Biology a level- genetic diversity

What is a mutation

a random change in DNA sequence

What is a gene mutation

a change in the base sequence of a gene

When do gene mutations occur

during DNA replication

What are homologous pairs

Pairs of chromosomes that carry the same genes in the same order, but may have different versions (alleles) of these genes

One chromosome is the maternal chromosome, and one is the paternal chromosome

When do mutations in the number of chromosomes arise

- chromosome non disjunction is the failure of chromosomes to separate

- this is either homologous pairs failing to separate in anaphase I or sister chromatids failing to separate in anaphase II

- anaphase I non disjunction means 2 daughter cells have an extra, and two have one less, anaphase II non disjunction means one cell has an extra, one has one less, and the other two are unaffected

- the result of this is that gametes are not the normal haploid number, and if these take part in fertilisation then a chromosome mutation occurs as the diploid cell with have the incorrect chromosome number

What is an example of a chromosome mutation

Down's syndrome.

Individuals have three copies of chromosome 21 rather than two.

What are the types of gene mutation

Deletion- one base is removed from the sequence

substitution- one base is replaced by another

Insertion- one base is added into the sequence

What is the impact of mutations on the protein

The change in primary structure of the protein (order of bases) may change the amino acid sequence coded for by the gene.

If the amino acid sequence is changed, this will mean a different polypeptide chain is produced, which could impact the tertiary structure.

Changes to the tertiary structure can change the shape of the active site which could stop it from forming an enzyme- substrate complex.

Why do not all base substitutions cause a change to the amino acids (proteins) produced

The genetic code is degenerate, which means multiple triplet codes code for the same amino acid.

This means that even if the triplet is changed, it may code for the same amino acid therefore have no impact on the protein.

Also, it may occur in a non coding section of the gene (intron) meaning it is not used to produce proteins.

Why are deletions and insertions more likely to change the amino acid sequence

They both cause a frame shift for all of the triplets after the triplet the mutation occurs in (as the genetic code is non overlapping therefore adding or taking away bases causes the triplet frames to move). This changes every single triplet, meaning it is likely at least one will not be degenerate, and thus make a diff amino acid.

Deletion= left frame shift

Insertion= right frame shift

What are chromosome mutations

Changes to the chromosome structure or number of chromosomes.

The main types include:

Polyploidy - when organisms have more than two sets of chromosomes, and is mostly seen in plants.

Non-disjunction - This occurs when homologous chromosomes fail to separate in meiosis, resulting in gametes with extra or missing chromosomes, and leads to individuals with extra or missing chromosomes in all cells.

What are mutagens

physical or chemical agents that can increase the rate of mutations

Eg: radiation (x rays and UV rays, tobacco)

What is meiosis

A type of cell division in which a parent cell divides to form four genetically unique haploid daughter cells

What is the process of meiosis (summary)

- DNA replication: the 46 chromosomes in the nucleus (all in a homologous pair) in the cell replicate to form 46 sister chromatids

- meiosis I: homologous chromosomes are separated (still as sister chromatids), with the mother or father chromosome in each going randomly to either cell (meaning that each new cell has 23 chromosomes still in sister chromatid form)

- meiosis II: the chromatids are separated

What are sister chromatids

A chromosome joined to its replicated chromosome by a centromere.

These can then align with their homologous pairs to form a pair where each chromosome is in sister chromatid formation.

What is a gamete

A sex cell that contains half of the normal genetic information of the cell (as it only contains chromosomes from the one creating the gametes)

Within this half of genetic material, it contains a random mix of the genetic material from the mother and father of the one creating the gametes, which might not have been expressed in the one creating the gametes.

How do gametes produce a new organism

When two gametes (one from the mother and one from the father) fuse together they create a diploid cell called a zygote, with an equal split of genetic information from the mother and father.

This zygote can then divide and eventually grow into a new organism.

What is the process of meiosis I

Prophase: chromosomes condense and homologous pairs pair up. Centrioles migrate to opposite poles and each begins to form spindle fibres.

Nucleolus and nuclear membrane breaks down.

