U A G

UA

C

UA A C

A U A C A

The polypeptide is released.

The polypeptide may combine with other polypeptides and will become variously coiled/folded to produce a protein.

G G C

U A C

U A AC A U

A C A

The tRNAs are recycled.

A U G C C G A U U G U A U G U U A G

The mRNA may be used again in this form, or it may be broken down into nucleotides which can be reassembled to produce a different polypeptide.

The ribosome is free to move along another mRNA.

Ribosomes work in groups so that many slide along a mRNA molecule simultaneously.

These groups are called POLYRIBOSOMES.

Each ribosome takes about 1 minute to travel along a mRNA molecule.

Transcription and translation

• DNAi animations

QUIZ What sort of chemical is helicase?

Why is DNA double stranded if one strand is redundant?

What is the difference between a polypeptide and a protein?

Give the three alternative stop codons.

What is the start codon?

Where in the cell are the ribosomes?

Give the primary structure (sequence of amino acids) of the polypeptide made in this animation.

What is the advantage of ribosomes operating as polyribosomes?

What are the similarities and differences between DNA replication and protein synthesis?

ANSWERSHelicase is an enzyme and therefore also a protein.

DNA is double stranded to permit replication.

Polypeptides have less than 100 amino acids, protein have more. A protein may consist of several polypeptides.

The three stop codons are UGA, UAG and UAA.

The start codon is AUG.

Ribosomes are located in the cytoplasm.

Give the primary structure of the polypeptide is methionine, proline, isoleucine, valine, cysteine.

Polysomes increase efficiency, they enable one mRNA molecule to produce many polypetides simultaneously.HINT - Think about the enzymes and nucleotides used, the end product and the location of the process.

Really good website

• http://www.le.ac.uk/ge/genie/vgec/he/index.html

Activity

• In pairs, complete activity 11.1

• Quick check qu 4-8 (pg 391)

• Ch 11 ch review qu 2, 3, 4, 9 (pg 414-416)

Reflection• Did the hands-on activity help cement your

understanding of transcription and translation? If not, how else do you think you could ensure your understanding?

• What learning was new today?• What learning was revision or built on what I already

know?• What did I find most challenging and what strategies

will I put in place to help me?• What percentage of the class did I spend on task and

how can I improve this if needed?

3.Genomics

EL: What a genome is and how gene expression is regulated

What is a genome?

• All of the genetic material (the base pairs) found in one complete set of an organism’s chromosomes.

• The study of genomes is called genomics.

Does genome size matter?COMMON NAME SPECIES NAME Approx GENOME SIZE

(millions of base pairs)

Fruit fly Drosophila melanogaster 180

Snake Boa constrictor 2100

Human Homo sapiens 3100

Onion Allium cepa 18000

Lungfish Protopterus aethiopicus 140000

Amoeba Amoeba dubia 670000

Why would a single celled animal like the amoeba need a genome that is about 200 times larger than the human genome?Ans: They carry a lot of junk DNA!

What is a gene?• Segment of DNA that codes

for formation of a protein

– Structural genes express structural and/or functional proteins.

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

Number of genes

Should we be offended that a mustard plant has as many genes as a human?

COMMON NAME SPECIES NAME No. GENES

Human Homo sapiens 25000

Mustard plant Arabidopsis thaliana 27000

Fruit fly Drosophila melanogaster 14000

Baker’s yeast Saccharomyces cerevisiae 6000

Gut bacterium Escherichia coli 4000

An Overview of Gene Structure

Coding RegionDNA sequence that will be transcribed from the template strand.

5’

5’3’

3’

Regulatory region

Promoter region

Terminator region

START

STOP

Gene Expression

• The expression of genetic information is one of the fundamental activities of all cells.

• Instructions stored in DNA are transcribed and translated into various RNA molecules.

Introns and Exons

• The coding region in eukaryotes contain: – introns - non-coding regions of DNA– exons - coding regions of DNA

• Prokaryotes do not have introns – why?– They don’t carry “junk DNA” due to short

replication cycles

DNA Template Strand

Pre-mRNA transcript of DNA template strand

Introns are spliced out by spliceosomes leaving only the sequences that will be expressed. This is an example of RNA processing. The introns usually are degraded. The result is a mature mRNA strand that will leave the nucleus to be translated.

