OCR A-Level Biology: Module 6

what are the two types of mutations

DNA and chromosome mutations

what is a DNA mutation

a random change in the quantity / arrangement of bases in an individual gene

what is a chromosome mutation

a random change in the structure of a chromosome

what can cause mutations

mistakes during DNA replication (e.g. with DNA polymerase) or mutagens: agents that cause mutations (e.g. asbestos or viruses)

what are the two different types of DNA mutation

substitution mutations and frameshift mutations (deletion / insertion of a nucleotide(s))

what is a silent mutation

when the same amino acid is coded for (because the genetic code is degenerate), so does not change the amino acid (e.g. GAA Glu -> GAG Glu)

what is a missense mutation

when a different amino acid is coded for (e.g. GAA Glu -> GAC Asp)

what is a nonsense mutation

when a stop codon is formed prematurely

why are frameshift mutations more harmful than point mutations

because they can change every amino acid past the mutation point

what are the four types of chromosome mutation

deletion, duplication, inversion, translocation

what is a deletion mutation

when part of the chromosome is deleted

what is an duplication mutation

when part of the chromosome is duplicated

what is an inversion mutation

when a sequence of genes is flipped back to front

what is a translation mutation

when part of one chromosome moves to another chromosome,

what are the three possible effects of mutations

beneficial, neutral or damaging

what is a beneficial mutation

when a new/superior gene is produced which could lead to the gain of a reproductive advantage e.g. the ability to digest lactose (this is how natural selection works)

what is a neutral mutation

has no effect, but changing the base sequence of a gene can lead to another version of the gene (an allele) e.g. eye colour

what is a harmful mutation

producing an inferior or no protein reduces the chance of survival and reproduction by causing disease and/or death e.g. causing sickle cell anaemia

what is a housekeeping gene

a gene whose protein is always expressed / 'switched on' (regulate important cell functions)

when can gene expression be regulated (turned on / off)

during transcription, after transcription and after translation

what can E. coli bacteria use instead of glucose for respiration

lactose (made from lactose permease and beta galactosidase)

what is an operon

a cluster of genes under the control of a single promoter (the promoter is where RNA polymerase binds)

what genes make up the lac operon in E.coli

I , P, O, Z, Y

what is the I gene on E.coli's lac operon

the regulatory gene

what are the two control sites on E.coli's lac operon

P (promoter region) and O (operator region)

what is the role of the promoter region on E.coli's lac operon

binding site for RNA polymerase for transcription of Z and Y

what is the role of the operator region on E.coli's lac operon

switches Z and Y on and off

what are the two structural genes on E.coli's lac operon

Z (beta galactosidase) and Y (lactose permease) which code for proteins

what happens if lactose is absent at E.coli's lac operon

the regulator gene (I) is expressed and produces the repressor protein (R), which binds to the operator region (O), partially covering the promoter region (P), so RNA polymerase cannot bind and Z & Y genes cannot be translated, so Z and Y are switched off

what happens if lactose is present at E.coli's lac operon

the inducer molecule (lactose) binds to the repressor protein, so the repressor protein leaves the operator region, unblocking it, so RNA polymerase & transcription factors can now bind to the promoter region and Z & Y can be transcribed, so mRNA is produced and beta-galactosidase & lactose permease are synthesised (so lactose is metabolised)

what is epigenetics

the study of changes in organisms caused by modification of gene expression rather than alteration of the genetic code itself

how can gene expression be modified during transcription

chromatin remodelling, histone modification or transcription factors

what is chromatin remodelling

when DNA is wrapped around histone proteins (to make chromatin), RNA polymerase cannot reach the genes and they cannot be transcribed, so altering the structure of chromatin to expose or hide certain genes determines whether or not they can be transcribed

what is histone modification

adding acetyl groups to histone proteins may help uncoil them, and adding methyl groups to histone proteins may stop DNA unwinding from them, which allows transcription to be started or stopped

what are transcription factors

small proteins that help RNA polymerase to bind to promoters and switch on genes (in bacteria cAMP receptor proteins also bind to DNA and helps RNA polymerase to bind)

how can gene expression be modified post-transcription

5' cap and poly-A tail, splicing or RNA editing

what is a 5' cap and poly-A tail

a 5' cap and poly-A tail can be added to protect mRNA from enzyme degradation

what is splicing

cutting out introns (non-coding areas)

what is RNA editing

base addition, deletion or substitution

how can gene expression be modified post-translation

cyclic AMP, protein kinases, addition of carbs and lipids or formation of disulphide bridges

what is cyclic AMP

cAMP activates proteins by altering their three dimensional structure

what are protein kinases

enzymes that add phosphate groups to proteins (involved with activation of enzymes)

what is addition of carbs and lipids

adding carbohydrates and lipids to proteins (like glycoproteins in the cell surface membrane)

why are fruit flies (Drosophila melanogaster) such good model organisms for genetic studies

because they are small, easy to keep, have a short life cycle, have been widely studied, and are more ethical to study than humans

what are homeobox genes

genes that control body plan

what is a body plan

the general structure of an organism

how are homeobox genes arranged

in clusters (in the same head-to-tail order as the body parts)

why are homeobox genes highly conserved (rarely mutated)

because they are vital, so mutations are often fatal

which factors can affect the expression of homeobox genes

stress and drugs in animals, light in plants

what is apoptosis

programmed cell death

which intrinsic signals can trigger apoptosis

hormones, growth factors and cytokines (immune hormones)

