Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
59 Cards in this Set
- Front
- Back
Define meiosis |
A reduction division in which the resulting daughter cellshave half the original number of chromosomes: Diploid 46 --> Haploid 23
|
|
What happens during interphase? |
During interphase:
1. Chromosomes are present as granular material calledchromatin 2. DNA replicates 3. New macromolecules and organelles aremanufactured in preparation for cell division Chromosomes now consist of two identical sister chromatids,joined at the centromere. Cell contains four, copies of each chromosome. |
|
What happens during prophase 1? |
Prophase is the longest phase of meiosis:
1. Chromatin condenses and undergoes supercoilingso chromosomes shorten and thicken. 2. Homologous chromosomes come to lie closetogether in a process called synapsis; the paired chromosomes are calledbivalents (one maternal; one paternal). 3. The bivalents become shorter and thicker byfurther spiralisation and each chromosome can now be seen to consist of twochromatids held together at the centromere. 4. The pairs of identical sister chromatids are theresult of DNA replication that occurred during interphase; each group of fourchromatids is termed a tetrad. 5. Non-sister chromatids wrap around each other andattach at points called chiasmata, here they swap sections of chromatids incrossing over 6. The nucleolus disappears and the nuclearenvelope disintegrates 7. A spindle forms, made of protein microtubules |
|
What happens during metaphase 1? |
1. Bivalents line up across the equator of thespindle attached to the spindle fibres at the centromeres (chiasmata stillpresent)
2. Bivalents are arranged randomly (randomassortment) with each member of a homologous pair facing opposite poles 3. This allows the chromosomes to independentlysegregate when they are pulled apart in anaphase 1 |
|
What happens during anaphase 1? |
1. During anaphase 1 spindle activity separates thehomologous pairs of chromosomes (centromeres do not divide)
2. The homologous pairs of chromosomes migrate tothe opposite poles of the cell |
|
What happens during telophase 1? |
Animal cells: two new nuclear envelopes form – one aroundeach set of chromosomes at each pole – and the cell divides by cytokinesis
There is a brief interphase and the chromosomes uncoil Plant cells: the cell goes straight from anaphase I intomeiosis II |
|
What happens during prophase 2? |
1. If a nuclear envelope has reformed, it breaksdown again
2. The nucleolus disappears, chromosomes condenseand spindles form. |
|
What happens during metaphase 2? |
1. The chromosomes arrange themselves on theequator of the spindle, they are attached to the spindle fibres at thecentromeres
2. The chromatids of each chromosome are randomlyassorted (arranged) |
|
What happens during anaphase 2? |
1. Centromeres divide and the chromatids are pulledto opposite poles by the spindle fibres
2. The chromatids randomly segregate |
|
What happens during telophase 2? |
Nuclear envelopes reform around the haploid daughter nuclei
Animals: the two cells now divide to give four haploid cells Plants: a tetrad of four haploid cells isformed |
|
Lists 6 major events that lead to variation in individuals of the next generation. |
1. Crossing over 2. Independent random assortment of Bivalents 3. Independent random assortment of (recombinant) chromatids 4. Random mating - choices of mates is random 5. Random fusion of gametes - chances of gamete fertilising another specific gamete 6. Chromosome mutations - any changes to DNA further increases the variation |
|
Describe the process of crossing over |
Occurs during prophase 1: - Homologous pairs of chromosomes (bivalents) form - Chiasmata form between homologous chromosomes - Alleles swap between paternal and maternal chromatids (Maybe 3/4 crossing over events per bivalent) |
|
Describe the process of random assortment of bivalents |
Occurs during metaphase 1: - Chromatids line up along the cell equator randomly and independent of each other |
|
Allele |
An alternative version of a gene |
|
Locus |
Specific position on a chromosome, occupied by a specific gene |
|
Genotype |
Combination of alleles in cells of an organism for a particular trait/characteristic |
|
Phenotype |
Physical characteristic expressed due to the influence of the genotype and the environment |
|
Dominant |
If a dominant allele is present then the trait is expressed in the phenotype |
|
Co-Dominant |
A characteristic where both alleles contribute to the phenotype The phenotype from the heterozygous individual expresses the characteristic of both alleles in the genotype |
|
Recessive |
