inheritance, variation, and evolution

what is sexual reproduction?

it involves the fusion of male and female gametes (sperm and egg cells in animals, pollen and egg cells in flowering plants)
there is mixing of genetic information which leads to variety in the offspring

what is meiosis?

form of cell division involved in the formation of gametes (haploid cells) in reproductive organs
chromosome number is halved (23 chromosomes)
it involves two cell divisions

outline the process of meiosis

1) copies of genetic information are made
2) copied chromosomes line up along the cell equator and DNA crossover occurs
3) the pair of chromosomes are separated and move to opposite ends of the cell - chromosome number is halved and the cell divides
4) chromosomes line up along the cell equator again
5) they move to opposite ends of the cell
6) cell divides for the second time - four unique haploid gametes are produced

compare meiosis and mitosis

mitosis is 1 cell division, meiosis is 2 cell divisions
mitosis makes clones, meiosis makes unique gametes
mitosis produces diploid cells, meiosis produces haploid cells
mitosis forms 2 identical cells, meiosis forms 4 different cells

why is meiosis important for sexual reproduction?

it increases genetic variation
it ensures the zygote formed at fertilisation is diploid

describe fertilisation and its resulting outcome

gametes join at fertilisation to restore the normal number of chromosomes
the new cell divides by mitosis
the number of cells increases
as the embryo develops, cells differentiate

what are advantages of sexual reproduction?

produces variation in the offspring
if the environment changes, variation gives a survival advantage by natural selection
natural selection can be speeded up by humans in selective breeding to increase food production

what are disadvantages of sexual reproduction?

time and energy are needed to find a mate - so fewer offspring are produced
two parents are required - makes reproduction difficult in endangered populations
reproduction is not always successful
favourable genes may not be passed down to the offspring

what is asexual reproduction?

involves only one parent and no fusion of gametes
no mixing of genetic information - leads to clones
only mitosis is involved

what is the process called when prokaryotic bacteria reproduce asexually?

binary fission

what type of plant reproduces asexually?

non flowering plants

what are organisms that can reproduce both sexually and asexually?

fungi, plants, malarial parasites

how do fungi reproduce both sexually and asexually?

sexually - to give variation
asexually - release spores

how do plants reproduce both sexually and asexually?

sexually - seeds
asexually - by runners (strawberry plants), or bulb division (daffodils)

how do malarial parasites reproduce both sexually and asexually?

sexually - in the mosquito
asexually - in the human host

what are advantages of asexual reproduction?

only one parent needed
more time and energy efficient as do not need to find a mate
faster than sexual reproduction
many identical offspring can be produced when conditions are favourable

what are disadvantages of asexual reproduction?

the offspring produced is a clone, any harmful mutation will also be passed on
no genetic variation - reducing probability of species being able to adapt to environmental change
the offspring have to compete for food/nutrients/space among the species

what is DNA?

a chemical which the genetic material in the nucleus of a cell is composed of

what is DNA made up of?

a polymer made of repeating nucleotide units
it is made up of two strands forming a double helix contained in structures called chromosomes

what is a gene?

a small section of DNA on a chromosome

what is a genome?

the entire genetic material of an organism

why is understanding the human genome important?

for the development of medicine in the future
- searching for genes linked to different types of disease
- understanding and treating inherited disorders
- tracing human migration patterns from the past

what does each nucleotide of DNA consist of?

a common sugar and phosphate group with one of four different bases attached to the sugar

what are the four bases in DNA?

A, C, G and T

what is the rule for complementary pairs of bases?

C is always linked to a G on the opposite strand and a T to an A

explain how a gene codes for a protein

a sequence of three bases (codon) codes for one amino acid
the order of amino acids determines the structure and function of protein formed

describe how nucleotides interact to form a molecule of DNA

sugar and phosphate molecules join to form a sugar-phosphate backbone in each DNA strand
base connected to each sugar
complementary base pairs are joined by weak hydrogen bonds

what is protein synthesis?

the formation of a protein from a gene

where are proteins synthesised?

on ribosomes

what do carrier molecules do?

they bring specific amino acids to add to the growing protein chain in the correct order

what happens when the protein chain is complete?

it folds up to form a unique shape
it enables the proteins to do their job as enzymes, hormones, or forming structures in the body (e.g. collagen)

what are the two stages of protein synthesis?