RNA Processing in Eukaryotic CellsINTRON INTRON

EXON EXON EXON

EXON EXON EXON

Spliceosome

Spliceosome

Genome to proteome

• The human genome has about 25,000 genes but our proteome (the total number of different proteins) is much larger (~100,000)

– 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 splicingPre-mRNA transcript

Possible mRNAs using different combinations of exons

Result: when each mRNA is translated, a different protein is produced.

EXON 1

EXON 1

EXON 1

EXON 2

EXON 2

EXON 2

EXON 2

EXON 3 EXON 4

EXON 3

EXON 3

EXON 4

EXON 4

EXON 4

PROTEIN 1

PROTEIN 2

PROTEIN 3

INTRONS

Activity:

• In pairs, complete activity 10.2 “Sequencing a genome”

• Quick check qu 15-17 (pg 362), 18-23 (pg 364), 24-27 (pg 370), 16&17 (pg 403)

• Chapter review qu 3, 5, 9 (pg 381-382)

Reflection

• What learning was new today?• What learning was revision or built on what I

already know?• What did I find most challenging and what

strategies will I put in place to help me?• What percentage of the class did I spend on

task and how can I improve this if needed?

4.Gene Regulation and Genetic Mutations

EL: How genes are switched on and offThe effects genetic mutations can have

on organisms

Gene Regulation• Each 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 your eye have the gene for producing fingernail protein (keratin) but this gene is not expressed.

• How do genes get switched on or switched off?

Why regulate gene expression?• 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 transcribing mRNA or making 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.

• E.g. the lac operon is a set of genes in bacteria used for lactose metabolism.

• Bacteria produce the enzymes to break down lactose to glucose and galactose only when lactose is present.

• http://www.sumanasinc.com/webcontent/animations/content/lacoperon.html

• http://pages.csam.montclair.edu/~smalley/LacOperon.mov

Gene regulation in Eukaryotes

• Still being investigated

• Proteins involved (like prokaryotes) – but more complicated– Enhancers: act as binding sites for activator proteins and

increase number of DNA polymerase molecules transcribing genes

– Chemical modification: eg presence or absence of histone proteins

Changing the Genetic InformationMutations

• A mutation refers to any permanent change in the DNA nucleotide base sequence of an organism.

• Mutations occur spontaneously and randomly throughout the lifetime of all organisms

• The effects of mutations vary depending on their location both within the chromosome (or gene) and the body of the organism.

This red delicious apple illustrates a somatic mutation. A mutation to the ovarian wall gives rise to a sector of yellow colored fruit. The mutation does not affect the seeds (germline) which give rise to the standard red delicious type.

Acquiring Mutations• Mutations in the DNA of an

organism can be caused by:

- Mistakes in DNA replication.• This is a natural process• 1 mistake in 1,000,000,000 bases• Proof reading enzymes correct most

mistakes

- Environmental factors that increase the rate of mutations are called mutagens.

• Radiation• Various chemicals• High temperatures

Can you inherit a mutation?

• Yes! If a mutation occurs in the cells that produce gametes (germ-line cells) the change will be passed onto the offspring.

• If a mutation occurs in any other cell of the body (somatic cells) it will not be inherited, but it may affect the individual during their lifetime.

The biological consequences of mutations

• Mutations may be beneficial, neutral or harmful!

• Mutations are a source of genetic variation – new alleles in a population – that may be selected for by environmental factors and confer an advantage on the organism.

• Mutations are a source of biological novelty for evolution.

Types of Mutations

• Point Mutations (i.e. spelling mistakes)- Changes in a single DNA nucleotide- These can occur within a gene’s coding region or within regulatory

regions of genes

• Block Mutations- Changes in a segment of a chromosome- These changes usually involve the rearrangement of a number of genes

• Chromosome Number Mutations- Changes in the number of chromosomes

Point Mutations• Single nucleotide substitutions may result in:1. Changed amino acid sequence2. No amino acid change because genetic code is

degenerate3. Results in “stop” instruction and formation of a new

allele.

Mutation: Substitute T instead of C

OriginalDNA

Mutant DNA

Point MutationsAs a reference for the following screens, the diagram below illustrates the transcription and translation of DNA without a point mutation.

Original Unaltered Code

Transcription

Amino acid sequence forms a normal polypeptide chain

Translation

Original DNA

mRNA

Amino acids

1. Changed amino acid sequence• A single base is substituted by another.

• Usually results in coding for a new amino acid in the polypeptide chain.

