6) Inheritance, variation and evolution

What is the difference between meiosis and mitosis?

Meiosis forms four non-identical cells

Mitosis forms 2 identical cells

What is sexual reproduction?

involves the fusion of male and female gametes, each containing genetic informatuon from the mother or father

What leads to variation?

A mixing of genetic information leads to variety in offspring

What are gametes in plants and animals? How are they formed? (3)

Plants - pollen and egg cells

Animals - sperm and egg cells

Formation of gametes involves meiosis

What is asexual reproduction? (4)

involves one parent and no fusion of gametes.

No mixing of genetic information

Leads to genetically identical offspring (clones)

Only mitosis is involved

Describe meiosis (3)

1) Cell makes copies of chromosomes. Doubles the amount of genetic info

2) Cell divides twice to form four gametes, each with a single set of chromosomes (23)

3) All gametes are genetically different from each other (chromosomes are shuffled)

How do gametes join at fertilisation? (4)

1) Gametes join at fertilisation to produce a cell with 46 chromosomes

2) Cell divides by mitosis, produces many copies

3) Embryo forms

4) Cells differentiate after this stage

List advantages of sexual reproduction (3)

1) Produces variation in offspring

2) if environment changes, variation gives survival advantage by natural selection (decreases chance of extinction)

3) Natural selection can be sped up by humans in selective breeding (e.g to increase food production)

List advantages of asexual reproduction (4)

1) Only one parent needed

2) Uses less energy and is faster as organisms do not need to find a mate

3) Faster than sexual reproduction

4) Many identical offspring can be produced when conditions are favourable

Give examples of organisms that reproduce by both methods (3)

1) Malarial parasites - reproduce asexually in human host (liver + blood cells), reproduce sexually in mosquito

2) Fungi - asexually by spores, sexually to give variation

3) Plants - sexually, but also asexually by runners (strawberries) or bulb division (daffodils) - can reproduce even if destroyed by frost/animals

What is the genome? (2)

the entire genetic material of that organism.

The whole human genome has now been studied and this will have great importance for medicine in the future.

Describe the structure of DNA (3)

1) Genetic material in the nucleus is composed of DNA

2) DNA is a polymer made up of 2 strands that make a double helix - made up of nucleotides.

3) Between the 2 strands are nitrogenous bases lined up in single rows - they form a series of complementary bases.

What is a gene? (3)

a small section of DNA on a chromosome.

A triplet of bases that codes for a specific protein.

Each gene codes for a particular sequence of amino acids - they join to make a protein

What are nucleotides made up of?

sugar, phosphate (form backbone), base

What are the 4 bases? (Name them in pairs)

A-T
G-C

(complementary base pairing)

How many bases code for an amino acid? (2)

3 (triplet)

The order of bases controls the order in which amino acids are assembled to produce a particular protein.

What are non-coding parts of DNA? (2)

they do not code for a protein sequence

-non-coding parts of DNA for proteins can switch genes on or off to control whether or not a gene is used to make a protein

What is protein synthesis?

the process of producing a protein from DNA

What does it mean if a gene is expressed?

It has been coded to make a protein

Describe protein synthesis (8)

Transcription:
1) DNA contains genetic code for protein - cannot move out of nucleus - too big

2) Two strands pull apart, mRNA nucleotides match to complementary base on strand

3) mRNA nucleotides are then joined together - creates new strand (mRNA strand) - template of original DNA.

Translation:
4) mRNA moves out of nucleus -\> cytoplasm -\> ribosomes

5) Bases on mRNA are read in 3s to code for an amino acid

6) Corresponding amino acids are brought to the ribosomes by carrier molecules

7) Amino acids connect to form a protein

8) When chain is complete - protein folds to form unique 3D structure

What can proteins be? (3)

Enzymes - biological catalysts that speed up rate of reaction

Hormones - chemical messengers that send signals around the body

Structural proteins - strong proteins to form strucures (e.g. collagen)

Why is the unique shape of proteins important?

enables the proteins to do their job as enzymes, hormones or forming structures in the body such as collagen

What is a mutation in DNA?

change in DNA sequence (bases)

What mutations occur in DNA? (3)

1) A base is inserted into the code - (changes way they're read, may change all amino acids coded for after insertion)

2) A base is deleted from code - may change all amino acids coded for after deletion

3) A base is substituted - only changes one amino acid or may not change it at all

How can mutations affect proteins? (4)

1) Affects the way it folds (structure)

2) Most mutations do not alter a protein or only do slightly

3) Change the shape - substrate won't fit active site so cannot act as a protein. Structural protein may lose its shape.

