Topic 3- Genetics

Define gene

a heritable factor that controls or influences a specific characteristic, consisting of a length of DNA occupying a particular position on a chromosome (locus)

Define allele

a version/variation of a gene

Describe a chromosome

structural unit made up of DNA and proteins

Describe a gene locus

the location of a gene on a chromosome

What is a snip/ SNP

Single nucleotide polymorphism- the exact positions where bases differ between alleles/ type of genetic variation

Define genome

the entire genetic material of an organism. It consists of DNA (or RNA in RNA viruses) and includes both the genes and the non-coding sequences

Difference between the genome of animals and plants

Animals have more chromosomes compared to plants

What is the Human Genome Project (HGP)?

an international 13-year effort, 1990 to 2003. Primary goals were to discover the complete set of human genes and make them accessible for further biological study, and determine the complete sequence of DNA bases in the human genome.

What is ‘junk’ DNA and what may be one of its functions

Non-coding DNA/ regions of DNA that are noncoding

Functions: regulating transcription and translation, producing different types of RNA, and protecting the ends of chromosomes

Outcomes of the Human Genome Project (HGP)

• It is now easier to study how genes influence human development. 

• It helps identify genetic diseases.

• It allows the production of new drugs based on DNA base sequences of genes or the structure of proteins coded for by these genes.

• It will give us more information on the origins, evolution and migration of humans

Definition of a mutation

a permanent change in the base sequence of DNA

What is a base substitution mutation

One base is substituted for another

may or may not result in the change of a single amino acid in the polypeptide

State the difference between mutations in general and base substitution mutations

Base substitution mutations is one base substituted for another which may or may not result in the change of a single amino acid in the polypeptide. While mutation in general and natural selection allowed all organisms to evolve from simpler ancestors.

Effects of sickle cell disease in terms of: haemoglobin production

• Causes abnormal haemoglobin to clump together, causing the red blood cells to turn sickle shaped

• Sickle shaped cells causes blockages in the blood flow- can lead to anemia

Effects of sickle cell disease in terms of: symptoms and mortality

• Fatigue

• Weakness

• Lung tissue damage- acute chest syndrome

• pain

• strokes

Some people inherit both a normal allele (Hb A) and a sickle cell (Hb S) allele. Such people do show very few symptoms of sickle cell disease. Identify parts of the world this genotype could be beneficial and explain why.

Having a recessive sickle cell allele creates resistance against malaria, this can be helpful in regions where malaria is highly infectious and dangerous

Cause of sickle cell anemia, including the name of differences in the Hb alleles

• caused by single base substitution mutation in the gene coding for one of the polypeptide chains in haemoglobin

• In the mutation, the sequence GAG (on the sense strand of DNA) is mutated to GTG. This results in a codon that codes for the amino acid VAL instead of GLU.

Describe the relationship between the number of genes in a species and the species complexity in structure, physiology and/or behavior.

In general, eukaryotes have more genes than prokaryotes. However, within plants and animals there is little correlation between complexity and the number of genes.

State similarities between alleles of the same gene

Alleles of the same gene are found at the same locus on homologous chromosomes, have mostly the same nucleotide sequence and code for the same general type of protein

State the difference between alleles of the same gene.​

Alleles of the same gene are slightly different from each other in the sequence of nucleotides. They can vary by just one base (i.e. A -->T), called a single nucleotide polymorphism (SNP) or by the insertion or deletion of a base.

State the source of new alleles of a gene

New alleles (versions) of a gene are formed through random mutation (changes) in the DNA sequence of the gene. Most new mutations arise due to errors in DNA replication.

Define missense mutation

When the change in base sequence has caused one different amino acid to be produced

Define nonsense mutatiom

The change in base sequence has caused a STOP codon to be produced, so the polypeptide produced is shortened

Define silent mutation

The change in base sequence has had no effect on the amino acid produced

Genotype

Set / combination of alleles

Diagram to show the two types of DNA present in a generalised prokaryote cell

Distinguish between the two types of DNA

plasmids - small extra circular DNA molecules

and

circular DNA- single circular chromosome

Explain why prokaryotes only possess a single chromosome.

