Unit 1 KA4-KA8

Question 1

Cellular differentiation

Answer 1

Cellular differentiation is the process by which a cell expresses certain genes to produce proteins characteristic for that type of cell.
This allows a cell to carry out specialised functions.

Question 2

Cellular differentiation in plants

Answer 2

-In plants, meristems are regions of unspecialised cells that can divide (self- renew) and/or differentiate
Two examples of meristem regions are the root and shoot tip.

Question 3

Cellular differentiation in animals

Answer 3

-In animals, unspecialised cells called stem cells can divide (self- renew) and differentiate into specialised cells.

Question 4

Stem cells

Answer 4

Stem cells are cells that have not undergone differentiation . A cell which has not yet become specialised is called undifferentiated

Question 5

Embryonic stem cells

Answer 5

-Embryonic stem cells come from very early embryos.
Since all of the genes in these cells are switched on, embryonic stem cells have the potential to differentiate into any type of cell which makes up the organism– they are pluripotent.

Question 6

Tissue(adult) stem cells

Answer 6

-Tissue (adult) stem cells are found in specific areas such as skin and bone marrow.
They are needed for growth, repair and renewal of the cells found in that tissue.
They replenish differentiated cells that need to be replaced e.g. skin and blood cells.
Since many of their genes have already been switched off, tissue stem cells can only differentiate into all the types of cell found in a particular tissue type. They are multipotent.

Question 7

Research Value of Stem Cells

Answer 7

Stem cell research provides information on how cell processes such as cell growth, differentiation and gene regulation work.
Stem cells can be used as model cells to study how diseases develop or for drug testing.

Question 8

Therapeutic Value of Stem Cells

Answer 8

Stem cells can be used therapeutically for the repair of diseased or damaged organs or tissue.
Stem cells from the embryo can be made to self-renew under the right conditions in the lab. For this reason they can be used in corneal repair and the regeneration of damaged skin.

Question 9

Example of therapeutic use of stem cells

Answer 9

• Skin grafts for badly burned patients
Adult skin cells can be removed from an area of healthy skin and cultured in the lab to produce new skin. The new skin can then be grafted onto the damaged areas of the patient.
• Corneal transplants
Stem cells found at the edge of the cornea of an undamaged eye can be removed, cultured and then transplanted onto the other damaged eye.

Question 10

Ethical issues

Answer 10

The extraction of embryonic stem cells results in the destruction of the human embryo. This means the loss of a potential human life and many people feel that this is unethical. Which is more important:-
• The discovery of new medical treatments to prevent/ease suffering, or
• To respect the value of human life?

Question 11

Structure of genome

Answer 11

The genome of an organism is its entire hereditary information encoded in DNA.
The genome is made up of genes (coding sequences) and other DNA sequences that do not code for proteins (non-coding sequences). Most of the eukaryotic genome consists of non-coding sequences.

Question 12

Coding sequences

Answer 12

• DNA sequences which code for the production of a protein (genes)

Question 13

Non coding sequences

Answer 13

• Sequences that are transcribed to RNA but are not translated (e.g. tRNA, rRNA)
• Those which regulate transcription

Question 14

Mutations

Answer 14

A mutation is a change in the DNA that can result in no protein or an altered protein being synthesised.
Mutations may affect a whole chromosome or may simply affect a single gene on a chromosome.

Question 15

Single gene mutation

Answer 15

A single gene mutation involves the alteration of a DNA nucleotide sequence as a result of substitution, insertion or deletion of nucleotides.

Nucleotide insertions and deletions result in frame-shift mutations.
Frame-shift mutations cause all the codons and all the amino acids after the mutation to be changed. This has a major effect on the structure of the protein produced.

Question 16

Substitution

Answer 16

• Substitution: involves the removal of one nucleotide and its replacement with another nucleotide containing a different base.

There are three types of nucleotide substitutions: missense, nonsense and splice-site mutations.

Question 17

Missense mutations

Answer 17

This can result in one amino acid being changed for another. This may result in a non-functional protein or have little effect on the protein.

Question 18

Nonsense mutations

Answer 18

Result in a premature stop codon being produced which results in a shorter protein

Question 19

Splice site mutations

Answer 19

Result in some introns being retained and/or some exons not being included in the mature transcript.

Question 20

Insertion

Answer 20

involves an additional nucleotide being added into the sequence.

Question 21

Deletion

Answer 21

involves the removal of a nucleotide from the sequence.

