prophase
early prophase and late prophase
the beginning of mitosis when the chromatin begins to form chromosomes
- centrioles split up and move to opposite poles - condensation of chromatin into chromosomes - nuclear membrane breaks down
metaphase (and prometaphase)
Alignment (ie meeting of the chrms)
- spindle fibres form - chromosomes align on metaphase plate - spindle fibres attach to centromeres
Prometa: discrete chrms visible; the nuclear envelope fragments
anaphase
the shortest phase of mitosis
- spindle fibres contract - sister chromatids separate to become daughter chromosomes - daughter chromosomes are pulled to opposite poles.
telophase
the chromosomes start to unravel
- chromosomes decondense back into chromatin - new nuclear membranes form
cytokinesis
after mitosis is complete (ie when 2 identical nuclei have formed) 2 daughters cells separate
euploidy and aneuploidy
having the correct number of chromosomes;
having the incorrect number
prophase 1 (very important); prophase 2
PROPHASE I
LEPTOTENE - condensation commences
ZYGOTENE - further condensation, pairing commences
PACHYTENE - further condensation, end of pairing, crossing over ( = chiasma formation)
DIPLOTENE/DIAKINESIS - further condensation, nuclear membrane breaks down
PROPHASE 2 -
metaphase 1; metaphase 2
METAPHASE I
-bivalents align on metaphase plate ie in pairs of homologous chrms (cf meta in mitosis in which indiv chrms line up on meta plate)
anaphase 1; anaphase 2
ANAPHASE I
- bivalents separate and move toward opposite poles
(disjunction)
telophase 1; telophase 2
TELOPHASE I - chromosomes reach poles and new nuclear membranes form
meiosis (meiosis 1 and 2)
produces gametes; involves 2 separate divisions, btwn which there is no interphase ie they are back to back
DIVISION II - phrophase II - telphase II are all identical to their mitotic counterparts but involve half the number of chromosomes.
Consequences:
(I) Production of haploid gametes
(II) New combinations of paternally and maternally derived chromosomes
(III) Produces genetic recombination via chiasma formation
This brings about a huge range of variation which allows for natural selection
metaphase 1; metaphase 2
METAPHASE I -bivalents align on metaphase plate
recombination (also “crossing over” or chiasma/ta)
changes the combination of genes on the chromosomes; with one crossover four possible gene combinations can be produced; one of the mechanisms of maintaining and propagating change and genetic variation
inheritance
the heritability of traits
allele
a version of a gene; linked with variation
uses of genetics
Disease diagnosis and cure (eg haemophilia, cystic fybrosis, sickle cell anaemia, Rh incompatability)
Crop and stock improvement (wheat, figs, peas, dogs, sheep)
Forensics (identity and paternity testing)
Conservation
Systematics (cf phylogeny)
Making sense of the Evolution module
systematics
concept of the relationships between species;
based on evolutionary history, in a modern sense; molecular systematics based on genetic features are more reliable
chromosome
(literally ‘coloured bodies’ in Latin); a condensed form of proteins and nucleic acids carrying genetic information; visible only during cell division.
the gene
from ‘pangen’, a shortening of Darwin’s name for a factor that conveys traits from parent to offspring, a gene is the smallest particle representing one hereditary characteristic; the fundamental physical and functional unit of heredity (from 1909)
chromatin
made of nucleic acids and proteins, like chromosomes, but looks different; it is spread out in the nucleus, relaxed
(sister) chromatid; daughter chromosome
one of two strands of the chromosome, joined by the centromere;
after they are pulled apart in Anaphase, they become daughter chromosomes
centromere
protein that joins that two sister chromatids
telomere
the ends of the chromatids; their degradation after many cells divisions links with aging and cancer formation
shapes of chromosomes: metacentric, acrocentric, telocentric
meta: chromatids joined roughly in the middle by the centromere
acro: joined nearer one end
telo: joined at the end, ie the telomeres
karyotype
the full array of chromosomes of an organism; the chromosomes can usually be sorted into pairs of similar shapes and sizes
pseudoautosomal regions (Y chrm) - Par1 and Par2
homologous to the X chrm, these allow the Y chrm to “pretend” to be an X during meiosis
SRY gene and gene Eif2s3y
switch genes on the Y chrm; these grow the penis and testes and are involved in the production of sperm respectively. (Women have all the genetic material to be male, except for these switches.)
The presence of the SRY gene determines a human’s phenotypic sex. In mammals, this SRY gene is BROKEN in that it is always on, leading to aggression and other dangerous behaviour
interphase
non-cell-division, comprising 3 parts: - G1 (period of cell growth before DNA is dup; all the materials needed for gen prod are made); S (period when DNA is dup ie when the chrms are dup); G2 (period after DNA dupd; cell prepares for div)
This always looks the same tho…
IPMATC (like “ipmatic”)
also “I play music at the concert”)
mnemonic to remember the cell division cycle/s: interphase, prophase, metaphase, anaphase, telophase and cytokinesis.
To remember the function of each phase, remember:
I is for interlude;
P is for prepare;
M is for meet;
A is for apart;
T is for tear
bivalents, ie bivalent chromosomes (meiosis)
homologous chromosomes which align on the metaphase plate during meiosis 1
incomplete dominance
three distinct phenotypes are evident. ie the heterozygote has a different phenotype from either homozygote
codominance
the phenotype of both alleles are expressed
sex-linked disorders
X-linked recessive conditions will be much more common in males than females;
- Fragile X syndrome
- Duchenne’s muscular dystrophy
- Haemophilia
- Red-Green Colourblindness
Inheritance of recessive X-linked traits:
- occurs entirely or primarily in males - skips generations - passed from carrier mother to son
Hardy-Weinberg Theorem
p^2 + 2pq + q^2 = 1
this gives us the genotypic frequencies: p^2 tells us homozygous dominant; 2pq tells us heterozygous; and q^2 tells us homozygous recessive.
P is the allelic frequency of the dominant allele, and q is the allelic frequency of the recessive allele.
p + q = 1
assumptions behind Hardy-Weinberg theorem
- ) population is large ie >30
- ) stable gene pool, ie no migration or gene flow
- ) no mutation (remember achondroplasia coming about by mutation)
- ) random mating (sexual selection throws this off; this has nothing to do with how using alleles are for survival)
- ) no natural selection
Hardy-Weinberg equilibrium
when the frequencies of alleles in a population remains constant;
this is thrown off by any of the following: evolution, mutation, selective mating, migrating, or extinction
random sampling effects (re factors that affect allelic freq over time)
this are all situations that “break” H-W equil due to small sampling sizes:
- genetic drift (random changes in allelic freq in a pop as a result of random variation in the reproductive success; for populations <30)
- bottleneck (occurs when a pop size is drastically reduced and then recovers; like genetic drift affecting a single generation)
- founder effect (a “bottleneck” in space, allelic freq is limited by population/s becoming isolated)
selection (re: factors that affect allelic freq over time)
- directional selection (when conditions favour indivs exhibiting one extreme of a phenotypic range; occurs when the environ changes OR when members of a pop migrate)
- stabilising selection (heterozygote advantage; acts against both extreme phenotypes and favours intermediate variants; maintains the status quo, eg birth weight of humans)
- disruptive/ diversifying selection (occurs when conditions favour indivs at both extremes of a phenotypic range over indivs with intermed phenos)