Characteristic in which the allele responsible is only expressed in the phenotype if there is no dominant allele present |
|
Homozygous |
Eukaryotic cell or organism that has two identical alleles for a specific gene |
|
Heterozygous |
Eukaryotic cell or organism that has two different alleles for a specific gene |
|
Explain the term linkage |
Genes for different characteristics that are present at different loci on the same chromosome are linked and inherited together. |
|
Explain the term crossing over |
When non-sister chromatids exchange alleles during prophase of meiosis 1. |
|
What is a test cross? |
A testcross or backcross is always used to see if genes are linked or unlinked. - A test cross is always a cross with a homozygous recessive genotype for both genes - This will allow all alleles to be expressed in the phenotypes - It will allow homozygous recessive genotypes to be expressed for both genes in the offspring if the parent is heterozygous for the two genes |
|
Sex-Linked (X-linked) |
Gene with its locus on one of the sex chromosomes, X or Y. As there are few genes on the Y chromosome, in humans, most sex-linked genes are on the X chromosome. |
|
Epistasis When is it referred to as dominant epistasis and when is it referred to as recessive epistasis? |
Where one gene may mask the expression of another If it’s the dominant allele causing the ‘hiding’, we call itdominant epistasis and if it’s the recessive allele causing the ‘hiding’ wecall it recessive epistasis. |
|
Give three ways that epistasis can occur
|
1. A gene may code for an enzyme with a complementary site for another protein 2. A gene may code for a transcription factor that regulates the expression of another gene 3. A gene may code for a competitive of non-competitive inhibitor for an enzyme synthesised from another gene |
|
Explain the general phenotypic ratios for epistasis |
As epistasis involves two genes, the epistatic crosses between two heterozygotic individuals for both genes results in a 9:3:3:1 ratio. All the epistatic phenotypic ratios will be variants of the 9:3:3:1 ratio. |
|
Predict phenotypic ratios in problems involving epistasis: Dominant Epistasis |
12:3:4 Grouping of the dominant phenotypes, genotypes (9:3 = 9+3 = 12) |
|
Predict phenotypic ratios in problems involving epistasis: Recessive Epistasis |
9:3:4 Grouping of the recessive phenotypes, genotypes (3:1 = 3+1 = 4) |
|
Predict phenotypic ratios in problems involving epistasis:
Complementary Epistasis |
9:7 Grouping of the intermediate and recessive phenotypes, genotypes (3:3:1 = 3+3+1 =7) |
|
Describe continuous variation Gives examples |
No distinct categories (quantitative) Polygenic - many genes contribute to the final phenotype (2 or more) - Each gene provides an additive effect on the phenotype Genes are not linked, they are on separate chromosomes Influenced by the environment Eg. Height, weight, etc. |
|
Describe discontinuous variation Give examples |
Distinct categories (qualitative) Monogenic - one gene contributes to the final phenotype - Epistasis if there is more than one gene at work Different alleles have a large effect on the phenotype Different genes at each different loci have large effects on the phenotype E.g. blood groups, ear lobe shape, etc. |
|
Explain that both genotype and environment contribute to phenotypic variation Whats the metaphor used to explain this? |
The phenotype is influenced by a combination of both the environment and the individuals genotype 'The genes load the gun and the environment pulls the trigger' Polygenic traits are more easily influenced by the environment that monogenic traits |
|
Explain why variation is essential in selection |
If there is no variation then all individuals have an equal chance of surviving This would mean if the environment changed then all would live and pass on their genes to the next generation or all would die and the species would become extinct Variation ensure evolution as some individuals of the species are more suitably adapted to the environment |
|
The Hardy-Weinberg model assumes that all allele and genotype frequencies will remain stable from generation to generation unless: |
1. The population is small 2. Mating is not random 3. There are mutations 4. There is migration in or out of the population 5. There is genetic drift 6. There is natural selection |
|
The Hardy-Weinberg model: Allele frequency |
p + q = 1 |
|
The Hardy-Weinberg model: Genotype frequency
|
p² + 2pq + q² = 1 |
|
What is a gene pool? |
The total number of different alleles within a population. - Populations have gene pools - Individuals have genomes |
|
What is a selection pressure? |
Environmental factor that confers greater chances of surviving and reproducing on some members of the population than on others. The environmental factor could be natural or artificial. |
