Biology: Topic 6: Inheritance, variation and evolution

What is the difference between meiosis and mitosis?

• Meiosis leads to non-identical cells being formed

• Mitosis leads to identical cells being formed

What does sexual reproduction involve?

The joining (fusion) of male and female gametes

What are the male and female gametes in animals?

Sperm and egg cells

What are the male and female gametes in FLOWERING plants?

Pollen and egg cells

What does sexual reproduction involve?

• The mixing of genetic information which leads

  to variety in the offspring

• The formation of gametes involves meiosis

What does asexual reproduction involve?

• Only one parent

• No fusion of gametes

• No mixing of genetic information

• Leads to genetically identical offspring (clones)

• Only mitosis is involved

How do the number of chromosomes change using meiosis in gametes?

• Meiosis halves the number of chromosomes in gametes

• Fertilisation restores the full number of chromosomes

What happens in meiosis?

Cells in reproductive organs divide to form gametes

What happens when a cell divides to form gametes?

• copies of the genetic information are made

• the cell divides twice to form four gametes, each with a single set of chromosomes

• all gametes are genetically different from each other

What happens to the number of chromosomes in fertilisation?

Gametes join to restore the normal number of chromosomes

What happens to the new fertilised cell?

• It divides by mitosis

• The number of cells increases

• As the embryo develops cells differentiate

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

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 do some organisms reproduce by?

Both sexual and asexual reproduction depending on the circumstances

How do malarial parasites reproduce?

• Asexually in the human host

• Sexually in the mosquito

How do many fungi reproduce?

• Asexually by spores

• Sexually to give variation

How do many plants reproduce?

• Sexually to produce seeds

• Asexually by runners such as strawberry plants, or bulb division such as daffodils

What is the genetic material in the nucleus of a cell composed of?

A chemical called DNA

What is DNA?

A polymer made up of two strands forming a double helix

What is DNA contained in?

Structures called chromosomes

What is a gene?

A small section of DNA on a chromosome

What does each gene do?

Codes for a particular sequence of amino acids, to make a specific protein

What is the genome of an organism?

The entire genetic material of that organism

The human genome

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

What is the importance of understanding the human genome?

• search for genes linked to different types of disease

• understanding and treatment of inherited disorders

• use in tracing human migration patterns from the past

What is DNA in relation to nucleotides?

A polymer made from four different nucleotides

What does each nucleotide consist of?

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

What four bases does DNA contain?

A, C, G and T

What is a sequence of three bases?

The code for a particular amino acid

What does the order of the bases control?

The order in which amino acids are assembled to produce a particular protein

What do long strands of DNA consist of?

Alternating sugar and phosphate sections

What is attached to each of these sugars?

One of the four bases

What is the DNA polymer made up of?

Repeating nucleotide units

Describe protein synthesis

• Proteins are synthesised on ribosomes, according to a template

• Carrier molecules bring specific amino acids to add to the growing protein chain in the correct order

How does the structure of DNA affects the protein made?

Describe how genetic variants may influence phenotype in coding DNA by altering the activity of a protein

Describe how genetic variants may influence phenotype in non-coding DNA by altering how genes are expressed

in a complementary strand, what is C always linked to?

G on the opposite stand

In a complementary strand, what is T always linked to?

A on the opposite stand

What happens when a protein change is complete

• It folds up to form a unique shape

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

How often do mutations occur?

Continuously

How do mutations affect proteins?

• Most do not alter the protein

• Or only alter it slightly so that its appearance or function is not changed

What do a few mutations code for?

An altered protein with a different shape

What might an altered protein with a different shape result in?

• An enzyme may no longer fit the substrate binding site

• A structural protein may lose its strength

Do all parts of DNA code for proteins?

No

What can non-coding parts of DNA do?

• Switch genes on and off

• Variations in these areas of DNA may affect how genes are expressed

What is a gamete?

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

What is a chromosome?

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

What is a gene?

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

What are alleles?

The different forms of the gene

humans have two alleles for each gene as they inherit one from each parent

What does a dominant allele mean?

Only one (out of the two alleles) is needed for it to be expressed and for the corresponding phenotype to be observed.

