Topic 4B - Diversity, Classification and Variation Flashcards
What are gametes?
They are the sperm cells and egg cells that join together at fertilisation to form a zygote, which divides and develops into a new organism
What is a ‘diploid cell’?
Cells that contain 2 of each chromosome, one from the mum and one from the dad (2n) e.g. normal body cells
What is a ‘haploid cell’?
A cell that only has one copy of each chromosome
What happens at fertilisation?
A haploid sperm fuses with a haploid egg to make a cell with the normal diploid number of chromosomes
- Half these chromosomes are from the father, and half from the mother
What is random fertilisation and why is it important?
It explains the way in which any sperm can fertilise any egg in sexual reproduction, to produce zygotes with different combinations of chromosomes to both parents
- Its important in introducing genetic variation within species
Where does meiosis take place?
In the reproductive organs
What does meiosis start with?
A diploid cell
What does meiosis produce?
4 haploid daughter cells
What are homologous pairs?
They are pairs of chromosomes that are the same size, have the same genes (only differing by alleles), and one chromosome in each pair comes from the mum and one from the dad
Describe the process of meiosis
DIAGRAM
- Before it starts, DNA unravels and replicates so there are 2 copies of each chromosome, called chromatids
- DNA condenses to form double-armed chromosomes, each made from 2 sister chromatids. Sister chromatids are joined in middle by centromere
- Meiosis I - Chromosomes arrange into homologous pairs
- Homologous pairs are separated, halving chromosome number
- Meiosis II - pairs of sister chromatids that make up each chromosome are separated (centromere divided)
- 4 genetically different haploid cells are produced
Why is meiosis needed?
- It is needed as without it, you’d get double the number of chromosomes when gametes fuse
- Introduces variation
What happens to chromatids during Meiosis I?
They twist around each over (in their homologous pairs) and bits of chromatids swap over
- The chromatids still contain the same genes but have a different combination of alleles
What are the 2 main events, during meiosis, that leads to genetic variation?
- Crossing over of chromatids
2. Independent segregation of chromosomes
How does the crossing over of chromatids’ lead to genetic variation?
It means that each of the 4 daughter cells contain chromatids with different alleles
- This leads to genetic variation
How does the ‘independent segregation of chromosomes’ lead to genetic variation?
- When homologous pairs are separated in Meiosis I, its completely random which chromosome ends up in which daughter cell
- So 4 daughter cells produced by meiosis have completely different combinations of those maternal and paternal chromosomes (Independent segregation)
- This ‘shuffling’ of chromosomes leads to genetic variation in any potential offspring
Give 5 differences between Mitosis and Meiosis
- Mitosis = produces cells with same number of chromosomes as parent cell /Meiosis = produces cells with half number of chromosomes as parent cell
- Mitosis = daughter cells are genetically identical to each other and parent cell /Meiosis = Opposite
- Mitosis = produces 2 daughter cells /Meiosis = produces 4
- Mitosis = 1 division /Meiosis = 2 divisions
- Mitosis = doesnt involve crossing over of chromatids or independent segregation, as theres no pairing/separating of homologous pairs/ Meiosis = does
What are chromosome mutations caused by?
They’re caused by errors during meiosis - sometimes, meiosis goes wrong and the cells produced contain variations in the numbers of whole chromosomes or parts of chromosomes
What can chromosome mutations lead to?
Chromosome mutations can lead to inherited conditions as the errors are present in the gametes (the hereditary cells)
Give an example of one type of chromosome mutation
Non-disjunction - a failure of the chromosomes to separate properly
What causes Down’s Syndrome
1- Down’s syndrome is caused by a person having an extra copy of chromosome 21 (or parts of it)
2- Non-dysjunction means chromosome 21 fails to separate properly during meiosis, so one cell gets an extra copy of 21 and another gets one
3 -When the gamete with the extra copy fuses to another gamete at fertilisation, the resulting zygote will have 3 copies of chromosome 21
In which division can non-dysjunction occur?
In both meiosis I and II
What are gene mutations?
They are changes to the DNA base sequence of chromosomes
What 2 types of errors can occur in mutations?
- Substitution: one base substituted with another
2. Deletion: One base is deleted
What affect can a mutation have on:
- Amino acids?
- proteins?
1- They could alter the sequence of amino acids that the gene codes for
2- They could alter the properties/function of the protein formed
Why dont all substitution mutations affect the order of amino acids?
The degenerate nature of the genetic code means that amino acids can be coded for by more than one DNA triplet
Why do all deletion mutations affect the order of amino acids?