• Chromosome is circular (compared to linear eukaryotic chromosomes)

• One copy of each gene, except during cell division (binary fission!)

• Chromosome is “naked” (i.e. not associated with histone proteins like eukaryotic chromosomes)

Outline Cairns’ technique and results for measuring the length of DNA molecules

Used autoradiography to visualize and measure DNA molecules in E. coli in 1963

• E. coli grown with thymine nucleotides that contained radioactive hydrogen isotope, tritium

• Thymine used so only DNA labeled

• Cells placed on dialysis membrane and lysed with lysozyme enzymes

• Coated in photographic emulsion and left in dark room for 2 months

• High energy electrons from tritium decay reacted with film emulsion, leaving dark lines showing DNA outline

Results: Image showed E. coli had circular chromosome, ~1,100 µm long (E. coli cell is only 2 µm)

Distinguish between eukaryote and prokaryote chromosomes.

Prokaryotic chromosomes are circular and not associated with histone proteins.

Eukaryotic chromosomes are linear and are associated with histone proteins.

Outline the three ways in which chromosomes can vary

• Length

• Position of the centromere

• Gene locus (location of the gene)

If chromosomes vary describe how individuals of a species are similar in terms of their DNA.

All individuals of a species possess the same chromosomes, with the same gene loci.

The number of chromosomes possessed by a species is known as the N number. State what the N number is for humans.

23

Give two reasons why the chromosome number can be used to characterise a species.

- Organisms with different numbers of chromosomes are unlikely to be able to interbreed successfully

- A chromosome number does reflect the complexity of an organism.

Haploid nuclei

A nucleus with only one chromosome of each type.

Diploid nuclei

A nucleus with two chromosomes of each type

List the types of cell in humans that are haploid and state the number of chromosomes present in the nuclei.

Gametes, 23

List the types of cell in humans that are diploid and state the number of chromosomes present in the nuclei

Somatic cells, 46

Diploid Chromosome Number Across Species (Human, Dog, Equine Roundworm, Chimpanzee, Asian Rice)

- Human - 46
- Dog - 78
- Equine Roundworm - 2
- Chimpanzee - 48
- Asian Rice - 24.

State what is meant by the term homologous chromosome.

Chromosomes of the same structure.

The chromosomes that make up a homologous pair have a different origin.
State where they originate and explain why this means that they can possess different alleles for a gene locus

- One maternal pair
- One paternal pair
- Therefore, the pairs can possess different alleles for the same gene locus, as they have different origins.

One pair of chromosomes in human cells is not always homologous.
State which pair

Sex chromosomes

Explain why sex chromosomes are not always homologous

The chromosome pairs segregate/separate during meiosis.

State the gene, if expressed, that causes the development of male characteristics and the chromosome it is located on.

SRY, located on short branch of the Y Chromosome

Explain why there is a 50:50 chance of human offspring being male or female.

Half the male sperm carry an X chromosome/ Half carry a Y chromosome.

Genome size

The total number of DNA base pairs in one copy of a haploid genome

Genome size in the selected organisms:

• Humans

• Virus- T2 phage

• Bacterium- E. coli

• Fruit fly

• Canopy plant

• Human - 3.2 billion base pairs

• T2 Phage - 4.6 million base pairs

• E. coli- 4.6 million base pairs

• Fruit Fly - 130 million base pairs

• Canopy Plant - 150 billion base pairs

Karyogram

A diagram or photograph of the chromosomes present in a nucleus (of a eukaryote cell) arranged in homologous pairs of decreasing length

Karyotype

A property of the cell described by the number and type of chromosomes present in the nucleus (of a eukaryote cell).

State three visual aspects of homologous chromosomes which can be used to identify them for the purpose of a karyotype?

• Banding patterns

• Size

• Centromere position.