Metaphase: sister chromatid chromosomes line up along the equator of the cell in their homologous pairs, and each becomes attached to the spindle fibres

Anaphase: homologous pairs are separated and pulled to opposite sides of the cell (sister chromatids stay together)

Telophase: the chromosomes reach the opposite sides of the cell, begin to uncoil, and a new nuclear envelope forms around each set of chromosomes

Cytokinesis: cytoplasm divides and two new cells form

what is the process of meiosis II

The same as meiosis I, except:

There is no DNA replication, meaning there are only 23 chromosomes in sister chromatid form (rather than 46)

- in metaphase the chromosomes now line up individually, rather than in their homologous pairs (like how they do in mitosis)

- in anaphase the sister chromatids are split apart

What is the differences between mitosis and meiosis

Function: mitosis is used for producing genetically identical cells, for growth and repair of body parts, whereas meiosis is used to produce gametes

Reproduction: mitosis is asexual, meiosis is sexual

Cells produced: mitosis produces diploid cells, meiosis produces haploid cells

Mitosis doesn't split homologous pairs, meiosis does.

What is genetic variation

differences that occur in individuals within a species (due to their differing combinations of alleles)

Not the same as genetic diversity, which is the diversity of the alleles themselves

How does meiosis create variation

Independent segregation, crossing over (occurs in meiosis I)

What is independent segregation and how does it create variation

- in meiosis I, homologous pairs line up opposite each other at the equator of the cell, forming a tetrad

- however, it is random which side of the equator the maternal and paternal chromosome lines up on for each pair

- this increases variation, as it means when the pairs are separated, the cells produced have a large possible number of combinations between maternal and paternal chromosome material

What is a tetrad

a pair of homologous chromosomes lined up next to each other

What is crossing over and how does it create variation

- occurs in meiosis one when the homologous pairs line up opposite each other along the equator

- due to their proximity to each other, parts of the chromatids of the mother and father can be twisted around each other at places on the chromatids. The points of contact between the chromatids are called chiasma

- this twisting causes tension, causing the chromatid to break. Where they break off, the can recombine with the other chromatid.

- this results in variation as it means there is a new combination of alleles on the chromatid. The new chromatids are called recombinant.

How can you identify where meiosis occurs in a life cycle

- meiosis is characterised by a diploid cell (2n, meaning 2 copies of each chromosome), to a haploid cell (meaning one chromosome with no homologous pair)

- this, meiosis occurs where a diploid cell becomes a haploid cell

What is random fertilisation and how does it create genetic variation

- there is already 2^n possible chromosome combinations in gametes (where n is the number of chromosome types in the organism= 23 in humans)

- however, during fertilisation, the egg and sperm randomly fuse, meaning that for each of these chromosomes the genetic variation is doubled. This can be represented as (2^n)^2

- this is before variation due to crossing over is even considered

What is genetic diversity

The total number of different alleles present for a gene in a given population (species)

What is a population

all of the organisms of the same species living in the same area at the same time

What is a gene pool

all the different alleles in a population

What is genetic diversity important

It increases variation in alleles between organisms, which means organisms have different phenotypes (observable traits)

- this allows natural selection to take place, as organisms with advantageous alleles will be more likely to survive and reproduce, causing evolution

- this means species have a better chance of surviving as a result

What causes genetic diversity

- mutations, as these can alter genes, creating new alleles

- gene flow, which is the transfer of genetic material from one population to another (either from the same population but a different area (eg: long haired bunnies in icy climates migrate and breed with short haired bunnies in a different climate) or different species in the same area)

what is an allele

A different form of a gene.

If the gene coded for a protein that controls a characteristic such as eye colour, the allele would be different versions of that protein such as blue or brown pigment.

What is evolution

the change in allele frequency in a population over many generations

this change is caused by natural selection

What is the process of natural selection

1) Organisms within a species show variation in their characteristics (alleles) due to genetic variation

This variation is increased due to random mutation.

2) Organisms that have alleles which code for characteristics best adapted to a selection pressure have an increased chance of survival and reproduction chance. Less well adapted ones die or fail to reproduce (this is survival of the fittest)

3) This means that these organisms pass the allele for the advantageous characteristic to their offspring. Ones that are non advantageous are less likely to be passed on, as the organisms die and don't reproduce.

4) This process is repeated, and over time there is a large proportion of organisms with this allele.

5) Over long periods, this process can lead to the evolution of a new species, and species becoming more adapted to their environment

What is the mode

most frequently occurring value

What is directional selection

- one of the extremes around the mode has the selective advantage. Eg: very few bacteria in the population have the the selective advantage of high resistance

- this only occurs when there is a change in the environment, as it means the original mean in the population is no longer the best surviving section.