4) Can also be mutations in non-coding parts of DNA that control whether genes are expressed

Define gamete

An organism's reproductive cell which has half the number of chromosomes

Define chromosome

A structure found in the nucleus which is made up of a long strand of DNA

Define gene

A short section of DNA that codes for a protein, and therefore contribute to a characteristic.

How are characteristics controlled by genes? (2)

Some characteristics are controlled by a single gene (fur colour in mice and red-green colour blindness in humans)

Most characteristics are the result of many different genes interacting

Define alleles

The different forms of the gene - humans have 2 alleles for each gene (one from each parent)

Define dominant allele

Only one is needed for it to be expressed and for the corresponding phenotype to be observed

Define recessive allele

Two copies are needed for it to be expressed and for the corresponding phenotype to be observed

Define homozygous

When both inherited alleles are the same

Define heterozygous

When one of the inherited alleles is dominant and one is recessive

Define genotype

The combination (genetic makeup) of alleles an organism has (e.g. Aa)

Define phenotype

The physical characteristics that are observed in the individual (e.g. eye colour)

How can probability of a genotype/phenotype in offspring be shown?

A punnett square

(capital \= dominant, lowercase \= recessive)

What is polydactyly and how is it caused?

Having extra fingers or toes

Caused by a dominant allele

What is cystic fibrosis and how is it caused? (4)

Disorder of cell membranes

Results in thick mucus in airways + pancreas

Caused by a recessive allele

Both parents need to be carriers or one must actually have cystic fibrosis to pass on

What is embryonic screening? (3)

Allows scientists to observe whether the child will have a genetic condition.

If embryo is developed in lab - cells can be taken from it and analysed

DNA from embryos in womb can also be extracted

What are the ETHICAL arguments FOR embryonic screening?

Reduces the number of people suffering

What are the ECONOMIC arguments FOR embryonic screening?

Treating disorders is very expensive

What are the SOCIAL arguments FOR embryonic screening?

there are many regulations in place to stop it getting out of hand

What are the SOCIAL arguments AGAINST embryonic screening? (3)

- Can lead to miscarriages

- Deicisions about terminating pregnancy have to be made

- May promote prejudice - suggests that those with genetic disorders are unwanted

What are the ECONOMIC arguments AGAINST embryonic screening?

expensive to carry out screening

What are the ETHICAL arguments AGAINST embryonic screening?

could encourage people to pick characteristics (designer babies)

How many pairs of chromosomes do ordinary human body cells contain?

23 pairs (46 in total)

How many pairs of chromosomes carry sex determining genes?

1 - the other 22 control characterisitics only

What are the male sex chromosomes?

XY

What are the female sex chromosomes?

XX

How can sex inheritance be shown?

Punnett Squares/Genetic cross diagrams

(50% chance)

What does the phenotype of an organism depend on? (3)

1) Genotype - genes it inherited. (Only identical twins have the same genotype)

2) Environment - examples include scars in animals, or small/yellow leaves in plants.

Some can result from both - e.g. weight. (Metabolism + how much you eat)

Describe mutations effects on phenotype (3)

1) Most have no effect on phenotype

2) Some will influence phenotype - unlikely to bring entire new phenotype

3) If mutation determines an advantageous phenotype - natural selection leads it to be the common phenotype quickly

What is evolution?

a change in the inherited characteristics of a population over time through a process of natural selection which may result in the formation of a new species.

What does the theory of evolution state?

all species of living things have evolved from simple life forms that first developed more than three billion years ago.

How does evolution occur through natural selection? (4)

- Mutations occur - provide variation

- If a mutation provides a survival advantage, the organism is more likely to live till breeding age

- Mutation will be passed onto offspring

- Over many generations, frequency of mutation will increase in the population

What happens if evolution makes two populations extremely different to eachother? (2)

- Can no longer interbreed to produce fertile offspring

- They have formed 2 new species (speciation)

What is selective breeding? (2)

the process by which humans breed plants and animals for particular genetic characteristics.

Humans have been doing this for thousands of years since they first bred food crops from wild plants and domesticated animals.