What does a recessive allele mean?

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

What is homozygous?

When both inherited alleles are the same

(i.e. two dominant alleles or two recessive alleles)

What is heterozygous?

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

What is a genotype?

The combination of alleles an individual has, e.g. Aa

What is a phenotype?

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

Give two examples of characteristics controlled by a single gene

• Fur colour in mice

• Red-green colour blindness in humans

The alleles present, or genotype operate at a molecular level to what?

Develop characteristics that can be expressed as a phenotype

When is a dominant allele expressed?

Always, even if one copy is present

When is a recessive allele expressed?

Only if two copies are present (no dominant allele)

What are most characteristics a result of?

Multiple genes interacting, rather than a single gene

What are inherited disorders caused by?

The inheritance of certain alleles

What is polydactyly?

• Having extra fingers and toes

• Caused by a dominant allele

What is cystic fibrosis?

• A disorder of cell membranes

• Caused by a recessive allele

How many pairs of chromosomes does the human body ordinarily contain?

23

What do 22 pairs of chromosomes control?

Characteristics only

What does one pair of chromosomes carry?

The genes that determine sex

What are the female sex chromosomes?

XX

What are the male sex chromosomes?

XY

How does the genome and its interaction with the environment influence the development of the phenotype of an organism?

What is variation?

Differences in the characteristics of individuals in a population

What might variation and be due to differences in?

• The genes they have inherited (genetic causes)

• The conditions in which they have developed (environmental causes)

• A combination of genes and the environment

What is there usually within a population of a species?

Extensive genetic variation

What do all variants arise from?

Mutations

What do variants do to phenotypes?

• Most have no effect on the phenotype

• Some influence phenotype

• Very few determine phenotype

How often does a mutation lead to a new phenotype?

Very rarely

If a new phenotype is suited to an environmental change, what can it lead to?

A relatively rapid change in the species

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 by natural selection state?

That 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 of variants that give rise to phenotypes best suited to their environment

When are two new species formed?

If two populations of one species become so different in phenotype that they can no longer interbreed to produce fertile offspring

What impact does selective breeding have on food plants and domesticated animals?

What is selective breeding (artificial selection)?

The process by which humans breed plants and animals for particular genetic characteristicsH

How long have humans been selective breeding for?

Thousands of years since they first bred food crops from wild plants and domesticated animals

What does selective breeding involve?

• Choosing parents with the desired characteristic from a mixed population

• They are bred together

• From the offspring those with the desired characteristic are bred together

  • This continues over many generations until all the offspring show the desired characteristic

Give some examples of when a characteristic is chosen for usefulness or appearance

• Disease resistance in food crops

• Animals which produce more meat or milk

• Domestic dogs with a gentle nature

• Large or unusual flowers

What can selective breeding lead to?

‘Inbreeding’ where some breeds are particularly prone to disease or inherited defects

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 plant crops been genetically engineered?

• To be resistant to diseases

• To produce bigger better fruits

How have bacterial cells been genetically engineered?

To produce useful substances such as human insulin to treat diabetes

Students should be able to explain the potential benefits and risks of genetic engineering in agriculture and in medicine and that some people have objections

What happens in genetic engineering?

Genes from the chromosomes of humans and other organisms can be ‘cut out’ and transferred to cells of other organisms

What are genetically modified (GM) crops?

Crops that have had their genes modified by being cut out and replaced

What do GM crops include?

Ones that are resistant to insect attack or to herbicides

What do GM crops generally show?

Increased yields

What do concerns about GM crops include?

• The effect on populations of wild flowers and insects

• Some people feel the effects of eating GM crops on human health have not been fully explored

What are the main steps in the process of genetic engineering?

• enzymes are used to isolate the required gene; this gene is inserted into a vector, usually a bacterial plasmid or a virus

• the vector is used to insert the gene into the required cells

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

What is cloning from tissue culture?

Using small groups of cells from part of a plant to grow identical new plants

Why cloning from tissue culture important?

For preserving rare plant species or commercially in nurseries

What is cloning from cuttings?

An older, but simple, method used by gardeners to produce many identical new plants from a parent plant