Mutation: Substitute T instead of C

Polypeptide chain with wrong amino acid

Original DNA

Mutant DNA

mRNA

Amino acids

2. No amino acid change

A single base is substituted by another.

Called silent or neutral mutations and produce little or no change in the phenotype.

A change in the third base of a codon still codes for the same amino acid.

Normal DNA

mRNA

Amino acids

Amino acid sequence from

the non-mutated DNA forms

a normal polypeptide chain

Mutation: Substitute C instead of T

Mutant DNA

mRNA

Amino acids

Despite the change in the last

base of a triplet, the amino acid

sequence is unchanged

3. Results in “stop” instruction and formation of a new allele

• A single base is substituted by another.

• This results in a new triplet that does not code for an amino acid.

• The resulting triplet may be an instruction to terminate the synthesis of the polypeptide chain.

Mutation: Substitute A instead of C

Original DNA

Mutant DNA

mRNA

Amino acids

Mutated DNA creates a STOP codon which prematurely ends synthesis of the polypeptide chain

Reading Frame Shift by Insertion or DeletionA single base is inserted, upsetting the reading sequence for all those after it.

• A reading frame shift results in new amino acids in the polypeptide chain from the point of insertion onwards.

• The resulting protein will be grossly different from the one originally encoded (it is most likely to be non-functional).

Large scale frame shift results in a new amino acid sequence. The resulting protein is unlikely to have any function.

Mutation: Insertion of C

Original DNA

Mutant DNA

mRNA

Amino acids

Trinucleotide Repeat Expansions• Many normal human genes contain multiple copies of a

three base sequence called a trinucleotide.

• These repeating sequences can expand in number. This mutation gives rise to several inherited conditions.

• The mutant allele that causes “fragile X syndrome” has 200 to 2000 repeats of the trinucleotide CGG, in contrast to 6 to 50 repeats in a normal person, in the untranslated region of the FMR1 gene.

Fragile X Syndrome• Occurs 1 out of every 4000 males and 1 out

of every 6000 females.

• Mutation of the FMR1 gene on the X chromosome leads to loss of the fragile X-metal retardation protein, FMRP. This FMRP protein is involved in the translation of a number of essential neuronal mRNA’s.

• Characteristics of this disease include:- Metal retardation- Shyness and limited eye contact- Elongated face- Large or protruding ears- Large testicles (macroorchidism)- Low muscle tone

Activity

• Quick check qu 28&29 (pg 373), 30&31 (pg 376), 20-24 (pg 412)

• Ch 10 ch review qu 11, 12, 13 pg 382-383

Reflection

• What learning was new today?• What learning was revision or built on what I

already know?• What did I find most challenging and what

strategies will I put in place to help me?• What percentage of the class did I spend on

task and how can I improve this if needed?

5. Genetic Mutations

EL: Chromosomal mutations and defects

Block Mutations• The rearrangement of blocks of

genes within a chromosome. Can occur during crossing over in meiosis

• The rearrangement of blocks of genes between non-homologous chromosomes (translocation). A piece of one chromosome is broken off and joined to another chromosome.

• Block mutations result in a new gene order along a chromosome. They can be highly disruptive!

translocations

Normal

Deletion: Pieces of chromosome are lost.

Duplication: Pieces of chromosome are repeated so there are duplicate segments

Inversion: Pieces of chromosome are flipped so the genes appear in reverse order.

Translocation: Pieces of chromosome are moved from one chromosome onto another.

.

• Aneuploidy – changes to the number of specific chromosomes

• Polyploidy- changes to the number of whole sets of chromosomes

Examples of Polyploid Plants

Name Number

Common wheat

6N = 42

Tobacco 4N = 48

Potato 4N = 48

Banana 3N = 27

Boysenberry 7N = 49

Strawberry 8N = 56

Changes to the Chromosome Number

Aneuploidy• Change to the number of specific

chromosomes.

• The extra or missing chromosome may be an autosomal or a sex chromosome.

• Such changes are due to non-disjunctions. These events are due to errors in chromosome segregation in meiosis.

• Pairs of homologous chromosomes may fail to separate in meiosis I or the centromere may fail to separate the sister chromatids in meiosis II.