Describe the process of selective breeding (4)

1) Parents with desired characteristics are chosen

2) They're bred together

3) From the offspring, those with desired characteristics are bred together

4) Process is repeated many times until all offsprng have desired characteristic

What are the problems of selective breeding? (4)

Inbreeding - breeding with similar characteristics means you're breeding closely related organisms.

Results in reduction of gene pool (number of different alleles reduce)

If environment changes or new disease arises - species can become extinct due to same genetic make-up (no survival advantages)

Smaller gene pool - greater chance of genetic defects (recessive characteristics more likely)

What type of characteristics are chosen for selective breeding? (4) (Examples)

• Disease resistance in food crops.

• Animals which produce more meat or milk.

• Domestic dogs with a gentle nature.

• Large or unusual flowers

What is genetic engineering?

a process which involves modifying the genome of an organism by introducing a gene from another organism to give a desired characteristic.

How have plants been genetically engineered?

to be resistant to diseases or to produce bigger better fruits.

How have bacteria cells been genetically engineered?

to produce useful substances such as human insulin to treat diabetes

Describe the process of genetic engineering (6)

1) Genes from chromosomes 'cut out' using restriction enzymes. Leaves 'sticky ends'

2) Virus/bacterial plasmid is cut using same restriction enzyme to create sticky ends. Also contains antibiotic marker gene

3) Loop and gene sticky ends are joined by DNA ligase enzymes

4) Combined loop placed in vector (e.g. bacterial cell) and allowed to multiply (contains modified gene)

5) the vector is used to insert the gene into the required cells

6) genes are transferred to the cells of animals, plants (meristematic cells) or microorganisms at an early stage in their development so that they develop with desired characteristics

Describe GM crops (2)

Engineered to be resistant to insects and herbicides

Results in increased yields as less crops will die

Describe GM in medicine (2)

May be possible to use genetic engineering to cure inherited disorders

Gene therapy - involves transferring normal genes into patients so correct proteins are produced

State benefits of genetic engineering (4) (3 agricultural ways)

1) Useful in medicine to mass produce certain hormones (in bacteria and fungi)

2) Improve agricultural yields (improves growth rates, crops can grow in different conditions, can make their own pesticide/herbicide)

3) Crops with extra vitamins produced in areas where they're difficult to obtain

4) Greater yields can solve world hunger (big issue due to overpopulation)

State risks of genetic engineering (4)

1) May have an effect on wild flowers/insects (GM crops infertile, may spread to other plants. Growing with herbicides + pesticides kills insects and plants which reduces biodiversity)

2) Worried that we don't understand effects of GM crops on human health

3) May lead to genetic engineering in humans - designer babies

4) Selection pressure - increased resistance in other species (super weeds and pests)

What is cloning?

Creating genetically identical copies of an organism

What are the methods of cloning? (4)

Tissue culture
Cuttings
Embryo transplants
Adult cell cloning

What is tissue culture? (4)

- Important to preserve rare plant species or commerically in nurseries.

1) Plant cells are taken
2) Placed in growth medium (nutrients + hormones)
3) Grow into new plants, clones genetically identical to parent.

Describe taking cuttings (3)

- Older, easier method of cloning

1) Cuttings (e.g. section of stem) are taken from a plant with desirable feature

2) Planted and produce clones - genetically identical to parent

Describe embryo transplants (4)

1) Sperm + egg cells from parents with desirable features are obtained

2) In lab, fertilised to form embryo

3) Embryo divides many times, inserted into host mother

4) Offspring is genetically identical to parents with desirable feature

Describe adult cell cloning (5)

1) Nucleus is removed from an unfertilised egg cell

2) Nucleus is removed from an adult body cell and placed in the denucleated egg cell

3) Electric shock stimulates the egg cell division to form an embryo

4) Embryo is implanted into womb

5) Offspring is born a clone of adult body cell

State the benefits of cloning (3)

1) Produce lots of offspring with desirable feature

2) Study of clones could help research into embryo development

3) May help extremely endangered species, or bring back extinct species

State the risks of cloning (3)

1) May lead to human cloning

2) clones have a low survival rate, and genetic problems

3) gene pool is reduced through producing clones (less likely that population will survive if a disease arises with low diversity in population)

Who proposed the theory of evolution? (3)

Charles Darwin,

as a result of observations on a round the world expedition,

backed by years of experimentation and discussion and linked to developing knowledge of geology and fossils

Describe the theory of evolution (3)

Individual organisms within a particular species show a wide range of variation for a characteristic.

• Individuals with characteristics most suited to the environment are more likely to survive to breed successfully.