Question 22

Chromosome mutations

Answer 22

A chromosome mutation may affect a change in the structure of a chromosome or a change in the number of chromosomes in a cell. The substantial changes in chromosome mutations often make them lethal.
There are 4 types:-
Deletion:- is where a section of a chromosome is removed
Duplication:- this is where a section of a chromosome is added from its homologous partner.
Translocation:- this is where a section of a chromosome is added to a chromosome, not its homologous partner.
Inversion:- is where a section of chromosome is reversed.

Question 23

Importance of mutations and gene duplication in evolution

Answer 23

Duplication allows potential beneficial mutations to occur in a duplicated gene whilst the original gene can still be expressed to produce its protein.
For example Coldwater fish have antifreeze protein in their blood. This is a result of a duplication of a gene which codes for a digestive enzyme. This gene then underwent a second mutation to allow the duplicated gene to produce an antifreeze protein.
This duplication allows the fish to keep the original gene which codes for the digestive enzyme but it now has an extra gene that produces the antifreeze protein.
This is beneficial as it means the fish can survive in colder water

Question 24

Evolution

Answer 24

Evolution is the changes in organisms over generations as a result of genomic variations.
It involves the processes of natural selection, inheritance and speciation.

Question 25

Selection

Answer 25

ection
Natural selection is a non-random increase in frequency of DNA sequences that increase survival and the non-random reduction in the frequency of deleterious sequences.
A deleterious DNA sequence will result in an organism being poorly adapted to the environment and many organisms
may die as a result. Thus there will be a reduction in frequency of these sequences throughout the generations and they may eventually disappear from a population.

Question 26

Stages of natural selection

Answer 26

• organisms produce more offspring than can be supported by the environment.
• these organisms struggle to survive due to lack of food, predation, disease etc.
• variation exists within a species.
• the organisms best adapted (suited) to the environment
(e.g. the ability to run fast
and escape predators) will survive, reproduce and pass on their

favourable characteristics to their offspring.
• the organisms less well adapted (suited) to the environment (e.g. slow animals who cannot escape predators) die and fail to pass on their less favourable characteristics.
• this process of selection - survival of the fittest and weeding out of the weakest - is repeated throughout the generations.

Question 27

The changes in phenotype frequency

Answer 27

result of stabilising, directional and disruptive selection
When you graph data for continuous variable (one that can be measured) in a large population you should get a “bell shaped curve” or normal distribution.

Question 28

Stabilising Selection

Answer 28

The average phenotype is selected for and the extremes of the phenotype range are selected against

e.g. babies of very low birth weight (one extreme) are more likely to develop life-threatening conditions at birth; babies of very high birth weight (other extreme) are more likely to experience complications during birth; babies within the average size range are more likely to be healthy and have a non-complicated birth. So average-sized babies have more chance of surviving and passing on their alleles.
Stabilising selection leads to a reduction in genetic diversity without changing the average value.

Question 29

Directional Selection

Answer 29

This type of selection is most common during a period of environmental change. One extreme of the phenotype range is selected for.

e.g. The dark form of peppered moth increased in number in industrial areas because they were camouflaged by the soot on the trees, not eaten by predators and survived to pass on their dark genes to offspring. So directional selection favoured a move from the light to the dark form.

Question 30

Disruptive Selection

Answer 30

In this type of selection, two or more phenotypes are selected for.
This results in the population becoming split into two distinct groups, each with
its own average value.
e.g. in salmon, larger male fish are better at competing for territories; however, smaller male fish with no territory can sneak into larger fish territories without being noticed and fertilise the eggs

Question 31

Vertical Transfer

Answer 31

Vertical gene transfer is where genes are transferred from parent to offspring as a result of by sexual or asexual reproduction.
Population after selection
Original population

Sexual reproduction involves two parents who differ genetically from one another. Offspring inherit different combinations of genes from each parent and so they show variation.
Asexual reproduction involves one parent. The offspring produced are genetically identical to the parent and to each other.
Vertical inheritance occurs in eukaryotes and prokaryotes.

Question 32

Horizontal Transfer

Answer 32

This is where genes are transferred between individuals in the same generation.
Natural selection is more rapid in prokaryotes. Prokaryotes can exchange genetic material horizontally, resulting in faster evolutionary change than organisms that only use vertical transfer.

