4B: Diversity, Classification & Variation Flashcards
What are gametes?
Sex cells:
Sperm cells in males and egg cells in females
What do gametes join together to make?
A zygote
What is the human haploid number?
23
What is the human diploid number?
46
Describe homologous chromosomes
- One maternal + one paternal
- Called homologous pairs
- Two chromosomes that carry the same genes but not necessarily the same alleles of the genes
- Have the same gene loci
What does the haploid number refer to?
HALF the number of homologous pairs
Define nondisjunction
The failed separation of chromosomes during meiosis I or chromatids in meiosis II
What happens if a gamete with an error is fertilised (nondisjunction)?
If the gamete with an error is the fertilised, the resulting embryo may not be viable and might die or else an individual may be born with a genetic disorder
In what 2 ways is meiosis a major source of variation?
- Independent assortment
2. Crossing over
What is a gene locus?
A fixed position on a chromosome that is occupied by the gene
When does crossing over occur?
In Meiosis I
What are the (3) stages of crossing over?
- A piece of chromatid from one chromosome touches a piece of chromatid from the homologous partner
- The chromatids of each pair become twisted around each other (crossing over). This creates tensions and causes portions of the chromatids to break off
- The broken portions might then rejon with the chromatids of its homologous partner (recombination)
Explain why no genetic material is lost in the crossing over of chromosomes
No genetic material is lost because the genetic material is exchanged between homologous chromosomes and at the same gene loci
- Meaning that the chromosomes will stay the same length
How does crossing over result in genetic variation in daughter cells?
- Crossing over allows the exchange of genetic material between homologous chromosomes
- This increases variation as it results in new allelic combinations that were not present in either parent chromosome
What is independent assortment?
In metaphase 1 homologous chromosomes randomly align along the equator of the cell
What is independent segregation?
Since the chromosomes align randomly, the combination of maternal/ paternal chromosomes that enter the daughter cells is also random
What does meiosis produce?
4 daughter cells that are genetically different from
each other
Describe the structure of a chromosome
Two sister chromatids joined together by a centromere
If you know the haploid number of an organism, what is the formula to work out the no. of possible combinations in the gametes?
Combinations = 2n (squared)
n= haploid number
Describe cross fertilisation
- The process of cross fertilisation provides further opportunity for variation
- The fusion of male + female gametes is an entirely random process
Where does meiosis take place?
Reproductive organs
Describe the 6 stages of meiosis
- Before meiosis starts DNA unravels and replicates (2 copies in each chromosome)
- The DNA condenses to form double-armed chromosomes (made from 2 sister chromatids)
- Meiosis I (1st division) the chromosomes arrange into homologous pairs
- The pairs are then separated, halving the chromosomes number
- Meiosis II (2nd division) - the centromere is divided producing separated sister chromatids
- 4 haploid, genetically different daughter cells (gametes) are produced
Are the cells that divide by meiosis diploid or haploid? What do they produce?
The cells that divide are diploid, but they produce haploid cells (with half the no. of chromosomes)
What are sister chromatids joined by?
The centromere
Describe independent segregation (4 points)
- Each homologous pair of chromosomes is made up of 1 maternal and 1 paternal chromosome
- When homologous pairs are separated (meiosis I) its random which chromosome from each pair ends up in which daughter cell
- So the 4 daughter cells produces have a random combination of maternal + paternal chromosomes
- Leads to genetic variation in offspring
When are homologous pairs separated?
Meiosis I
How many divisions occur in meiosis compared to mitosis?
2 divisions in meiosis but only 1 in mitosis
What happens during the 2 divisions of meiosis?
Separates homologous pairs and then the sister chromatids
In meiosis, how many chromosomes will each daughter cell have?
23 whole chromosomes (1 from each homologous pair)
What do genetic mutations involve?
Gene mutations involve a change in the base sequence of chromosomes
Describe substitution mutations
When a nucleotide is replaced by another nucleotide (with a different base)
Give an example of a substitution mutation.
What effect could this have on the protein?
E.g:
G T C –> codes for the amino acid Glutamine
Then if the base C is replaced by G:
G T G –> codes for histidine
Therefore the polypeptide produced will change. The significance of this depends on the role of the original amino acid.