Down’s SyndromeAn extra chromosome 21

Non-Disjunction: Meiosis INon-disjunction Meiosis I

Meiosis II

n+1 n+1 n–1 n–1Gametes

• A non-disjunction in meiosis I occurs when homologous chromosomes fail to separate properly during anaphase I.

• One gamete receives two of the same sort of chromosome and the other gamete receives no copy.

Non-Disjunction: Meiosis II

• A non-disjunction in meiosis II occurs when sister chromatids fail to separate properly during anaphase II.

• One gamete receives two of the same sort of chromosome and the other gamete receives no copy.

• Some gametes are unaffected.

Meiosis I

Meiosis II

Non-disjunction

n+1 n–1 n nGametes

Turner’s Syndrome 44 + X• Occurs in 1 out of every

25,000 births

• Only one X chromosome is present and is fully functional.

• Common symptoms include:– Short stature– Swelling of hands an feet– Broad chest– Low hairline– Webbed neck

• Occurs in about 1 in 500 to 1000 births.• Characteristics vary widely.• Males are normally sterile• Some degree of language impairment• Youthful build• Rounded body type• Some degree of gynecomastia• Hypogonadism

Mildly impaired IQ (intelligence)

Poor beard growth

Frequently some breast development (low levels of testosterone)

Chest hair is sparse

Penis and testes underdeveloped, low levels of testosterone. Always infertile.

Female type pubic hair pattern

Limbs tend to be longer than average

Osteoporosis

Sex chromosomes: XXY

Klinefelter’s Syndrome 44 + XXY

Polyploidy• Organisms that have more

than two sets of chromosomes

• Very common in plants because they can reproduce asexually, but rarer in animals.

• Polyploidy can result in “instantaneous speciation”.

• Polyploid plants are usually more robust and sturdy than diploid plants.

• In our long history of plant cultivation we have selected out such plants because they produced a higher yield and were less subject to disease.

• As a result many of our crops today have been bred to a high level of ploidy.

• Wheat was the first crop to be domesticated originating in SW Asia about 10,000 years ago.

• Today, bread wheat is a hexaploid.

An Expert in Polyploidy• The twenty different species

worldwide differ widely in their chromosome number as they exhibit a range of polyploidy.

• The haploid number is 7

• Strawberry species an be:– Diploid– Tetraploid– Hexaploid– Octoploid– decaploid

As a general rule, strawberry species with more sets of chromosomes tend to be more robust and produce larger plants with larger berries.

Fitness of Mutations

• The fitness of a mutation describes its value to

the survival and reproductive success of the

organism. A mutation may turn out to be:

• Lethal: Many mutations are lethal and embryos are non-viable.

• Harmful: Non-lethal mutations, e.g. Down syndrome and sickle cell disease, may be expressed as effects that lower fitness.

• Silent (neutral): Most point mutations are probably harmless, with no noticeable effect on the phenotype.

• Beneficial (useful): Occasionally mutations may be useful, particularly in a new environment, e.g. insecticide resistance in insects, antibiotic resistance in bacteria.

Mutations: Key Points• All new alleles originate by

mutation.

• New alleles introduce genetic variation: the raw material on which natural selection can act.

• Most mutations occur in somatic cells and are not inherited.

• Only mutations in gametes can be inherited.

Evolutionary Significance of Mutations

• Polyploidy can result in the formation of “instant species” by creating a barrier to chromosome pairing at meiosis (common in plants).

• Fusion of chromosomes (a form of translocation) may reduce chromosome number. This can result in reproductive isolation and a new species.

Example: Fusion of chromosomes may have taken place during the course of human evolution.

The chromosome number in the great apes is 2N = 48, whereas in humans 2N = 46.

Human Chimpanzee Gorilla Orangutan

11

12

132

1212

11

Possible fusion of two chromosomes to create the No. 2 chromosome in humans.

Note the similar banding patterns of chromosomes from related primate species.

Activity:

• Complete Activity 11.2 “Changing your genes”

• Quick check qu 14&15 (pg 400)

• Biochallenge pg 413

• Ch 11 ch review qu 5, 7, 8, 11, 13, 14, 15

Reflection

• What learning was new today?• What learning was revision or built on what I

already know?• What did I find most challenging and what

strategies will I put in place to help me?• What percentage of the class did I spend on

task and how can I improve this if needed