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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
    •          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)
  • Egg cells
  • Pollen
  • Egg cells

 

Advantages Disadvantages
  • Produce variation in offspring
  • 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
  • Advantages of asexual selection

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
  • 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
 Advantages of sexual selection

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

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

(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
  • Amino acids make up a protein

Explain how DNA is responsible for structure of proteins. (3)

  • DNA carries coded info which controls order of amino acids to form specific proteins
  • A sequence of 3 bases is coded for 1 amino acid
  • …

Explain how mutation could causes enzyme not to work. (5)

  • A sequence of 3 bases is coded for 1 amino acid
  • Mutation changes DNA code from CAG to TAG
  • Changes order of bases, change order of amino acids, change shape of protein that change active site
  • So enzyme no longer fits substrate

6.1.6 Genetic inheritance

Gamete An organism’s reproductive cell (egg in female & sperm in male), which has half no of chromosomes
Chromosome A structure found in nucleus which is made up of long strand of DNA
Gene Small section of DNA on chromosome that codes for a particular sequence of amino acids to make specific protein that determine characteristics
Allele Different forms of a gene controlling a characteristic & occupying same site on homologous chromosome
Dominant
eg B		 Only expressed if recessive allele is not present

Will show a characteristic if inherited from one or both parents
Recessive
eg b		 Only expressed if dominant allele is not present.

Will show a characteristic only if inherited from both parents
Homozygous
eg BB or bb		 A genotype with two same alleles
Homozygous dominant

eg BB

 A genotype with two dominant alleles
Homozygous recessive
eg bb		 A genotype with two recessive alleles
Heterozygous
eg Bb		 A genotype with one dominant & one recessive allele
Genotype A genotype with one dominant & one recessive allele
Phenotype Physical characteristics of an organism as described by words

Explain why it looks similar to its parents. (1)

  • DNA passed from parents

Explain why not identical to its parents. (2)

  • DNA from 2 parents
  • Different alleles
  • Environmental effect e.g. mutations

Why might your prediction not proved right? (1)                    

It’s down to chance / only a prediction

6.1.7 Inherited disorders

Polydactyly
  • Have extra fingers / toes
  • Caused by dominant allele
Cystic fibrosis
  • A disorder of cell membranes
  • Caused by recessive allele

Solution – Embryo screening

  • Embryos are tested to see if alleles for inherited disorders are present

Describe how embryos can be screened for the alleles that cause genetic disorders. (4)

  • DNA isolated from embryo
  • Fluorescent probe mixed with embryo DNA
  • Probe then binds with embryo DNA
  • UV light to show alleles / gene for disorder
Advantages Disadvantages
  • ↑chance of having a baby without disorder
  • Disorder won’t be passed on to future generation
  • ↓ppl suffer
  • There’re many regulations to stop it going too far
  • Use more than 1 embryo so more chance of success
  • Spare embryo have potential use – could be used in stem cell treatment
  • Operation hazard eg infection
  • Ethical – potential damage to embryo
  • Expensive
  • Large no of embryo created but only small no are implanted – remaining embryos destroyed
  • Promote prejudice as it suggests ppl with genetic disorders are unwanted
  • Encourage ppl to pick characteristics
  • Embryos might not develop

6.1.8 Sex determination

Human body cell – 23 chromosomes

  • 22 pairs – control characteristics
  • 1 pair – carriers genes that determines sex
  • Sex chromosomes – Male (XY), Female (XX)
  • What is the chance of having a female child? Give the reason for your answer. (2)
    • 50% coz 2 out of 4 boxes are XX