B6: Inheritance, variation and evolution Flashcards
give three features of sexual reproduction
- the production of gametes by meiosis
- a gamete from each parent fuses to form a zygote
- gamete have different genetic information so zygote is unique
what are gametes?
- sex cells ( sperm cell and egg cells in animals ; pollen and egg cells in flowering plants )
- haploid ( half the number of chromosomes)
give three features of meiosis
- form of cell division involved in the formation of gametes in reproductive organs
- chromosome number is halved
- involves 2 divisions
what must occur prior to meiosis?
interphase:
- copies of genetic information are made during this process
why is meiosis important for sexual reproduction?
- it increases genetic variation
- it ensures that the zygote formed at fertilisation is diploid
Describe fertilisation and its resulting outcome
- gametes join together to form a zygote
- the new cell then divides by mitosis
- as the embryo develops, cells differentiate
give three advantages of sexual reproduction
- genetic variation in offspring,
- species can adapt to new environments due to variation, which gives them a survival advantage
- Humans can speed up natural selection through selective breeding, which can increase food production
Describe the disadvantages of sexual reproduction (2)
- two parents are required. this makes reproduction difficult in endangered populations or in species which exhibit solitary lifestyles
- more time and energy is required so fewer offspring are produced
give 2 features of asexual reproduction?
- involves mitosis only
- produces genetically identical daughter cells
describe the advantages of asexual reproduction (3)
- only one parent is required
- more time and energy efficient as do not need a mate
- lots of offspring can be produced in a short period of time, when conditions are favourable ( enabling the rapid colonisation of an area and reducing competition from other species)
give a disadvantage of asexual reproduction
- no genetic variation ( except from spontaneous mutations)
- reducing the probability of a species being able to adapt to environmental change
- species may only be suited to one habitat
Describe the circumstances in which malarial parasites reproduce sexually and asexually
- sexual reproduction in the mosquito ( the gut)
- asexual reproduction in the human host ( liver and red blood cells)
Describe the circumstances in which fungi reproduce sexually and asexually
- asexual reproduction by spores
- sexual reproduction to give variation
Describe the circumstances in which plants reproduce sexually and asexually
- sexual reproduction to produce seeds
- asexual reproduction by runners (e.g strawberry plants) or bulb division ( e.g daffodils)
Describe the structure of
DNA
- A polymer made up of repeating units called nucleotides.
- The polymer is made up of 2
strands that is twisted into a double helix
Define genome
- the entire genetic material of an organism
give three reasons why understanding the human genome important?
- searching for genes linked to different types of disease
- understanding and treating inherited disorders
- tracing human migration patterns from the past
what is a chromosome?
- a long coiled molecule of DNA that carries genetic information in the form of genes
- DNA + protein
how many chromosomes do human body cells have?
46 chromosomes ( 23 pairs)
How many chromosomes do human gametes have?
23 chromosomes
Define gene
- A small section of DNA that codes for a particular sequence of amino acids (that undergo polymerisation) to make a specific protein
what are the monomers of DNA
nucleotides
what are DNA nucleotides made up of
- common sugar
- phosphate group
- one of four bases : A,T,C , G
state the full names of the four bases in nucleotides
- Adenine
- Thymine
- Cytosine
- Guanine
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 (A pairs with T, C pairs with G) joined by weakhydrogenbonds.
Explain how a gene codes for a protein (3)
- A sequence of three bases in a gene forms a triplet.
- Each triplet codes for an amino acid.
- The order of amino acids determines the structure and function of protein formed
Why is the ‘folding’ of amino acids important in proteins such as enzymes?
- the folding of amino acids determines the shape of the active site
- which must be highly specific to to the shape of its substrate
define protein synthesis
the formation of a protein from a gene
what are the two stages of protein synthesis
- transcription
- translation
what does transcription involve?
the formation of mRNA from a DNA template
explain the process of transcription
- DNA double helix unwinds
- base sequence of the gene is copied into a complementary template molecule ( mRNA)
- mRNA moves from the nucleus into the cytoplasm
what does translation involve:
- a ribosome joins amino acids in a specific order dictated by mRNA to form a protein
describe and explain translation
- mRNA attaches to a ribosome
- amino acids are brought the the ribosome on carrier molecules (tRNA)
- Ribosome reads the mRNA bases in triplets.
- A polypeptide chain is formed from the sequence of amino acids which join together to from a protein
what is a mutation?
- A random change in the base sequence of DNA ( DNA Structure)
Describe the effect of a gene mutation in coding DNA
- if changes sequence
- if not
If a mutation changes the amino acid sequence:
- protein structure and function may change e.g. an enzyme may no longer fit its substrate binding site or a structural protein may lose its strength).
If a mutation does not change amino acid sequence:
- there is no effect on protein structure or function.
What is non-coding DNA?
DNA which does not code for a protein but instead controls gene expression.
Describe the effect of a gene mutation in non-coding DNA
- Gene expression may be altered,
- affecting protein production and the resulting phenotype.
what are alleles
different forms of the same gene
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 copes are needed for it to be expressed
define homozygous
- when an organism has two copies of the same allele ( two recessive or two dominant)
define heterozygous
- when an organism has two different versions of the same gene ( one dominant or one recessive)
define genotype
the genes present for a trait
define phenotype
- the visible characteristic
what is the problem with single gene crosses?
- most characteristics are controlled by multiple alleles rather than just one
what is an inherited disorder
- a disorder caused by the inheritance of a certain allele
give two examples of inherited disorders:
- is it dominant or recessive
- describe what it is
- Polydactyly - caused by a dominant alleles - having extra fingers or toes
- Cystic fibrosis caused by a recessive allele - disorder of cell membranes
how are embryos screened for inherited disorders?
- during IVF , one cell is removed ( from an 8-inch embryo) and tested for disorder-causing alleles
- if the cell doesn’t have any indicator alleles, then the originating embryo is implanted into the uterus
give three ethical issues concerning embryo screening?
- it could lead to beliefs in society that being disabled or having a disorder is associated with inferiority
- the destruction of embryos with inherited disorders is seen as murder and against the idea of sanctity of life
- it could be viewed as ‘’ designer babies’’ as it may be for parents convenience rather than the child’s wellbeing
what are the economic issues concerning embryo screening?
- consideration of costs of treatment and medication of the child with the inherited disorder
- financial support may be needed
give two social issues concerning embryo screening?
- if embryo has an inherited disorder and is discarded off, this can prevent a child and its parents from potential suffering
- social care for children with inherited disorders may need to be considered
define gene therapy
- the insertion of a normal allele into the cells of a person with an inherited disorder to functionally replace the faulty allele
give two ethical issues concerning gene therapy
- some people believe that it is playing god
- the introduced genes could enter sex cells and so be passed to future generations
what are sex chromosomes and examples..
a pair of chromosomes that determine sex:
- males have an X and a Y chromosmes
- females have two X chromosomes