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Mitosis, meiosis, other stuff


Used for sexual reprod­uction, combines genetic material from two parents to produce geneti­cally unique offspring.
Halves the number of chromo­somes in gamete, occurs in germline cells.
2 sets of division - Meiosis I and Meiosis II
Prophase I
Chromo­somes condense
Homologs pair up
Crossing over occurs
Metaphase I
Homologous pairs line up along metaphase plate
Anaphase I
Homologous pairs are separated
Sister chromatids stay together
Telophase and cytoki­nesis follow same steps as Mitosis
Genetic variation in meiosis
Crossing over
Occurs in Prophase I
Homologous pairs come together
Internal chromatids cross at chiasma
Genetic info is swapped
Indepe­ndent assortment
Occurs in Metaphase I
Homologous pairs line up randomly
Gametes do not receive just paternal or maternal information
More than 8 million combin­ations
There is not further duplic­ation of DNA between meiosis I and meiosis II, meiosis II is the same as mitosis except 4 daughter cells are produced.


Mitosis is the process used for growth and repair of cells as well as reprod­uction of unicel­lular organisms.
It comes after interphase and before cytoki­nesis, order can be remembered using Isaac please make another two cells.
Replicated DNA condenses into chromo­somes wrapped around histones
Centrioles produce spindle fibres and migrate to poles
Nuclear membrane breaks down
Nucleolus disappears
Spindle fibres attach to chromo­somes at centromere
Spindle fibres line the chromosome up on the metaphase plate
Spindle fibres contract
Sister chromatids are pulled to the poles
Chromo­somes reach the poles
New nuclear membrane forms
Spindle fibres break down
Chromosomes condense


Smallest unit of life. Contains organe­lles.
If if metabolic rate is greater than material exchange rate then the cell will die.
No nucleus
No membrane bound organelles
1 circular chromosome
Commonly contains cytoplasm, nucleoid, plasmids, ribosomes, cell wall, slime capsule, flagella and pili
All multic­ellular organisms
Membrane bound organelles.
Shares plasma membrane, cytoplasm, DNA and ribosome features with prokar­yotes.

Mechanisms of change

Change in allele frequency in an gene pool
Random change in DNA sequence
Sexual reprod­uction
New gene combin­ations and alter allele frequency
Gene flow
Immigr­ation or emigration of alleles in a gene pool
Genetic drift
Random event changes compos­ition, larger effects in smaller populations
Bottleneck effect- dramatic decrease in popula­tion, unique alleles are lost
Founder effect- small portion of population move, smaller gene pool
Natural selection
Selection pressures leave fittest phenotypes
Mutation alone does not have a large effect when combined meaningful changes can be made.

Evolution types

Evolution type:
New species result from the same ancestral species, allopatric speciation results in homologous struct­ures.
Opposite of divergent, similar selection pressures cause similar adapta­tions. Result in analogous features
Evidence of evolution can include a variety of different studies about an animal
Fossil record
Order of species in time scale
Study of historical distri­bution of species past and present. Geogra­phi­cally closer together means it is more likely they are similar.
Analysis of features to see if they are homologous
Studying the simila­rities between the base pairs. DNA hybrid­isation can be used.

Cell cycle

Process of growth and division to form new cells
3 stages - interp­hase, meiosi­s/m­itosis, cytoki­nesis
G1- cell growth; cytosol, proteins and organelles
G0- arrest of cell cycle, dying or damages cells
Synthesis- replic­ation of DNA
G2- contin­uation of growth and prepar­ation
Mitosis or meiosis see respective sections
Differs from animal to plant
Divison of cytoplasm and organelles before splitting of cells
Animals have cleavage furrow pinching the cells apart
Plants have a cell plate that turns into the cell wall

Simple inheri­tance

Complete dominance
Dominant allele masks the recessive allele
Recessive allele only has an effect when homozygous
Dominant is capital letter, recessive lowercase.
Incomplete dominance
Neither allele is wholly dominant
Heterozygous individual will show a blending
Writing alleles- common base used supers­cript capitals used for allele.
Both alleles are dominant only one expressed in each cell
Inheri­tance patterns:
Autosomal dominant
Find two parents with trait having child without
Autosomal recessive
Check for two parents without having a child with
X-linked dominant
Occurs in every generation of family, all daughters have the trait when father has and mother doesn't.
X-linked recessive
Does not occur in every generation of family, do all females with trait have father with.
Occurs in every generation of family, only in males, passing from father to son.
Some genetic disorders include Down's syndrome which is trisomy for chromosome 21, Turner - monosomy for X without Y. Klinef­elter two X chromo­somes and a Y