- this change in selective advantage causes the modal trait to change, as the trait causes more with it to survive. Over time, this then means that the majority of the population has this trait, therefore the mode has shifted towards this extreme.

Eg: all bacteria with high resistance survive, and over time having high resistance becomes the mean.

what are the two types of natural selection

directional selection and stabilising selection

What is stabilising selection

- when one of the modal traits has the selective advantage ie: most bunnies in a population have the advantageous trait of long hair

- this occurs when there is no change in the environment

- this means that the modal trait remains the same, however individuals at the extremes decrease (standard deviation decreases)

What is continuous variation

Continuous variation is where a characteristic can have any value over a range, from minimum to maximum

What is an adaptation

an adaptation is a trait that helps an organism survive and reproduce in its specific environment

What are the types of adaptations an organism may possess

anatomical: physical features of an organism such as body size and physical features such as fur length

physiological: internal body processes and functions that help an animal to survive, such as number of enzymes produced or water able to be conserved

behavioural: the way an organism behaves in its environment eg: hunting strategies, huddling together for warmth

What is an example of stabilising selection

- human birth weights

- babies born with extreme birth weights (very high or very low) are selected against as it is harder for them to survive, reproduce, and pass on their characteristics

- this means over time, the modal birth weights will stabilise towards the mean, as these values have higher survival chances

What is an example of directional selection

- antibiotic resistance in bacteria

- in the absence of antibiotics, the number of bacteria with antibiotic resistance is low

- however, when the environment changes and an antibiotic is now present, these bacteria have a survival advantage, and thus reproduce and pass on this trait

- this means over time the gene becomes more common in the population, and thus the mean shifts in this direction

Are the base sequences on different alleles different

Generally the base sequence for a gene for different alleles is different.

This means that the proteins produced, though carrying out the same function, are slightly different due to their different primary structures.

thus, one might have an advantage for performing the function.

What is random selection

It is the opposite to natural selection, where each individual has an equal chance of being chosen and surviving.

This contrasts natural selection, which favours individuals with advantageous traits.

How do prokaryotic cells divide?

Binary fission

This includes bacteria. It doesn't include fungi (eg: yeast) as these aren't prokaryotic though they are unicellular.

What is the process of binary fission

1. The circular DNA (kind of like a chromosome but not the same) replicates and both copies attach to the cell membrane

2. The plasmids (extra smaller circular loops of DNA) also replicate

3. The cell membrane grows between the two DNA molecules and pinches inwards, dividing the cytoplasm in 2

4. A new cell wall forms between the separating cells, dividing the original cell into two identical daughter cells, containing a copy of the DNA and a random plasmid assortment

How can the growth/ division rate of cells be increased

- increase glucose to increase rate of respiration for energy release for growth processes

- increase temperature to increase enzyme activity

- increase phosphate concentration to increase ATP and DNA production, to ensure that DNA replication can occur and thus the cells can divide

How do viruses replicate

- viruses are parasitic, meaning they can only reproduce by infecting living cells known as host cells using the following process:

1. The virus uses attachment proteins on its surface to bind to complementary receptor proteins on the host cell

2. Once bound to the host, the virus injects its nucleic acid into the host cell

3. The viral nucleic acid is taken up by the cells protein making machinery (ribosomes) causing it to produce new viral proteins and new copies of the viral nucleic acid

4. The newly made proteins are assembled into new virus particles in the ER and Golgi, where they are then released through bursting the cell, or leaving through the cell membrane. If they leave through the cell membrane, they take a part of the membrane with them, which becomes their envelope

How do you calculate population size after divisions

If the organism splits into two every division=

number of organisms in the population x 2^ how many generations (how many times it divides)

This is because every time it divides, the population doubles (hence the x2 part) and then that results then doubles etc.. That can be represented with a power sign.

What does n and 2n mean in genetic variation

n= the number of homologous pairs in the organism: this is what the value is for number of chromosomes in a gamete (as the number of chromosomes halves)

2n= the number of chromosomes in the diploid organism

How do you find the number of possible gamete combinations in an organism after meiosis

2^how many chromosome pairs are in the organism (n)

When are log values used

When there is a very large range between the top and bottom value, which means that a graph cannot be plotted properly to show trends as some values would appear as zero due to the scale being so large on the Y axis

How do you work out the original value from a log value and vice versa

To get from an original value to log, you press the log button and put the original value in brackets

To get from the log back to original, you do 10 to the power of the log