• The characteristics that have enabled these individuals to survive are then passed on to the next generation.

Why were Darwins ideas (published in On the Origin of Species (1859)) controversial? (3)

• the theory challenged the idea that God made all the animals and plants that live on Earth

• there was insufficient evidence at the time the theory was published to convince many scientists

• the mechanism of inheritance and variation was not known until 50 years after the theory was published.

What was Lamarck's theory? (2)

that changes that occur in an organism during its lifetime can be inherited.

Now understood that this is wrong (changes in environment are not passed on in sex cells)

How was Darwin's theory supported? (3)

Supported by genetics - provided a mechanism for beneficial characteristics caused by mutations to be passed on

Fossil evidence showed how developments in organisms arose slowly

Knowledge of how resistance to antibiotics evolves in bacteria

What is speciation and what does it result in?

1) Process of a new species developing through the selection of different alleles

2) Increases genetic variation until new offspring cannot breed with old population to produce fertile offspring

Who developed the theory of speciation? (Evolution by natural selection)

Alfred Russel Wallace

Describe Wallace and the development of his theories (5)

1) Independently proposed theory of evolution by natural selection

2) Published joint writing with Darwin in 1858, prompted Darwin to publish On the Origin of Species in 1859

3) Worked worldwide gathering evidence for theory

4) Best known for work on warning colouration in animals and theory of speciation

5) Much pioneering work on speciation - more evidence over time has led to our current understanding.

What is the process of speciation? (6)

1) Variation exists within population as a result of genetic mutations

2) Alleles which provide survival advantage are selected through natural selection

3) Populations of species can become isolated

4) Different alleles may be advantageous in new environment, so they're selected

5) Over time - selection of different alleles increases genetic variation between 2 populations

6) When they cannot breed to produce fertile offspring, a new species has formed.

Who was Gregor Mendel and what did he do? (4)

carried out breeding experiments on plants (peas).

Observed that inheritance is determined by hereditary units (now known as genes)

One unit received from each parent - can be dominant and recessive - not mixed together

Ideas weren't recognised until after his death.

What are fossils?

the 'remains' of organisms from millions of years ago, which are found in rocks.

What are fossils formed from? (3)

1) Parts of organisms that have not decayed because one or more of the conditions needed for decay are absent (oxygen/moisture)

2) When parts of the organism are replaced by minerals as they decay, forms a rock structure of the original part.

3) Preserved traces of organisms, they harden and form a cast (footprints, burrows, rootlet traces)

What are fossils used to show? (3)

1) How the anatomy of organisms has changed over time

2) Compare how closely related 2 organisms are by looking at similarities

Can be used to develop evolutionary trees

Why can't fossils show us how life started on Earth? (2)

1) Traces left have been destroyed by geological activity

2) Most early life forms were soft-bodied so they decayed completely, there are few fossils of them

What is extinction?

when there are no remaining individuals of a species still alive.

List factors that contribute towards extinction (6)

1) Changes in environment which the species cannot adapt quickly to

2) New predators

3) A new disease

4) Competition with advantageous mutations

5) Catastrophic event can wipe out a species

6) Destruction of a habitat

When are bacteria labelled as resistant?

when they're not killed by antibiotics which previously were used as cures against them.

How do resistant bacteria develop? (5)

1) Bacteria reproduce at a fast rate

2) Mutations develop randomly, can result in new genes such as for antibiotic resistance. Creation of a new strain.

3) Exposure to antibiotics - selection pressure. Those with antibiotic resistant genes survive and those without die.

4) Those with Antibiotic resistance reproduce and pass on advantageous gene to offspring.

5) Population of antibiotic resistant bacteria increases.

Why would a new bacterial disease affect people?

Bacterial diseases spread rapidly - because people aren't immune to new resistant bacteria, there's no treatment for it.

What is MRSA?

A strain of bacteria that is resistant to antibiotics (methicillin)

What is MRSA also known as? Why?

A 'superburg' - resistant to many different types of antibiotics

Where is MRSA commonly found? How is it spread here?

Hospitals - it spreads when doctors/nurses move to different patients

How can you slow the development of resistance in bacteria? (4)

1) Don't give antibiotics for viral/non-serious infections.

2) Specific antibiotics given for specific bacteria

3) Patients must complete full course of antibiotics - if not, some bacteria may survive and mutate for resistance.

4) Antibiotics used less in agriculture