Diversity and Selection Flashcards
1
Q
Diploid Body Cells
A
- Normal body cells have the diploid number (2n) of chromosomes- meaning each cell contains 2 of each chromosome (a pair), one from mum and one from dad
- Chromosomes that make up each pair are the same size and have the same genes, although they could have different versions of the genes known as alleles (these pairs are called homologous pairs)
- Humans have 23 homologous pairs and so 46 chromosomes in total so diploid number for humans is 46
2
Q
Gametes in sexual reproduction
A
- Gametes are sperm cells in males and egg cells in females
- Gametes have a haploid number of chromosomes- they only contain one copy of each chromosome in a homologous pair
- Haploid number for humans is 23
- In sexual reproduction 2 gametes join together at fertilisation to form a zygote, which divides and develops into a new organism
3
Q
Fertilisation
A
- Haploid sperm fuses with a haploid egg, making a cell with the normal diploid number of chromosomes
- Half these chromosomes are from the father (sperm) and from the mother (egg)
- Any sperm can fertilise any egg- fertilisation is random
- Random fertilisation produces zygotes with different combination of chromosomes to both parents
- This mixing of genetic material in sexual reproduction increases genetic diversity within a species
4
Q
Meiosis definition
A
- Cell division that takes place in the reproductive organs of multicellular, eukaryotic organisms
- Cells that divide by meiosis are diploid to start with, but the cells that are formed from meiosis are haploid- chromosome number halves
- Meiosis in humans and mammals produce gametes directly
- In other organisms it produces haploid cells which later divide by mitosis to become gametes
- Without meiosis, you’d get double the number of chromosomes when the gametes fused which isn’t good
5
Q
Meiosis
A
- Before meiosis starts, the DNA unravels and replicates so there are two copies of each chromosome, called chromatids
- DNA condenses to form double-armed chromosomes, each made from 2 sister chromatids
- Sister chromatids are joined in the middle by a centromere
- Meiosis (first division)- chromosomes arrange themselves into homologous pairs
- Homologous pairs are then separated, halving the chromosome number
- Meiosis (second division)- pairs of sister chromatids that make up each chromosome are separated (centromere is divided)
- 4 haploid cells that are genetically different from each other are produced
6
Q
How to create genetic variation in gametes?
A
Crossing over of chromatids and independent segregation of chromosomes
7
Q
Crossing over of chromatids
A
- During meiosis I, homologous chromosomes come together and pair up
- Chromatids twist around each other and bits of chromatids swap over
- Chromatids still contain the same genes but now have a different combination of alleles
- Means that each of the 4 daughter cells formed from meiosis II contain chromatids with different alleles
8
Q
Independent segregation of chromosomes
A
- Each homologous pair of chromosomes in your cells is made up of one chromosome from your mum (maternal) and one from you dad (paternal)
- When homologous pairs are separated in meiosis I, it’s completely random which chromosome from each pair ends up in which daughter cell
- 4 daughter cells produced by meiosis have completely different combinations of those maternal and paternal chromosomes
- Shuffling of chromosomes leads to genetic variation in any potential offspring
9
Q
How many possible combinations in diploid species?
A
2n possible combinations of maternal and paternal chromosomes (n is number of homologous pairs) e.g. for humans there are 2^23 possible combinations
10
Q
What do Zygotes divide by in humans?
A
Mitosis
11
Q
Outcomes of mitosis and meiosis
A
- Mitosis= Produces cells with the same number of chromosomes as the parent cell, daughter cells are genetically identical to each other and to the parent cell, produces 2 daughter cells
- Meiosis= Produces cells with half the number of chromosomes as the parent cell, daughter cells are genetically different from one another and the parent cell, produces 4 daughter cells
12
Q
What is a gene mutation?
