Bothwell Flashcards
What did Darwins theory or natural selection require?
Offspring to inherit information (I.e. Height, eye colour) from parents, but also required information to vary slightly from generation to generation.
Why did medal use pure breaking plants instead of something like humans to measure offspring?
- most parents I.e. Humans give rise to too much variation, making it difficulty to tell exactly what info each parent passes on.
What are pure breading plants?
They have been in read for several generations to produce strains that display very little variation in phenotype from one generation to the next.
- you can predict exactly how fast they will grow, how big they’ll be and what they’ll taste like etc.
- perfect for mental as you know exactly what information will be passed from parents to offspring.
How did Mendal’s work help rule out ‘blending inheritance’ ?
By showing that when you crossed two pure breading lines, all of the offspring looked like one line, rather than a mix of both.
I.e. If mixing one tall pure breading with one small pure breading
Either get a tall plant or small one. Not a medium one.
In summary what was Mendal’s suggestions?
Each phenotype (= the trait you can see) is determined by genotype (=the material of hereditary you can’t see).
- the genotype consists of 2 alleles (one from the father, one form the mother).
- it’s the exact combination of these alleles that determines the phenotype of the organism.
What is the name given to an organism when both alleles in the genotype are the SAME?
Homozygous.
What is the name given to an organism when both alleles in the genotype are DIFFERENT?
Heterozygous.
What is the most common from of dwarfism called and what is it caused by?
Achondroplasia, result of dominant mutation in a growth signalling molecule.
What is the ‘principle of segregation’ ?
When we cross 2 F1 individuals, we find that their alleles are divided into offspring randomly.
= only one parental allele is given, at random, to each gamete the parent produces.
What is the ratio when we cross Tt X Tt ?
3:1
T: t
What did Mendal’s do to discover the principle of random assortment? And what is it?
Looked at what happened when he crossed pure breading plants with 2 traits of interest ( we call these ‘dihybrid crosses’).
- the alleles of different genes are allocated to gametes independently of each other.
What is the ratio seen with a dihybrid cross?
9:3:3:1
If the organism displays a dominant trait what can its genotype be either?
How can we work out which one it is?
Homozygous dominant or heterozygous.
- we cross the mystery-genotype-plant with a homozygous recessive trait strain, and look at the ratio of offspring produced.
- this is known as a ‘test cross’.
What is ‘modern synthesis’?
It provides a mathematical description that links evolution and genetics.
What is Mendelian genetics?
Uses the term gene to refer to a unit of information, this information can be passed on to offspring according to various rules.
What did Walter Flemming do, and how did this serve as a breakthrough for genetics?
Used aniline dyes to show chromosomes in the nucleus divide when the cell does; a process that Flemming called mitosis.
- we now know the cell passes through 4 main cycles of the ‘cell cycle’
What is the G1 phase?
Most cells are in G1 phase, this is the normal, diploid, cell going about its business.
What is the S phase?
A number of signals hormones, environmental stress and so on can tell a cell that it’s time to divide.
- the first step to nuclear division is DNA replication, which occurs during the DNA SYNTHESIS phase.
- the genome remains relatively open during the S phase, to allow the DNA machinery to access the DNA.
- lasts a couple of hours in euk. Minutes in prok.
What is ‘n’ used to refer to?
Used to refer to the genetic content of a cell.
1n = the number of alleles present for a gene locus in a haploid cell.
What does ‘C’ refer to?
Used to refer to the DNA content of the cell.
- 1C = the amount of DNA present in a haploid cell.
Using n and C notation what would a diploid cell have during G1?
2n and 2C.
Using n and C notation what would a cell in G2 have? And why?
2n and 4C. (There’s twice as much DNA in the G2 cell, as its replicated, but the same amount of genetic information- there’s still only 2 alleles per gene locus).
What are the 5 stages of the M phase (mitosis)?
- the cell equally divides its contents between 2 daughter cells.
- Prophase
- Prometaphase
- Metaphase
- Anaphase
- Telophase
What happens in prophase?
The chromosomes condense and become visible. Remember they have replicated, so each chromosome consists of 2 copies (sister chromatids) held together at the centromere.
What happens in prometaphase?
The nuclear envelope breaks down.
What happens during metaphase?
All of the chromosomes line up along the middle of the nucleus (at the equatorial plate.)
What happens during anaphase?
each chromosome is pulled apart into its 2 chromatids.
- one chromatid goes into one daughter cell and the other goes into the other daughter cell. The chromatids are pulled apart by a spindle that attaches to the centromere.
What happens during telophase?
A nuclear envelope forms around each of the 2 daughter nuclei and the daughter chromosomes relax and decondense.
What did Theodor Boveri do?
He used microscopy to look at the patterns of chromosome inheritance in ployspermic sea urchin embryos.
- he showed that if you don’t get the correct assortment of chromosomes into cells at mitosis then the daughter cells die.
What is the structure of chromosomes at metaphase?
(Paired sister chromatids)
- sister chromatids (each of which is a double helix).
- short arm = p arm (petit)
- long arm = q arm
Sister chromatids are joined at the centromere.
What is the structure of chromosomes after telophase?
Sister chromatids have separated leaving one condensed chromosome.
What are G bands on chromosomes?
(Condensed chromosome, Giemsa stained).
- Euchromatin (white bits)
- Heterchromatin (black bits)
How do chromosomes exist in diploid organisms?
In homologous pairs.
- Each of your (diploid) cell carries 2 complete copies of the genome.
- every chromosome you get from mum you get one with the same basic structure from dad.
Why evolutionarily are chromosomes dived up?
Genomes are too big and unwieldy to carry about in one DNA strand so evolution has divided them into smaller chunks.
