VCE BIOLOGY UNIT 4EXAM REVISION

Gary Simpson

Contents Chromosomes, Genes and Alleles Structure of DNA Protein Synthesis –

Transcription/Translation Genetic Code Cell Reproduction Mitosis Meiosis Karyotypes Inheritance at one gene locus Pedigree Analysis Inheritance involving two gene loci Variation

Mutation Genotype/Phenotype/

Environment Restriction Enzymes Recombining DNA Vectors Polymerase Chain Reaction Gene Probes Gel Electrophoresis DNA Profiles DNA Sequencing Applications of Gene Technology

Chromosomes, Genes and Alleles

Chromosomes – are composed of DNA Genes – short pieces of DNA Alleles – the alternate forms of a gene Gene Classification – genes are classified by whether

they are structural, regulatory or homoeotic Gene Activity – embryonic stem cells are totipotent,

that is all the genes can express themselves, as cells specialise genes become switched off and switched on and so the ability of cells to be cloned from specialised cells for other purposes does not exist.

Structure of DNA Comprised of a sugar

unit and a phosphate molecule that forms the backbone of the double helix and nitrogen bases which join the two strands together. Adenine binds with

Thymine Cytosine binds with

Guanine.

Protein SynthesisTranscription

RNA polymerase attaches to a region of DNA and unwinds the double strand.

nucleotides align with complimentary bases to form a single stand of pre mRNA

the introns are removed to form mRNA a methyl cap and a poly A tail are added

to produce operational mRNA the mRNA exits the nucleus via the

nuclear pores.

Protein SynthesisTranslation

The operational mRNA travels to the Ribosomes within the cytosol.

tRNA in triplets (anti-codons) carrying amino acids are attracted from the cytosol to bind with complimentary codons on mRNA

Amino acids combine to form a protein.

DNA RNA

Sugar Unit Deoxyribose Oxyribose

Nitrogen bases A, T, G, C A, U, G, C

Double/single strand

Double Single

Location Nucleus Mostly Cytoplasm

Comparison of DNA and RNA

Protein SynthesisSummary

DNA Complimentary Strand - ATG GTC GCC GGC AGA TGA DNA Template Strand - TAC CAG CGG CCG TCT ACT

mRNA - AUG GUC GCC GGC AGA UGA

tRNA - UAC CAG CGG CCG UCU ACU

Amino acid sequence - Start, Val, Ala, Gly, Arg, Stop

DNA CodeSecond Position

First position U C A G

Third position

  Phe Ser Tyr Cys U

U Phe Ser Tyr Cys C

  Leu Ser STOP STOP A

  Leu Ser STOP Trp G

  Leu Pro His Arg U

C Leu Pro His Arg C

  Leu Pro GluN Arg A

  Leu Pro GluN Arg G

  Ileu Thr AspN Ser U

A Ileu Thr AspN Ser C

  Ileu Thr Lys Arg A

  Meth Thr Lys Arg G

  Val Ala Asp Gly U

G Val Ala Asp Gly C

  Val Ala Glu Gly A

  Val Ala Glu Gly G

Information exists in three base sequences

Code is not overlapping (bases are read 3 at a time)

Code is universal (all organisms share the same nucleotides)

Code is redundant (more than one codon can code for the same amino acid)

Code is unambiguous (each codon only codes for one amino acid)

Cell Reproduction Multiple cell organisms

grow by cells copying themselves and dividing, this requires the copying of the DNA.

The Cell Cycle G1 Phase – cell growth

prior to DNA replication.

S Phase – DNA replication

G2 Phase – the cell prepares for division into two

M Phase – Mitosis – the nucleus divides

C Phase – Cytokinesis – the division of the two cells is completed.

Mitosis Mitosis occurs in all organisms that are actively

growing. During this process one cell divides and produces 2 daughter cells that are identical to the parent cell. These cells have two complete sets of chromosomes and are therefore described as diploid or 2n cells.

Meiosis Process that occurs in the gonads (ovaries and

testes) to produce 4 haploid cells called gametes (eggs and sperm).

Two stage process. First division is the same as mitosis (one diploid cell makes two diploid cells) the second division reduces the two diploid cells to four haploid cells.

Prophase 1 – highest potential for single gene mutation as the chromosomes are most fragile as they are copied.

Metaphase 1 – crossing over may occur during random assortment of chromosomes at the equator in preparation for independent assortment.

Mitosis v MeiosisMITOSIS MEIOSIS

Number of replications 1 1

Number of divisions 1 2

Number of daughter cells produced

2 4

Haploid/Diploid Diploid Haploid

Types of cells produced Somatic Gametes

Karyotypes A Karyotype is

essentially a map of an individuals chromosomes. It is created by analysing a tissue sample.

Tissue can be sampled using chorionic villus sampling, mouth swab, amniocentesis, sperm, mucous, skin, or hair samples.

Segregation of Alleles Gregor Mendel developed a series of laws

to explain the separation of alleles into different gametes.

1st Law states that the two alleles separate into different gametes

2nd Law states that the separation of alleles for one gene are independent of the separation of alleles for another gene.

Inheritance at a single gene locus

Homozygous – two alleles identical Heterozygous – two alleles not identical Test Cross – used to determine the

genotype of unknown individuals. One crosses an unknown individual with an individual known to be homozygous recessive.