How to deduce gender and chromosomal abnormalities from a karyogram

Deducing Sex:

XX = Female

XY = Male

Deducing Chromosomal Abnormalities

• Errors in Meiosis can lead to the formation of zygotes with abnormal chromosome numbers (In humans, this is any number that is not 46 – 44 autosomes, 2 sex chromosomes)

• Trisomy: Having a third (extra) chromosome

• Monosomy: Having only one chromosome

Meiosis

process by which sex cells (gametes) are made in the reproductive organs

Differences between mitosis and meiosis

Reduction division

Number of chromosomes is reduced from diploid to haploid (they are halved)

Prophase I

• DNA supercoils and chromosomes condense

• The homologous chromosomes associate with each other to form bivalents (synapsis)

• nuclear membrane dissolves

• centrioles migrate to the poles of the cell

• Crossing-over between non-sister chromatids can take place. This results in recombination of alleles and is a source of genetic variation in gametes

Metaphase I

• Random orientation occurs - each bivalent aligns independently and hence the daughter nuclei get a different mix of chromosomes

• The bivalents line up at the equator

Anaphase I

• Homologous pairs are separated and pulled to opposing poles

• Spindle fibres contract

• This is the reduction division – the bivalent is split and half the chromosomes move towards each pole

Telophase I

• The nuclei are now haploid (N) not diploid (2N): they each contain one pair of sister chromatids for each of the species’ chromosomes

• cytoplasm begins to divide by cytokinesis

• New nuclei form

• Chromosomes decondense

Prophase II

• Nuclear membranes dissolves

• Chromosomes, consisting of two sister chromatids, condense

• No crossing-over occurs

• centrioles move to opposite poles (perpendicular to previous)

Metaphase II

• Spindle fibres form and attach at the centromeres.

• Pairs of sister chromatids align at the equator

Anaphase II

• The sister chromatids are separated. The chromatids (now called chromosomes are pulled to opposing poles

• Spindle fibres contract and the centromeres are split

How to identify whether a chromosome is homologous

• Length

• Position of the centromere

• Banding

Why is meiosis important?

It ensures that all organisms produced via sexual reproduction contain the correct number of chromosomes

Meiosis I

first cell division- homologous pair of replicated chromosomes separates

Meiosis II

second cell division- sister chromatids separate to produce 4 haploid daughter cells

Telophase II

• Four new haploid nuclei are formed

• Nuclear membrane reforms

• Chromosomes decondense

• Cytokinesis begins, dividing the cells

• End result is four haploid gamete cells

• Fertilisation of these haploid gametes will produce a diploid zygote

Interphase

• In the S-phase of the interphase before meiosis begins, DNA replication takes place

• Chromosomes are replicated and these copies are attached to each other at the centromere

• The attached chromosome and its copy are known as sister chromatids

• Following S-phase, further growth and preparation take place for meiosis

Gregor Mendel

He established the principles of inheritance via experimentation (he crossed large numbers of pea plants)

What were the findings/discoveries from Mendel’s experiment

• Organisms have heritable factors (genes)

• Parents contribute equally to inheritance by supplying one version of the gene each (alleles)

• Gametes contain only one allele of each gene (haploid)

• Fusion of gametes results in zygotes with two alleles of each gene (diploid)

It is now known that the separation of the two alleles of each gene into separate haploid gametes occurs via meiosis

Genotype

The combination of alleles of a gene carried by an organism

Phenotype

The expression of alleles of a gene carried by an organism

Dominant allele

An allele that has the same effect on the phenotype whether it is present in the homozygous or heterozygous state

Codominant allele

Pairs of different alleles that both affect the phenotype when present in a heterozygote

Homozygous

Having two copies of the same allele

Heterozygous

Having two different alleles

Carrier

Heterozygous individuals who possess one copy of the faulty/recessive allele but do not develop disease symptoms

Autosomal genes

Genes which are not on the sex-chromosome

Many genetic diseases in humans are due to recessive alleles of autosomal genes

Sex-linked inheritance

traits or disorders influenced by genes on the X chromosome

What is meant my multiple alleles

occurs when more than two gene/allele forms exist for the same locus

State the genotype and phenotype of a blood group which is an example of codominance.