What do homologous chromosomes have in common?
- the gene loci are in the same order on each member of a pair, but the alleles at a given gene locus don’t have to be the same.
What did Nettie Steven’s and Edmund Wilson do?
Uses chromosomes spread to look at male and female karyotypes and identify sex chromosomes.
Why does mitosis fit with Mendal’s principle of segregation?
In mitosis each daughter cell receives all the alleles present in the parent. The principle of segregation suggests that gamete cells only receive half the alleles present in the parent.
What did Edouard Van Beneden discover?
Another mechanism of cell division, found a second mode of division that began with 2 lines of chromosomes lining up at metaphase rather than one.
What are the 2 lines of chromosomes at the Equatorial plate made up of?
Formed by homologous chromosomes pairing up with each other, and the process that this starts with was named ‘meiosis’.
What are the 3 main differences between meiosis and mitosis?
I. They happen in different places- mitosis in somatic cells, meiosis in germ cells.
II. Mitosis has one cell division, but meiosis actually has 2 cell divisions.
- one immediately after the other, Meiosis I and Meiosis II.
- so mitosis produces 2 daughter cells, but meiosis produces four.
- both mitosis and meiosis start with the same amount of DNA in the cell (= 2n/ 4C produced in S phase).
> mitosis divides once so produces diploid (2n, 2C) daughter cells.
> meiosis produces haploid (n, C) gametes.
III. Homologous chromosomes don’t associate with each other during the mitotic prophase. But they DO associate with each other during the prophase of the first meiosis division (=Meiosis I), as you can see in the right-handed cell of the figure above, hence the Van Beneden’s two lines of chromosomes- each line contains one member of a homologous pair. This pairing up of homologous chromosomes drives an incredibly important process called recombination.
What is genetic?
Comes from Greek roots meaning ‘race’ or ‘kind’ and therefore the study of how and why offspring become the same ‘kind’ of organisms as their parents.
What happens during meiosis 1?
- appart from the prophase of meiosis 1 (M1) the phases are basically the same as mitosis.
What happens during prophase of meiosis 1?
The chromosomes condense BUT homologous chromosomes also pair up = the big difference. They do so in 5 sub-phases.
During prophase of meiosis 1 what are the 5 sub phases?
A. Leptotene- ‘thin threads’ chromosomes become visible.
B. Zygotene- ‘yoked threads’ homologous chromosomes pair up (synapsis)
C. Pachytene- ‘ thick threads’ crossover occurs.
D. Diplotene- Homologous chromosomes move slightly appart.
E. Diakinesis- Homologous pairs move to the equatorial plate.
What happens after prophase of meiosis 1?
Prometaphase
- nuclear membrane breaks down
What happens in metaphase of meiosis 1?
Homologous pairs are now all lined up on the equatorial plate.
What happens during anaphase of meiosis 1?
Homologous chromosomes seporate, so that each daughter nucleus is now haploid (unlike in mitosis where sister chormatids separate the daughters are diploid).
What happens in telophase of meiosis 1?
Events of prometaphase are reversed and nuclear envelope reforms.
What happens in meiosis II?
M1 produces 2 daughter cells ( the two n, 2C cells)
Both of these cells undergo meiosis II.
Basically just like meiosis the sister chromatids seporate to give a total of 4 gametes.
What happens if two different gene loci are on the same chromosome?
And are filled by two particular alleles then those alleles will be inherited together. This is known as linkage.
When an experiment was carried out into linkage, the results produced a 10:2:2:2 ratio, what was the reason for this?
The result sits half way between mono/di hybrid crosses. (Half between independent and complete linkage)
- they proposed that although the genes were found on the same chromosome one or both of these alleles sometimes crossed over.
What is genetic mapping?
The likes hood of crossover between any two genes gives some idea of how far apart they are.
What was Morgan’s and Sturtevants ‘stroke of genius’ regarding the didstance between 2 genes on a chromosome?
- they suggested that alleles found together on chromosomes were linked but those chromosomes were quite fragile and could break appart. They realised that the further appart any two genes are on a chromosome the more likely they’d be separated by random chromosomal breaks and crossover.
- this meant they could use the frequency of linkage breaking of alleles as an indication of how far those alleles were from one another.
What are crossovers?
Recombination between homologous chromosomes in prophase I.
- they occur when homologous pairs line up (synapsis) during zygotene.
How do crossovers occur?
When they’re sitting close together the double helix that makes up one sister chromatid in one homologous chromosome, can UNWIND AND INVADE the double helix that makes up the sister chormatid in the other homologous chromosome, this forms a holiday junction.
- cutting of the holiday junction results in 1 of 2 things:
> non crossover products
> recombination that leads to crossover products, therefore generate new combinations of alleles.
SEE diagram on page 7
3/9 section Bothwell
Explain how Mendelian genetics gives us a ‘null hypothesis’
Gives us a model we can use to redact how information is passed from parents to offspring; it allows testable predictions to be made about how many offspring will have XYZ phenotype based on a number of assumptions.
- we can then compare those predictions with observations- if the predictions aren’t met, then we need to reconsider our assumptions.
What 3 assumptions are made by the Mendelian model?
I. Genes are on nuclear chromosomes
II. Only one gene locus determines a trait
III. Alleles are either dominant or recessive.
How is number III of Mendelains assumptions flouted in nature?
( alleles are either dominant or recessive)
Alleles can have a wide range of effects; they can be incompletely dominant, co-dominant or they can show varying degrees of expressivity.
How is number II of Mendelains assumptions flouted in nature?
( Only one gene locus determines a trait)
More than one gene can be involved; this is called gene interaction.