A
- Change in the DNA base sequence of chromosomes
- Order of DNA bases in a gene determines the order of amino acids in a particular protein so if a mutation occurs in a gene, the sequence of amino acids it codes for (and the protein formed) could be altered
13
Q
Substitution
A
One base is substituted with another
14
Q
Deletion
A
One base is deleted
15
Q
Effects of mutations
A
- Degenerate nature of the genetic code means that some amino acids are coded for by more than one DNA triplet
- This means that not all substitution mutations will result in a change to the amino acid sequence of the protein- some substitutions will still code for the same amino acid
- However, deletions will always lead to changes in the amino acid sequence beacuse it will change the number of bases present, which will cause a shift in all the base triplets after it
16
Q
Mutagenic Agents
A
- Mutations occur spontaneously- but mutagenic agents can increase rate of mutations
- Examples include ultraviolet radiation, ionising radiation, some chemicals and some viruses
- Mutagenic agents increase probability of a mutation occuring
17
Q
Chance
A
Possibility of something happening
18
Q
Probability
A
Measure of how likely events are to happen
19
Q
Chromosome Mutations
A
- Meiosis can go wrong and cells produced contain variations in the whole number of chromosomes or parts of chromosomes
- Caused by errors in meiosis
- Lead to inherited conditions because the errors are present in the gametes (hereditary cells)
- One type of chromosome mutation is called chromosome non-disjunction (failure of chromosomes to separate properly) e.g. Down’s syndrome is caused by having an extra copy of chromosome 21
20
Q
What is Genetic Diversity?
A
- Number of different alleles of genes in a species or population
- Large number of different alleles in a population means a large variety of different characteristics and a high genetic diveristy
- If a population has low genetic diversity, it might not be able to adapt to a change in the environment and the whole population could be wiped out by a single event (disease)
- Genetic diversity is what allows natural selection to occur beacuse some characteristics are more advantageous than others
21
Q
What is genetic diversity increased by?
A
- Mutations in the DNA forming new alleles (some can be advantageous, while others lead to problems)
- Different alleles being introduced into a population when individuals from another population migrate into it and reproduce (known as gene flow)
22
Q
Gene Pool
A
Complete range of alleles in a population
23
Q
Do all members of a species have the same genes?
A
Yes, diversity only occurs in the form of different alleles of those genes
24
Q
Genetic Bottleneck
A
- Event that causes a big reduction in a population
- This reduces the number of different alleles in the gene pool and so reduces genetic diversity
- Survivors reproduce and a larger population is created from a few individuals
25
Q
Founder Effect
A
- Describes what happens when just a few organisms from a population start a new colony and there are only a small number of different alleles in the initial gene pool
- Frequency of each allele in the new colony might be very different to the frequency of those alleles in the original population (e.g. rare allele in original population might be more common in new colony- may lead to higher incidence of genetic disease)
- Founder effect can occur as a result of migration leading to geographical separation or if a new colony is separated from the original population for another reason (religion)
26
Q
Process of Natural Selection
A
- Some mutations produce alleles that are beneficial to an organism, helping the organism to survive in certain environments
- When the allele codes for a characteristic that increases the chances of an organism surviving, it’s frequency within the population can increase (known as N.S)
- Not all individuals are as likely to reproduce as each other (have different reproductive success)
- Individuals that have an allele that increases their chance of survival are more likely to survive, reproduce and pass on their genes (including the beneficial allele), than individuals with less advantageous alleles
- Means that greater proportion of next generation inherits beneficial allele
- They, in turn, are more likely to survive, reproduce and pass on their genes
- Frequency of the beneficial allele in the population increases from generation to generation
- Over generations this leads to evolution as the advantageous alleles become more common in the population
- Natural selection leads to organisms becoming better adapted to their environment and alters allele frequency in a population
27
Q
Evolution
A
- Gradual change in species over time
- Led to huge diversity of living organisms on earth
- Adaptation and selection are both key factors in evolution
28
Q
Adaptations
A
- Features that help organisms to survive in their environment (behavioural, physiological and anatomical)
- Organisms that are well adapted to their environment have a selective advantage over less-well adapted organisms
29
Q
Behavioural Adaptations
A
Way an organism acts that increases its chance of survival (playing dead, dancing before mating)
30
Q
Physiological Adaptations
A
Processes inside an organism’s body that increase its chance of survival (hibernation, bacteria producing antibiotics)
31
Q
Anatomical Adaptations
A
Structural features of an organism’s body that increase its chance of survival (thick layers, streamline shape)
32
Q
Directional Selection
A
- Individuals with alleles for characteristics of an extreme type are more likely to survive and reproduce
- Could be a response to an environmental change
33
Q
Stabilising Selection
A
- Individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce
- Occurs when environment isn’t changing, and it reduces the range of possible characteristics
34
Q
Interpreting data on effects of selection
A
- If variation stays roughly the same but the mean changes, must be directional selection
- If mean stays the same but the variation decreases, must be stabilising selection