1) transcription
2) translation

what does transcription involve?

the formation of mRNA from a DNA template

outline the process of transcription

1) the DNA double helix unwinds
2) RNA polymerase binds to a specific base sequence of non-coding DNA in front of a gene and moves along the DNA strand
3) RNA polymerase joins free RNA nucleotides to complementary bases on the coding DNA strand
4) mRNA formation is complete. the mRNA detaches and leaves the nucleus

what base cannot be paired like usual in transcription?

if there is an A on the DNA strand, it pairs with the U on the mRNA strand instead of a T

what does translation involve?

a ribosome joins amino acids in a specific order dictated by mRNA to form a protein

outline the process of translation

1) mRNA attaches to a ribosome
2) ribosome reads the mRNA bases in codons. each codon codes for one amino acid which is brought to the ribosome by a tRNA molecule
3) the complete amino acid chain detaches from the ribosome and folds up to form a protein with a unique shape

what is a mutation?

a random and spontaneous change in the structure of a gene, chromosome or number of chromosomes
mutations occur continuously
most have little impact on the phenotype

what are mutations caused by?

errors when the DNA is copied prior to cell division
ionising radiation
chemical mutagens - tar from cigarette smoke

what structure can mutations affect?

cause a change in a protein
if the protein is an enzyme it can cause it to have a different shape so it will not fit with the substrate
if it is a structural protein, it may lose its strength

what are the two types of mutation?

substitution mutation and insertion/deletion mutation

what is substitution mutation?

where one or more bases are changed in the DNA sequence
may not change the protein as some base triplets code for the same amino acid

what is insertion/deletion mutation?

where a base is added or removed from the DNA sequence
may result in a different amino acid being synthesised and possibly a different protein

how are mutations inherited?

mutation in a cell which becomes a gamete that is fertilised
the characteristics caused by the mutation will be in the zygote - could be expressed in the offspring

what are impacts of an inherited mutation?

mutation may have no effect
harm embryo’s development + could cause death of embryo
if mutation stops protein from working, could cause disease
mutation may help the organism survive better by gaining a characteristic that is advantageous

what is non-coding DNA?

DNA which does not code for a protein, but instead controls gene expression

how is the sex of a human determined?

by the 23rd pair of chromosomes

what are the sex chromosomes in females?

XX

what are the sex chromosomes in males?

XY

what is the genotype?

the genes present for a trait

what is the phenotype?

the physical characteristic determined by the genotype

what are the characteristics controlled by a single gene?

fur colour in mice, eye colour, red-green colour blindness in humans

what are alleles?

different versions of the same gene

what is the name for different alleles?

heterozygous - genes coding for different versions of the same characteristic

what is the name for same alleles?

homozygous - genes coding for the same version of the same characteristic

what is a dominant allele?

a version of a gene where only one copy is needed for it to be expressed

what is a recessive allele?

a version of a gene where two copies are needed for it to be expressed

when are alleles expressed?

a dominant allele is always expressed, even if only one copy is present (e.g. BB, Bb)
a recessive allele is only expressed if two copies are present (e.g. bb)

what are Punnett squares used for?

used to determine all possible genotypes from a particular cross

how are dominant alleles represented in a Punnet square?

using upper case letters (e.g. brown eye colour is dominant so it is B)

how are recessive alleles represented in a Punnett square?

using lower case letters (e.g. blue eye colour is recessive so it is b)

what are possible genotypes for eye colour alleles?

BB (brown, homozygous dominant)
Bb (brown, heterozygous)
bb (blue, homozygous recessive)

what are two examples of inherited disorders?

polydactyly and cystic fibrosis

what is polydactyly?

having extra fingers or toes caused by a dominant allele (Qq and QQ)

what is cystic fibrosis?

a disorder of cell membranes caused by a recessive allele (ff)

what can embryonic screening be used for?

to detect genetic disorders of an IVF embryo before implantation or of an embryo in the womb
methods include amniocentesis, CVS, and the IVF process

what are arguments for embryonic screening?

prevents suffering 
treating a lifelong disorder is expensive
may result in reduction of prevalence of a disease in a population
laws are in place to stop it from being abused

what are arguments against embryonic screening?

can lead to pregnancy being terminated 
people with genetic disorders are perceived as undesirable - could lead to prejudice 
very expensive
people may take advantage of process for offspring to have desirable traits (designer babies) 
risk to mother + embryo

what is gene therapy?

