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AQA GCSE Biology notes Topic 6 – Inheritance, variation and evolution
Topic 6 – Inheritance, variation and evolution
Table of Content
6.1 Reproduction .............................................................................. 2
6.1.1 Sexual and asexual reproduction ................................................................................2
6.1.3 Advantages and disadvantages of sexual and asexual reproduction (biology only) ...2
6.1.2 Meiosis ........................................................................................................................3
6.1.4 DNA and the genome ..................................................................................................3
6.1.5 DNA structure (biology only) ......................................................................................4
6.1.6 Genetic inheritance.....................................................................................................5
6.1.7 Inherited disorders......................................................................................................6
6.1.8 Sex determination.......................................................................................................7
6.2 Variation and evolution .............................................................. 7
6.2.1 Variation .....................................................................................................................7
6.2.2 Evolution .....................................................................................................................7
6.2.3 Selective breeding .......................................................................................................9
6.2.4 Genetic engineering ....................................................................................................9
6.2.5 Cloning (biology only) ...............................................................................................11
6.3 The development of understanding of genetics and evolution .. 13
6.3.1 Theory of evolution (biology only) ............................................................................13
6.3.2 Speciation (biology only) ...........................................................................................13
6.3.3 The understanding of genetics (biology only) ...........................................................14
6.3.4 Evidence for evolution ..............................................................................................14
6.3.5 Fossils ........................................................................................................................14
6.3.6 Extinction ..................................................................................................................15
6.3.7 Resistant bacteria .....................................................................................................15
6.4 Classification of living organisms ............................................... 16
1
, AQA GCSE Biology notes Topic 6 – Inheritance, variation and evolution
6.1 Reproduction
6.1.1 Sexual and asexual reproduction
6.1.3 Advantages and disadvantages of sexual and asexual reproduction
(biology only)
Sexual reproduction (meiosis)
• Joining of male & female gametes that produce genetically different offspring
o Gametes formed by meiosis
§ Mixing of genetic info leads to variety in offspring
§ Requires energy as some animals & plants produce gametes in high no. & process of
courtship requires energy
Animals Flowering plants
• Sperm (contains 46 chromosomes) • Pollen
• Egg cells • Egg cells
Advantages Disadvantages
• Produce variation in offspring • Advantages of asexual selection
• If environment changes, variation gives a
survival advantage by natural selection
• Natural selection can be sped up by human
in selective breeding to ↑food production
Asexual reproduction (mitosis)
• Involves only one parent & no fusion of gametes
• No mixing of genetic info – produce genetically identical offspring (clones) by mitosis
• E.g. Bacterial reproduce asexually when divide by binary fission
Advantages Disadvantages
• Only one parent needed • Advantages of sexual selection
• More time & energy efficient as don’t need
to find mate
• Faster than sexual reproduction
• Many identical offspring can be produced
when conditions are favourable
Some reproduce both sexually & asexually depending on the circumstances
Malarial parasite • Sexually – in human host
• Asexually – in mosquito
Fungi • Sexually – by sporesto give variation
• Asexually – by spores
Plants • Sexually – produce seeds
• Asexually – produce tiny plantlets on runners e.g. strawberry plants
– bulb division e.g. daffodils
2
, AQA GCSE Biology notes Topic 6 – Inheritance, variation and evolution
6.1.2 Meiosis
• Only takes place in reproductive organs
• Male - testes
• Females – ovary
Describe how meiosis halves no of chromosomes in gametes.
1. All chromosomes made copies of their DNA
2. Chromosome pairs line up and exchange pieces of DNA (DNA crossover)
3. Cell divides into 2
4. Chromosomes line up
5. Both cells divide one more time to form 4 gametes, each with a single set of chromosomes
6. DNA is exchanged so all gametes are genetically different from each other
Describe how fertilisation restores full no of chromosomes.
• In sexual reproduction (fertilisation), gametes join together
• Cell now has normal no of chromosomes
Explain why sexual reproduction could produce new variety of onion. (3)
• Meiosis produces 4 gametes from an original cell
• Fusion of gametes in fertilisation
• These gametes are genetically different from each other & have different alleles, produced by
meiosis which causes variation
6.1.4 DNA and the genome
3
, AQA GCSE Biology notes Topic 6 – Inheritance, variation and evolution
(See 6.1.5 DNA structure (biology only) for more notes)
6.1.5 DNA structure (biology only)
Structure of nucleotide DNA polymer
Describe where DNA is found in a human cell. (2)
• In chromosome in nucleus
What is a section of DNA which codes for one specific protein called? (1)
• A gene
In which part of an animal cell is DNA found? (1)
• Nucleus
Describe the function of DNA. (3)
• DNA carries coded info which controls order of amino acids to form specific proteins
Describe difference between alleles & genes (2)
• Alleles - different forms of gene controlling a characteristic & occupying same site on homologous
chromosomes
• Genes - units of DNA on chromosomes carrying info that determines characteristics
Why is sequence of compounds A,C,G & T in gene important? (2)
• They are codes for order of amino acid which make a specific protein
Protein synthesis
• Protein - polymers of amino acids
• Sequence of bases in gene determines order of amino acids (aka codes for sequence)
• Specific order of amino acids determines shape of protein
• Shape of protein determines function eg enzyme (amylase), structural protein (collagen),
hormones (insulin)
Describe stages of protein synthesis
• Proteins are synthesised on ribosomes, according to a template
• Carrier molecules bring specific amino acids to add to growing protein chain in correct order
• When protein chains are complete, it folds up to form a unique shape. This unique shape enables
proteins to work as enzymes, hormones or forming structures in body e.g.collagen
In the cell, where are proteins synthesised? (1)
• Ribosomes
Describe how protein is synthesisd. (3)
• A sequence of 3 bases is coded for 1 amino acid
4
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