Inheritance at a single gene locus Monohybrid cross –

alleles of only one gene at a single locus

Punnett Squares are used to determine the chance of each type of offspring occurring.

Mother’s Allele 1 (A)

Mother’s Allele 2 (a)

Father’s Allele 1 (A)

AA Aa

Father’s Allele 2 (a)

Aa aa

Inheritance at a single gene locus Complete

Dominance – the affect of one allele masks the affect of the other allele.

For example, the Allele B for large noses is dominant to the allele b for small noses. 50% of the offspring will have large noses and the other 50% will have small noses.

Allele 1B

Allele 2b

Allele 1 b Bb bb

Allele 2 b Bb bb

Inheritance at a single gene locus

Co-dominance, two alleles are both dominant over a third.

For example, in human ABO blood groups both A and B are dominant over type O blood.

Phenotype Genotype

A Type Blood IA IA , IA i

B Type Blood IB IB , IB i

AB Type Blood IA IB

O Type Blood i i

Inheritance at a single gene locus

Incomplete Dominance – the affect of the two alleles are blended

For example, if an allele for white flowers (W) is mixed with an allele for red flowers (R) then the offspring have pink flowers.

Allele 1 W

Allele 2 W

Allele 1 R

RW RW

Allele 2 R

RW RW

Inheritance at a single gene locus

Lethal Alleles – the presence of some alleles can cause the death of the individual, either as an embryo or before they reach reproductive age.

Sex linkage – is when a gene is found on either the X or Y chromosome, then it is said to be sex linked and its pattern of inheritance is different to autosomal loci.

Autosomal recessive inheritance Things to remember:

For an individual to express an autosomal recessive trait both copies of the allele must be present

If both parents affected then all offspring will also be affected.

Recessive traits tend to skip generations with few individuals affected.

Autosomal dominant inheritance Things to remember:

For an autosomal dominant trait to be expressed only one copy of the allele is required.

Usually present in each generation, many affected individuals.

If the homozygous dominant condition is lethal the phenotypic and genotypic ratios will be different – good exam trick!

X-linked recessive inheritanceThings to remember:

Males only require one allele to express the trait, therefore more males than females will show the trait.

Females require both alleles to show the trait.

Heterozygous females are described as carriers of the trait

X-linked dominant inheritanceThings to

remember: Any individual with

the trait must have a parent with the trait

Females may be heterozygous and show the trait

This condition is much harder to detect, so again a good exam trick question!

Inheritance involving two gene loci

Independent assortment of chromosomes. If two genes are found on different chromosomes they are said to be inherited independently.

AB Ab aB ab

AB AABB AABb AaBB AaBb

Ab AABb Aabb AaBb Aabb

aB AaBB AaBb aaBB aaBb

ab AaBb Aabb aaBb aabb

Variation Continuous and discontinuous Polygenes Sources of variation include mutations,

environmental influences, random fertilisations, independent assortment, crossing over during meiosis

Discrete Variation

ContinuousVariation

Mutations Mutations are a change in the DNA

sequence that may or may not have a significant impact on the phenotype.

Can occur naturally or caused by high temperatures, chemicals and radiation

Mutations can be responsible for new alleles and variations in the phenotype of a species.

Point Mutation This is caused by a change to one base:

Template DNA TAC TTC ACG GCA ATC CGA

Deletion TAC (T)TCA CGG CAA TCC GA?

Addition T(T)A CTT CAC GGC AAT CCG A??

Substitution TA(T) TTC ACG GCA ATC CGA

Inversion TAC TTC A(GC) GCA ATC CGA

Chromosomal Mutation This is caused by a change to a

complete chromosome or large part of it: Deletion – whole chromosome or section

of it removed Duplication – whole chromosome or

section of it duplicated Inversion – a region of the chromosome is

turned around Translocation – a region of one

chromosome is added to another.

Genotype and Phenotype Genotype refers to the alleles carried by an

individual. Eg Bb Phenotype refers to the visible characteristic that an

organism displays. Eg. Bb means Big nose.

Environmental Influence on Phenotype

The phenotype of an individual is the sum of the genotype and the effects of the environment.

Reverse Transcriptase This enzyme is used to make copy DNA

(cDNA) from mRNA. As mRNA contains no introns only the

coding regions of a gene it is very useful when inserting genes into other organisms.

Restriction Enzymes Restriction enzymes, or endonucleases,

are naturally occurring chemicals which can cut strands of DNA at known positions, called recognition sequences.

They can produce blunt end cuts or sticky end cuts.

Recombination Once a piece of DNA has been cut with a

restriction enzyme then it can be recombined with itself or with other strands of DNA using DNA ligase.

Vectors

Polymerase Chain Reaction

Gene Probes

Gel Electrophoresis

DNA Profiles Short Tandem Repeats

(STR) are sections of non-coding DNA of between 2 and 5 bases which are inherited and are display distinctive patterns within families.

Variable Nucleotide Tandem Repeats (VNTR) are longer than 5 bases, but also display distinctive patterns within families.

DNA Sequencing It is possible to map

the location of genes by determining the position of each nucleotide

Applications Medicine Malnutrition Agriculture Conservation Forensic Science