In substitution mutations, what happens if the amino acid coded for is important in forming bonds that determine the tertiary struc of the protein?
- If the amino acid is important in forming bonds that determine the tertiary structure of the final protein
- then the replacement amino acid may not form the same bonds
- The protein may be a different shape
- Which means that it may not function properly
E.g enzymes
What will happen if the base is substituted by another base that codes for the same amino acid?
No change
E.g:
GTC becomes GTT –> but both triplets code for glutamine
How, and when, do chromosome mutations occur?
Mutations in the number of chromosomes can arise spontaneously by chromosome non-disjunction during meiosis.
Describe deletion mutations
- When a nucleotide is lost from the normal DNA sequence
- Has drastic results:
A deletion of just 1 nucleotide base causes the entire sequence to be read differently because each base has been shifted to the left by one base (frame shift)
What can increase the rate of gene mutation?
Mutagenic agents
Give 3 example of mutagenic agents
X-rays, UV radiation and radioactive substances
What are chromosome mutations?
Changes in the structure or number of whole chromosomes
What are the 2 types of chromosome mutation?
- Changes in whole sets
- Changes in no. of individual chromosomes
Describe the type of chromosome mutations where changes to the whole set are made.
This occurs when organisms have 3 or more sets of chromosomes rather than the usual 2 –> called polyploidy
In which kingdom does polyploidy mostly occur?
Plant kingdom
What is genetic diversity?
The number of different alleles of genes in a population
What can genetic diversity be increased by?
- Mutations in DNA
- Gene flow
What is gene flow?
When different alleles are introduced into a population. Happens when individuals from another population migrate into them and reproduce
Genetic diversity allows what process to occur?
Evolution by natural selection
What can reduce genetic diversity?
Genetic bottlenecks
What is a genetic bottleneck?
- An event that causes a big reduction in a population e.g when a large no. of organisms within a pop. die before reproducing
- The survivors reproduce and a larger population is created from few individuals
What do genetic bottlenecks reduce?
The no. of different alleles in the gene pool - so reduced genetic diversity
What is the founder effect a type of?
Genetic bottleneck
What is the founder effect?
- When just a few organisms from a population start a new colony and there are only a small no. of different alleles in the initial gene pool
- Allele frequency may be very different to original population = may lead to a higher incidence of genetic disease
What is the process of natural selection?
- Random mutation creates a new allele
- The new allele provides the organism with an advantage over others in the population.
- The organisms with beneficial mutations are more likely to obtain available resources + grow more rapidly
- Therefore they are more likely to survive and reproduce
- Reproduction of these organisms passes on the mutated allele to their offspring
- This process continues over time, increasing the frequency of the mutated, advantageous allele and decreasing the frequency of the non-advantageous or old alleles in the population
What are the 3 types of adaptations that natural selection can lead to?
- Behavioural
- Physiological
- Anatomical
What is a behavioural adaptation? give an example
Ways an organism acts that increase it’s chance of survival & reproduction
e.g possums can ‘play dead’ to escape attack from a predator
What is a physiological adaptation? give an example
Processes inside an organism that increase its chance of survival
e.g Brown bears hibernate over winter (lowers metabolic rate) to conserve energy
What is a anatomical adaptation? give an example
Structural features of an organism’s body that increase its chance of survival
e.g Whales have thick blubber to keep them warm in the cold sea
What does natural selection alter in a population?
Allele frequency
Describe directional selection
Where individuals with alleles for an extreme type of phenotype are more likely to survive and reproduce
- Could be a response to an environmental change
- Results in the bell curve shifting in the direction of selection
Describe stabilising selection
Where individuals with alleles for average phenotypes are more likely to survive and reproduce
- Occurs when environment is not changing
- Reduces the range of possible characteristics
- Results in a narrow bell curve
Describe how bacteria evolving antibiotic resistance is an example of directional selection
- Some individuals in a pop. have alleles that give them resistance to an antibiotic
- The pop. is exposed to antibiotic, killing bacteria without the resistant allele
- The resistant bacteria survive and reproduce without competition, passing on the allele that gives then resistance to their offspring
- After some time, most organisms in the pop. will carry the antibiotic resistance allele
Describe how bacteria evolving antibiotic resistance is an example of directional selection
- Some individuals in a pop. have alleles that give them resistance to an antibiotic
- The pop. is exposed to antibiotic, killing bacteria without the resistant allele
- The resistant bacteria survive and reproduce without competition, passing on the allele that gives then resistance to their offspring
- After some time, most organisms in the pop. will carry the antibiotic resistance allele
Give an example of stabilising selection
Human birth weight
Describe how human birth weight is an example of stabilising selection
- Humans have a range of birth weights
- Very small babies are less likely to survive
- Giving birth to very large babies can be difficult and result in complications, so they are less likely to survive
- Conditions are most favourable for medium-size babies, so the weight of human babies tends to shift towards the middle of the range
What is phylogeny?