Transport across the membrane

The membrane is a phosph­olipid bilayer enclosing the cell. Made of phosph­oli­pids, proteins and choles­terol.
The phosph­olipid bilayer has outwards facing negatively hydrop­hilic phosphate heads and inwards facing lipid tails that are hydrop­hobic.
Types of transport:
Simple diffusion
Down the concen­tration gradient, occurs do to the random movement overtime, passive.
Can transport gases and small lipophilic molecules
Facili­tated diffusion
Down the concen­tration gradient through transport proteins, passive.
Channels transport small charged ions
Carrier proteins transport small uncharged molecules like glucose
Net movement of water across a semipe­rmeable membrane. Water moves from low solute concen­tration to high. Membrane must be permeable to water but not solute. Dilates solution. Hypo to hyper
Active transport
Requires ATP uses carrier proteins as pumps. Against the gradient. Sodium potassium pump for muscle and nerves.
Bulk transport
Substances to large to pass through proteins. ATP used to form vesicles which move between membrane and golgi.


The evolution of two or more species from a single species to the point the viable offspring cannot be produced.
Isolating mechanisms prevent gene flow
Ecolog­ical- different areas
Temporal- breeding season differs
Behavioural- different mating behaviours
Mechanical- physical charac­ter­istics are incompatible
Gamete isolation- female reprod­uctive tract is fatal to sperm
Zygote mortality- fertil­isation occurs but zygote doesn't develop
Inviability of zygote- develops to embryo but not further
Sterile- offspring cannot reproduce
Types of specia­tion:
Occurs due to a geogra­phical barrier and assumes no gene flow between popula­tions
Same geographic areas habitat prefer­ences differ. Gene flow can occur.

DNA stucture

Double stranded double helix shape composed of nucleo­tides. Strands run from 5' to 3' and are antipa­rallel.
3 main components in a nucleo­tide. The nitrog­enous bases hold strands together
Nitrog­enous bases
Adenine, thymine, cytosine, guanine.
Form weak hydrogen bonds with comple­mentary base.
Chargaff's rule states A goes with T and C with G.
A and G are purine with1 ring.
C and T are pyrimi­dines with 2 rings.
A and T, 2 H bonds, C and G, 3 H bonds
Phosphate group
Forms strong backbone with sugar molecules
Sugar group
Deoxyr­ibose sugar bound to phosphate at 5', nitrog­enous base at 1' and previous nucleotide at 3'.
Semi-c­ons­erv­ative replic­ation as one strand from every new helix is from parent strand.

Protein synthesis

DNA to protein - 3 stages; Transc­rip­tion, mRNA proces­sing, Transl­ation
RNA polymerase unwinds dsDNA
Adds RNA nucleo­tides comple­mentary to template strand in 5' to 3' direction.
Stops when polymerase reaches stop.
dsDNA reanneals and primary RNA peals off
mRNA processing
Primary RNA transcript transf­ormed into a form that can exit nucleus
Leaves through a pore intera­cting with ribosome either in cytoplasm or on RER
Small ribosomal subunit bonds to mRNA at 5'
tRNA bonds to mRNA, the anticodon is comple­mentary to codon.
Large ribosomal unit attaches
Next tRNA molecule enters the ribsosome
Complementary amino acids form polype­ptide chain at opposite end
Translation stops when a STOP is reached.
Proteins are large biological macrom­ole­cules, made up of amino acids, 3D shaped vital for correct functi­oning.


Differs from DNA as it has a ribose sugar rather than deoxyr­ibose, and uracil instead of thymine. It is also single stranded
Types of RNA:
DNA is copied to RNA so it can be translated
Clover leaf shaped adapter molecule carrying amino acid
Important part of ribosome


Changes to DNA base sequence. Can be induced from exposure to mutagens or sponta­neous.
Point mutations can be substi­tution insertion or deletion
Silent mutations
Have no effect, base is changed but codon does not
Can be minor- only one codon is altered, Major - frameshift where every acid past point is altered. Sickle cell anaemia
Premature stop is created can result in cystic fibrosis.
Mutation can only be inherited if it occurs in a germline cell and is passed on.

Natural selection

Mechanism by which evolution occurs
Removes less fit genes from gene pool, increasing allele frequency of fitter phenot­ypes.
Better phenotype means increased chance for reprod­uction thus higher allele frequency.
Can result in co-evo­lution if two species provide the selection pressure for
each other.


Trace fossils are not the body but can be excrements or similar. Body fossils are bones or parts of the body preserved.
The process of fossil­isation starts with organism death, the soft tissue decays and buried under sediment. The sediment solidifies and is exposed.
The process requires hard parts, quick burial, low oxygen enviro­nments.
Types of fossils:
Organic matter is turned into minerals
Organism completely decomposes and leaves and empty space
Cast fossil
An empty space filled with rocks
Dating of fossils:
Relative dating
Looks at the rock layer the fossil was found in and surrou­nding fossils with known age ranges
Absolute dating
Uses radioi­sotopes which decay over time and measuring the level of different elements and the isotopes.
Transi­tional fossils are highly sought after as they can show traits from both the ancestral and modern forms of the organism.


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