Phenotype: Blood group AB

Genotype: I^ A, I^B

Explain why blood group AB is an example of codominance

Because both the type ‘A’ allele and type ‘B’ allele are dominant and so both alleles are expressed

Recessive allele

An allele that only has an effect on the phenotype when present in the homozygous state

Explain why genetic diseases are very rare in humans

• There are a large number of genes present in the human genome

• Most conditions are autosomal recessive (need 2 recessive alleles to be expressed)

• the probability that both parents have a mutation on the same gene is very small

State two examples of sex-linked genetic disorders

• haemophilia

• red-green colour blindness

Explain why sex-linked disorders are more common in males than females

• Because sex-linked traits are those which are carried on the X-chromosome in the non-homologous region

• Alleles in this regions are expressed whether they are dominant or recessive, as there is no alternate allele carried on the Y chromosome

Explain why human females can be homozygous or heterozygous for sex-linked genes, where males cannot.

Females have two X chromosomes (males only have 1), so they will have two copies of each X-linked gene

State the normal function of the gene associated with hemophilia.

Blood clotting

results from a lack of clotting factors - globular proteins, which act as enzymes in the clotting pathway.

Describe the signs and symptoms of hemophilia.

• Excessive bleeding

• Wounds don't stop bleeding

• Blood clotting disorder.

Suggest reasons why the frequency of some disease-related alleles might be increasing in the population.

• May be maintained by gene flow.

• having an allele causing disease increases an organisms fitness (ability to survive and reproduce)
  -ex: sickle cell anemia are immune to malaria, can survive and reproduce

State the definition of a mutation.

A change in an organisms genetic code

Mutations can cause a change in a gene allele, which can be harmful. Occasionally mutations can be beneficial. Some changes however are ‘silent’, i.e. they don’t cause a change in the trait. Explain how this is possible.

Neutral - Due to degenerate nature of DNA.

because they do not alter the polypeptide or only alter it slightly so that its structure or function is not changed

Mutagens are agents that cause gene mutations. List three types of mutagens.

• carcinogens

• high-enery radiation (x-rays)

• ultraviolet light

Distinguish between mutations that can affect an individual during their lifetime and those which can lead to genetic diseases.

Mutations that affect an individual: Mutations that occur in body (somatic cells) and remain within the organism

Mutations which can lead to genetic diseases: Mutations that occur in gametes can be inherited by offspring

Impacts and supporting evidence of accident at Chernobyl nuclear power station

Impacts:

• A large area of pine forest downwind of the reactor turned brown and died.

• Horses and cattle near the plant died from radiation damage to their thyroid glands.

• Bioaccumulation of radioactive caesium in fish (Scandinavia and Germany) and lamb (Wales)

Evidence:

Drinking water (and milk) contaminated with radioactive iodine - at least 6,000 thyroid cancer cases attributed to radioactive iodine.

Limitations of the evidence /what cannot be concluded from accident at Chernobyl nuclear power station

No clear evidence to support an increase in the rate of leukemia and other cancers – in part due to the widely dispersed variable radiation and measures taken in European populations.

Impacts and supporting evidence of release of a nuclear bomb at Hiroshima

Impacts:

• Radioactive isotopes released into the environment exposing humans and other organisms to potentially dangerous levels of radiation.

Evidence:

• Elevated rate of Leukemia (with the greatest impact in children and young adults)

• Elevated rates of other cancers

Limitations of the evidence /what cannot be concluded from release of a nuclear bomb at Hiroshima

No evidence of stillbirth or mutations in the children of those exposed to radiation

Mendel’s experiment

• carefully transferred pollen from one pea plant to the reproductive parts of another

• He collected the pea seeds from these plants and grew them in favourable conditions to find out their characteristics

• He also cross-bred offspring peas in order to find out which, if any characteristics would appear in future generations

• Mendel investigated the height of pea plants, the colours of their flowers and the smoothness of their seed coat

Huntington’s disease

• a genetic condition that develops as a person ages

• experience neurological degeneration; they lose their ability to walk, talk and think

Cystic fibrosis

• a genetic disorder of cell membranes caused by a recessive allele (f) of the CFTR gene located on chromosome 7

• causes secretions which are usually thin to become thick

• CFTR codes for the production of chloride ion channels (required for secretion of sweat, mucus and digestive juices)