Question 33

Speciation

Answer 33

A species is a group of organisms capable of interbreeding and producing fertile offspring and which does not normally breed with other groups.
Speciation is the generation of new biological species by evolution as a result of:
1. isolation of populations

2. mutation
3. natural selection

Question 34

Importance of isolation barriers

Answer 34

They prevent gene flow between sub-populations during speciation. Different barriers lead to different types of speciation.

Geographical (sea, river, desert, mountain)-allopatric
Behavioural (reproductive differences)-sympatric
Ecological (changes in habitat such as pH and humidity)-sympatric

Question 35

Allopatric Speciation

Answer 35

• Isolation
Sub-populations become isolated by a geographical barrier such as a sea, river, desert or mountain.
• Mutation
Different mutations will occur at random within each sub-population resulting in new variation, but because of the barrier, these mutations will not be passed between the sub-populations.
• Natural Selection
The selection pressures acting on each sub-population are different (depending on climate, predators, disease etc.)

If the mutation is of benefit to the organism, the organism will survive and pass on that beneficial characteristic to its offspring (natural selection) - thus the mutation will become more common within that sub- population.
Over a very long period of time, and after more mutations and natural selection, the gene pools of the sub-populations will be so different that they will not be able to interbreed successfully and produce fertile offspring (even if the barrier is removed) - they are now separate distinct species.

Question 36

Sympatric Speciation

Answer 36

Sympatric speciation occurs when an ecological or behavioural barrier prevents gene flow between populations. Thus the populations are genetically isolated from one another.
• Ecological Barriers
These are caused by changes in habitat conditions (e.g. humidity, pH) and one sub-population may live in a particular type of habitat that others may not survive in.
• Behavioural Barriers
These are reproductive differences between sub-populations can prevent interbreeding e.g. there may be a lack of attraction between males and females of different subgroups.

Question 37

Genomics

Answer 37

Genomics is the study of genomes.

Question 38

Genomic Sequencing

Answer 38

In genomic sequencing the sequence of nucleotide bases can be determined for individual genes and entire genomes.
Computer programs can be used to identify base sequences by looking for sequences similar to known genes.

Question 39

Comparison of Genomes from Different Species

Answer 39

Comparison of sequence data requires bioinformatics – computer and statistical analyses.
In addition to the human genome many other genomes have been sequenced:-
• Disease-causing organisms, such as bacteria and viruses
• Pest species, such as the organism that causes malaria
• Species that are important model organisms for research – they have
genes equivalent to human genes responsible for inherited diseases and may provide the key to the development of treatments for these diseases

Comparison of genomes reveals that many genes are highly conserved across different organisms e.g. humans and whales are very distant relatives, yet the base sequences of many of their genes are very similar.
Highly conserved DNA sequences can be used to find out how close or distant the relationship between two groups is; the greater the number of conserved DNA sequences they have in common, the more closely related they are.

Question 40

Phylogenetics

Answer 40

Phylogenetics is the study of evolutionary history and relationships.
It uses genomic sequence data to determine the evolutionary relatedness
amongst groups of organisms.
Sequence divergence is then used to estimate time since lineages diverged (split).
A phylogenetic tree can be drawn using these sequence data results in combination with fossil evidence.

Question 41

Molecular Clocks

Answer 41

These are used to show when species diverged during evolution. They assume a constant mutation rate (this causes groups to become
genetically different)
These groups then separate due to the differences in the DNA sequences or amino acid sequences.
Therefore, differences in sequence data between species indicate the time of divergence from a common ancestor.
The higher the number of differences (mutations) found, the longer the time since the groups diverged.

Question 42

Events in the evolution of life

Answer 42

Evidence from molecular clocks and phylogenetics are used to determine the main sequence of events in evolution.
The sequence of events in the evolution of life can be determined using sequence data and fossil evidence.
Comparison of sequences provides evidence for three domains of life from the first universal ancestor:-
1. bacteria (traditional prokaryotes)
2. archaea (prokaryotes that survive in extreme environments
such as hot springs)
3. eukaryotes (plants, animals & fungi)

Question 43

Personal Genomics and Health

Answer 43

Many diseases have a genetic risk and since individuals all have slight variations in their genome sequences (largely due to mutations), analysis of these sequences could:-
• assess their risk of developing certain diseases or conditions
• assess the chances of a particular treatment being successful.
Pharmacogenetics is the use of genome information in the choice of drugs.
An individual’s personal genome sequence can be used to select the most effective drugs and dosage to treat their disease (personalised medicine)
So knowing the genome sequence could be used to predict which medicines, and which dosages, will be most effective in one person compared to another.