The study of the evolutionary history of groups of organisms. It attempts to arrange species into groups based on evolutionary history
What can a phylogeny tree show?
Common ancestors between organisms
Describe a phylogeny tree
- Uses hierarchy where smaller groups are placed within larger groups - no overlap
- 1st branch represents a common ancestor of all the family members
- Different groups diverged from the common ancestor, forming new branches
- Closely related species diverged away from each other more recently (their branches are close together)
e. g humans and chimpanzees are closely related because they diverged recently
What is taxonomy?
The science of classification, it involves naming and organising organisms into groups
What are the 8 taxonomic ranks? (in order)
Domain Kingdom Phylum Class Order Family Genus Species
What are the 3 domains?
Eukarya, Bacteria and Archaea
Describe the taxonomic groups
- Organised in a hierarchy
- 8 groups
- As you move down the hierarchy, there are more groups at each level but fewer organisms in each group
- As you move down, the organisms in each group are also more closely related
What is the definition of a species?
A group of organisms that look similar and are able to produce fertile offspring
What naming system is used in classification?
Binomial
What language does the binomial naming system use?
Latin
Describe the binomial naming system
All organisms are given 1 name in latin (in 2 parts)
- Names are always written in italics (or underlined if handwritten)
Describe the 1st name in the binomial naming system
The genus, begins with a capital letter
Describe the 2nd name in the binomial naming system
The species, begins with a lower case letter
What does the binomial naming system help to avoid?
Confusion from using the common names of organisms
Why is courtship behaviour carried out by organisms?
To attract a mate of the right species
- essential for the survival of that species
Why is courtship behaviour species specific?
It allows members of the same species to recognise each other, preventing interbreeding and making reproduction more successful
Why can courtship behaviour be used to classify organisms
Because it is species specific
What is the difference between a taxa and a taxon?
Taxa - groups used to classify organisms (plural)
Taxon - individual group
What does courtship behaviour allow organisms to do?
- Recognise own species
- Form a pair bond
- Synchronise breeding
- Recognise when their mate is capable of breeding
Name 3 advances that can clarify evolutionary relationships
- Genome sequencing
- Immunological comparisons
- Comparisons of amino acid sequences
What is variation?
The differences that exist between organisms
What is interspecific variation?
Variation between different species
What is intraspecific variation?
Variation between individuals of the same species
What can variation be caused by?
Genetic or environmental factors.
Or a combination of both
When studying variation, why does the sample have to be random?
- To make sure its not biased
- To ensure any variation in the sample isn’t due to chance
What does standard deviation tell us?
How much the values in a single sample vary.
- It measures the spread of values about the mean
What is biodiversity?
The variety of living organisms in an area
What is a habitat?
The place where an organism lives
What is a community?
All the populations of different species in a habitat
What is species diversity?
The no. of different species + the no. of individuals of each species within the community
What is genetic diversity?
The variety of genes possessed by the individuals that make up a population of a species
In the formula for the index of diversity (d) what do N and n stand for?
d = N (N − 1) / T n (n − 1)
N = Total number of organisms of all species n = Total number of organisms of one species
What is species richness also a simple measure of?
Biodiversity
What does an index of diversity measure?
Biodiversity
What is species richness?
A measure of different species in a community
How do you compare genetic diversity? (4 ways)
- Compare observable characteristics
- Compare DNA base sequences
- Compare mRNA base sequences
- Compare amino acid sequences
How can agriculture reduce biodiversity? (5 ways)
- Deforestation
- Hedgerow removal
- Pesticides
- Herbicides
- Monocultures
What are the 2 types of selection?
Stabilising and directional