the process by which normal alleles are inserted into the chromosomes of an individual who carries defective alleles

how did mendel study inheritance and what did he discover?

carried out breeding experiments on plants
observed that the inheritance of each characteristic is determined by ‘units’ that are passed on to descendants unchanged

why was mendel’s work not recognised until after his death?

his approach was a novel concept, other scientists hadn’t come up with similar ideas
he did not communicate his ideas well

how did mendel’s work contribute to greater understanding of genetics?

chromosomes and Mendel’s ‘units’ behaved in similar ways
‘units’ (genes) were located on chromosomes

what is variation?

the differences in the characteristics of individuals in a population

how can variation occur?

genetic inheritance (mutations, only some affect the phenotype)
environmental factors
a combination of genes + environment

what did charles darwin do?

he proposed the theory of evolution by natural selection
published his ideas in On the Origin of Species (1859)

what is evolution?

a gradual change in the inherited traits within a population over time

why was there controversy on charles darwin’s theory of evolution?

challenged the idea that God made all the animals and plants that live on Earth
insufficient evidence to convince many scientists
mechanism of inheritance and variation was not known until 50 years after the theory was published.

what is darwin’s theory of evolution by natural selection?

individuals within a species have variation
some organisms are better suited to the environment due to advantageous traits and are more likely to reproduce
they are more likely to pass on the genes for these traits to offspring
this is repeated over many generations resulting in evolution

what did alfred wallace do?

he proposed the theory of speciation

what is speciation?

the formation of new biological species

what can speciation be caused by?

isolation of individuals
mutations
natural selection

what is the process of speciation through the isolation of individuals?

two groups of a population become geographically isolated
different mutations occur in each group, leading to variation in isolated populations
different environmental conditions, so different adaptions are favoured by natural selection
different characteristics evolve in each group as time passes
the groups’ phenotypes become so different that they can no longer interbreed to produce fertile offspring
they are now separate species

what are different ways of geographical isolation?

a geographical barrier (rivers, mountains)
movement of land-masses by continental drift

what is selective breeding?

the process where humans breed plants or animals for particular genetic characteristics

what is the process of selective breeding?

1) parents are chosen with the desired characteristic
2) they are bred together
3) from the offspring, those with the desired characteristic are bred together
4) this continues over many generations until all the offspring show the desired characteristic

what desired characteristics make plants suitable for selective breeding?

disease resistance in food crops, large or unusual flowers

what desired characteristics make animals suitable for selective breeding?

animals which produce more meat or milk, domestic dogs with a gentle nature

what can selective breeding lead to?

‘inbreeding’ - some breeds are particularly prone to disease or inherited defects

what are advantages of selective breeding?

creates organisms with desirable features e.g. crops produce a higher yield, cows produce a greater supply of milk, plants produce larger fruit, domesticated animals

what are disadvantages of selective breeding?

reduction in the gene pool → reduced variation
inbreeding results in genetic disorders
development of other physical problems e.g. respiratory problems in bulldogs
potential to unknowingly select harmful recessive alleles

what is genetic engineering?

modifying the genome of an organism by introducing a gene from another organism to result in a desired characteristic

what has GM been used for?

plant crops to be disease-resistant/produce bigger fruits
bacterial cells to produce human insulin to treat diabetes

what can GM be hoped to use for in the future?

overcome inherited disorders (e.g. cystic fibrosis)

what are the stages of genetic engineering?

1) selection of desired characteristic
2) restriction enzymes are used to isolate the required gene for the characteristic
3) gene is inserted into a vector (bacterial plasmid or virus)
4) the vector inserts the gene into the required cells
5) genes are transferred to the cells of organisms an early stage in their development so that they develop with desired characteristics

what is a vector?

a structure that delivers the desired gene into the recipient cell

what are advantages of GE insulin?

not limited by slaughter of animals - previously insulin was obtained by pigs
large quantities made quickly
no risk of transferring infections

what are ethical issues of GE?

long-term effects of GM crop consumption is unknown
negative environmental impacts (reduction in biodiversity)
ethically wrong to create new life forms

how can plants be cloned?

cuttings and tissue culture

how can animals be cloned?

embryo transplants and adult cell cloning

how are plants cloned using tissue culture?

1) select a plant with desired characteristic
2) cut multiple small sample pieces from meristem tissue
3) grow in a petri dish containing growth medium with nutrients and auxins
4) when developed into tiny plantlets, transfer to compost for further growth