DIAGRAM
Because deletions will change the number of bases present, which will cause a shift in the base triplets after it
What are mutagenic agents? (give examples)
Things that can cause an increase in the rate of mutations e.g. ultraviolet radiation, ionising radiation some chemicals and some viruses
What is genetic diversity?
The number of different alleles of genes in a species/population
- It allows natural selection to occur
What is genetic diversity, within a population, increased by?
- Mutations in the DNA - forming new alleles
2. Gene flow = when individuals from another population migrate and introduce different alleles to a population
What does genetic diversity allow to take place?
It allows natural selection to occur
What is a genetic bottleneck?
An event that causes a big reduction in a population e.g. sudden deaths of organisms in populations
- This reduces the number of different alleles in the gene pool and so reduces genetic diversity
- The survivors reproduce and a larger population is created from a few individuals
Give an example of a genetic bottleneck event in the 1800s
Northern Elephant Seals
After being hunted in late 1800’s, their pop. fell to 50 seals. Since then, they’ve produced a population of 170,000
- BUT this new pop has very little genetic diversity compared to the southern elephant seals who never suffered such a large reduction in numbers
Define ‘gene pool’
the complete range of alleles in a population
What is the founder effect?
A type of genetic bottleneck that describes when few organisms from pop start new colony & there are only a small number of different alleles in initial gene pool
What usually causes the founder effect?
- Migration
2. If colony is separated from original pop due to reason such as religion
Why may the founder effect lead to a higher incidence of genetic disease?
Frequency of each allele might be very different to frequency of alleles in original pop. e.g. allele that was rare in original pop might be common in new colony.
- This may lead to higher incidence of genetic disease
Give an example of a flounder effect situation?
The Amish
Pop remained isolated due to religious beliefs, so few new alleles have been introduced and hence, shows little genetic diversity. (may be linked to pop having an unusually high incidence of certain genetic disorder)
When are mutations bad?
When mutated alleles lead to harmful/undesirable characteristics
when are mutations good?
When mutated alleles contribute to natural selection and allow an organism to adapt in order to survive in certain environments
How does natural selection work?
- Individuals with an allele that increases their chance of survival are more likely to survive, reproduce and pass on their genes
- This means that a greater proportion of next generation inherits beneficial allele
- They, in turn, are more likely to survive, reproduce and pass on genes
- Frequency of beneficient allele increases from generation to generation
- Over generations, this leads to evolution as advantageous alleles become more common in pop
Name 3 types of adaptations in natural selection
- Behavioural adaptations
- Physiological adaptations
- Anatomical adaptations
Define behavioural adaptations (GIVE EXAMPLE)
Ways an organism acts that increases its chance of survival and reproduction
- e.g. possums ‘play dead’ when under threat by a predator, to escape attack
Define physiological adaptations (GIVE EXAMPLE)
Processes inside an organisms body that increases its chance of survival
e.g. brown bears hibernate over winter. They lower rate of metabolism to conserve energy, so they dont need to look for food in months when its scarce
Define anatomical adaptations (GIVE EXAMPLE)
Structural features of an organism’s body that increases its chance of survival
e.g. whales have thick fat layer for warmth in cold sea
Name 2 types of natural selection
- Stabilising selection
2. Directional selection
Define directional selection
It is where individuals with alleles for a characteristic of an EXTREME type are more likely to survive and reproduce.
- This could be in response to an environmental change
Give an example of directional selection
DIAGRAM FOR PARENT AND OFFSPRING
Antibiotic resistance
1) some individuals in a pop have alleles that give them resistance to an antibiotic
2) the pop is exposed to the antibiotic, killing bacteria without the resistant allele
3) the resistant bacteria survive and reproduce without competition, passing on the allele that gives antibiotic resistance to their offspring
4) After some time, most organisms on the pop will carry the antibiotic resistance allele
Define stabilising selection
Stabilising selection is where individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce
- it occurs when the environment isn’t changing and it reduces the range of possible characteristics
Give an example of stabilising selection
(DIAGRAM FOR PARENT AND OFFSPRING
Human Birth Weight
- Humans have a range of birth weight
- Very small babies are less likely to survive - partly because they find it hard to maintain body temp
- Giving birth to large babies can be difficult - so large babies are less likely to survive too
- Conditions are most favourable for medium-sized babies - so weight of human babies tends to shift towards middle
look at questions on interpreting data on the effects of selection
look in book