which extrinsic signals can trigger apoptosis

nitric oxide

how does nitric oxide trigger apoptosis

by punching holes in the mitochondrial membranes (so the cell is starved of ATP)

why is apoptosis important during the development and growth of an organism

because it is used to get rid of harmful, ineffective or excess cells (e.g. extra white blood cells after the specific immune response)

describe the process of apoptosis

normal cell -> cell shrinkage (enzymes break down the cytoskeleton and the cytoplasm & chromatin become dense) -> membrane blebbing -> nuclear collapse and continued blebbing -> apoptotic body formation -> lysis of apoptotic bodies by phagocytosis (so no harmful cell components are released into the surrounding tissue)

what is a gene

a length of DNA that codes for one or more polypeptides (e.g. genes for eye colour)

what is a locus

the place on a chromosome which is occupied by a gene (e.g. the short arm of chromosome number three)

what is an allele

an alternative version of a gene (e.g. blue / green / brown eye colour)

what is a genotype

the alleles an individual has (e.g. BB)

what is a phenotype

the observable characteristics of an organism (e.g. blue eyes)

what does homozygous mean

eukaryotic cells or organisms which have two identical alleles on a particular chromosome (e.g. BB)

what does heterozygous mean

eukaryotic cells or organisms which have two different alleles on a particular chromosome (e.g. Bb)

what is a dominant characteristic

a characteristic for which the alleles responsible are expressed in the phenotype even if the characteristic is heterozygous, so only one copy is required (e.g. B)

what is a recessive characteristic

a characteristic for which the alleles responsible are only expressed in the phenotype if no dominant allele is present, so two copies are required (e.g. b)

what is codominance

a characteristic where both alleles contribute to the phenotype (e.g. blood types A and B)

what is linkage

when genes for different characteristics are present at different places on the same chromosome (e.g. in pea plants the genes for height and texture are both found on chromosome four)

what is sex linkage

a gene with its locus on one or more of the sex chromosomes

what are monogenetic characteristics

characteristics that are controlled by only one gene (e.g. flower colour in violets)

what are polygenetic characteristics

characteristics that are influenced by more than one gene (e.g. human skin colour)

which two types of factors contribute to phenotypic variation

genetic and environmental factors

give an example of an environmental factor which contributes to phenotypic variation in plants

etiolation: when plants grow abnormally long and spindly because they aren't getting enough light

give an example of an environmental factor which contributes to phenotypic variation in animals

body mass is partly genetic, but is also strongly influenced by diet (e.g. not eating enough nutrients lowers body mass)

how should a genetic diagram be drawn

parent phenotype -> parent genotype -> gametes -> genetic diagram -> offspring (F1) genotypes -> offspring (F1) phenotypes -> phenotypic ratio

how are alleles for autosomal (not sex-linked), codominant and sex linked genes written

each allele is represented by a capital letter in superscript

what is the chi-squared test

a statistical test to find out whether the difference between observed and expected data is small enough to be down to random chance

what does a chi-squared test require

a large sample size with raw data and no zero score

what can the chi-squared test be used for

the results can be analysed and used to build scientific theories e.g. assign alleles as dominant or recessive

what is the null hypothesis for a chi-squared test

there is no significant difference between the observed value and the expected value, so this phenotypic ratio must be correct

what do chi-squared test results mean

if the value for x^2 exceeds the critical value (for P = 0.05), the null hypothesis can be rejected and there IS a significant difference between the expected and observed values

how can the degrees of freedom value for a chi-squared test be calculated

(number of categories) - 1

what can the Hardy-Weinberg principle be used for

to calculate allele frequencies in populations

what does the Hardy-Weinberg principle assume

there is a large population size, the mating within the population is random, there is no selective advantage for any genotype and there is not mutation migration or genetic drift

which letter is used to represent the dominant allele in the Hardy-Weinberg principle

p (so AA = p^2)

which letter is used to represent the recessive allele in the Hardy-Weinberg principle

q (so aa = q^2)

how is the heterozygous genotype referred to in the Hardy-Weinberg principle

2pq

which factors can affect the evolution of a species

stabilising selection, directional selection, genetic drift, genetic bottleneck and founder effect

what is stabilising selection

when the environment is stable so individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce (so the curve narrows)

what is directional selection

when there's a change in environment so individuals with characteristics of an extreme type are more likely to survive and reproduce

what is disruptive selection

when the extremes are selected for and the norm is selected against (the opposite of stabilising selection)

what is genetic drift

when the alleles in a population change due to chance

where is genetic drift most obvious

in small populations e.g. islands and tribes

what is a genetic bottleneck

when an event/change drastically reduces the population size (reduces the 'gene pool')

what is the founder effect

when alleles are randomly lost according to what is / isn't inherited

what is artificial selection

a process carried out by humans to choose desirable characteristics to be passed on in plants and animals for our benefit

how artificial selection different from natural selection

humans are the driving force of artificial selection, not the environment

which characteristics have been selectively bred for in animals

increased meat / milk quantity / quality, calm temperament and increased egg size

which characteristics have been selectively bred for in plants

increased fruit size, resistance to disease and increased nutritional content

describe the process of artificial selection

a male and female organism which display the desired characteristic are selected and bred together to produce offspring, then the offspring which also display the desired characteristic are selected and bred together, with the process being repeated over many generations

what is interbreeding

breeding together related individuals

what issues does inbreeding cause

adverse health problems, loss of alleles from a population (so decreased genetic biodiversity), and the offspring are genetically similar so equally vulnerable to the same diseases