|
Give some environmental factors that act as selection pressures |
Biotic: - Disease - Food - Competition (interspecific or intraspecific) Abiotic: - Water availability (soil or pools) - Mineral availability - Space - Light - Temperature |
|
Give some human factors that acts as selection pressures |
- Size of yield - Quality of yield - Progeny - Disease resistance - Temperament |
|
Explain how environmental factors can act as stabilising forces of natural selection: Stabilising selection |
Stabilising selection Favours individuals with a general phenotype Maintains the populations mean phenotype by selecting against the phenotypic extremes E.g. Calf birth weights Too small = difficulty carry our thermoregulation Too large = they cannot pass through the pelvic girdle |
|
Explain how environmental factors can act as evolutionary forces of natural selection: Directional selection |
Directional selection Favours individuals with phenotypes at one extreme and selects against individuals the other extreme Allele frequency is shifting in one direction E.g. Giraffe neck lengths |
|
Explain how environmental factors can act as evolutionary forces of natural selection:
Disruptive selection |
Disruptive selection
Favours individuals at both extremes and selects against those with a general phenotype Supports speciation E.g. Galapagos Finches beak size |
|
Explain how genetic drift can cause large changes in small populations |
- The change in allele frequency in a population, as some alleles pass to the next generation while others disappear - Chance or random selection of individuals and hence their alleles - Individuals are selected against by natural disasters and chance events as opposed to their suitability to the environment Accelerates evolution Most exaggerated in small populations E.g. Earthquake |
|
What is an isolating mechanism? |
A mechanism that divides population of organisms into smaller sub groups. |
|
How will isolating a population affect it? |
Isolating a population will: - Decrease the gene pool - Increase inbreeding which could lead to an increase in homozygous recessive genotypes within a population - Increase the effects of genetic drift |
|
Explain the role of isolating mechanisms in the evolution of new species with reference to ecological (geographic) mechanisms |
When two species or populations occupy different habitats within the same environment Can be separated by a physical barrier, such as a mountain range, river, road systems, island etc. Example: Galapagos finches |
|
Explain the role of isolating mechanisms in the evolution of new species with reference to reproductive mechanisms
|
When the reproductive structures are no longer physically compatible Example: Great Dane will not mate with a Chihuahua |
|
Explain the role of isolating mechanisms in the evolution of new species with reference to seasonal (temporal) mechanisms
|
When two species or populations live within the same area but are reproductively active at different times Example: American frog species - Wood frog: fertile in April - Tree frog: fertile in June - Bull frog: fertile in July |
|
Explain the role of isolating mechanisms in the evolution of new species with reference to behavioural mechanisms
|
When two species or populations evolve different courtship displays which are essential for successful mating Example: bird of paradise |
|
What is the biological species concept? |
Can interbreed to produce fertile offspring Reproductively isolated Have the same: - Morphology - Physiology - Anatomy - Embryology - Behaviour - Occupy the same ecological niche |
|
What is phylogeny? |
All living things have RNA / DNA / proteins Comparisons to relatedness of these molecules can determine relatedness of organisms Haplotypes can also be used for comparisons: - Holotypes are particular base sequences - Can be 1 locus, several loci or an entire chromosome - They are a combination of alleles at different loci transmitted together |
|
What is a monophyletic group? What is a clade? |
A group of organisms with similar haplotypes Shows the common ancestor and all descendants A monophyletic group can be 1 or more clades |
|
What is a paraphyletic group? |
Only linear more like traditional classification |
|
What is natural selection? |
Mechanism for evolution - Organisms that are well adapted to their environment - More likely to survive and reproduce - Passing on the alleles for favourable characteristics |
|
What is artificial selection? |
- Humans select organisms with desirable characteristics - They reproduce, passing on the alleles for favourable characteristics |