Molecules, Cells and Variation - 1.5 +1.6 Flashcards
Mitosis
- Type of nuclear division that produces genetically identical daughter cells.
- During, DNA replicates in the parent cell, and is distributed so each daughter cell is identical.
- Only source of variation in the cells is via mutations
Why is mitosis important?
For growth and repair of tissues, and in asexual reproduction.
Somatic cell
Any cell in an organism other than the reproductive cells. During mitosis, the chromosome homologous pairs line up separately from each other.
Stages of mitosis
Interphase Prophase Metaphase Anaphase Telophase
Interphase
Represents the non-dividing cell and includes the stages G1, S and G2. Cell is carrying out its normal cellular functions. Towards the end, the cell prepares for cell division by carrying out a number of processes:
- DNA content is doubled via DNA replication.
- Other cell organelles replicate
- RNA and protein synthesis also occurs.
- ATP content increases, as cell division is active
Prophase
1) Chromatin organises into chromosomes. Each chromosome now consists of two genetically identical sister chromatids,
2) Each chromosome condenses by spiralisation.
3) Centrioles move to opposite poles of the cell. Nucleoli and nuclear membrane break down.
Why does condensation of chromosomes occur during prophase?
Prevents tangling during cell movements.
Metaphase
1) Centrioles at the poles form a spindle across the cell.
2) Chromosomes move to equator of cell and attaches to spindle via its centromere, with its sister chromatids orientated towards opposite poles of the cell.
3) There is no association between members of each homologous pair on the spindle
What do spindle fibers consist of?
Microtubules
Not present in plant cells
Anaphase
1) Centromere splits and sister chromatids separate.
2) Sister chromatids are pulled to opposite poles of the cell by contraction of the spindle microtubules.
3) Numerous mitochondria supply energy for this
Telophase
1) Nuclear membranes reform around chromatin and organelles.
2) Sister chromatids represent chromosomes of each new cell, identical in genetic content to each other and to the parent cell.
3) Two distinct cells form as cytoplasmic cleavage occurs and a cell membrane forms between the cells
Cytokinesis
- Last stage of the cell cycle before entering G1 this is cell division to produce two daughter cells
- Organelles move towards the poles of the cell.
- Cell membrane constricts at the equator of the cell
What occurs in the final stages of plant cell mitosis?
A cell plate forms where new cell wall material is organised to form a new primary cell wall that eventually fuses with the old cell walls separating the two cells.
Importance of meiosis
- Ensures haploid gametes are produced from a diploid cell. Diploid number restores during fertilisation.
- Ensures constant number of chromosomes at each generation.
- Produces genetic variation in gametes.
Process of meiosis
- A cell divides twice, DNA replication occurs only once.
- Number of chromosomes is reduced from 2n to n.
- A cell produces 4 cells which are genetically varied.
- Cells produced usually function as gametes.
What processes in meiosis produce genetically different daughter cells?
- Independent segregation of homologous chromosomes
- Genetic recombination by crossing over
Genetic recombination
Exchange of genetic material either between multiple chromosomes or between different regions of the same chromosome
What occurs during the first meiotic division?
Homologous chromosomes pair, then separate so one member from each pair enters the gamete. This is independent from the separation of another pair. So, the chromosomes randomly associate in a gamete.
Crossing over
Only occurs during prophase of the first meiotic division. The two members of each homologous pair form a bivalent. Chromatids then intertwine and genetic recombination occurs.
Bivalent
During meiosis, when 2 members of each homologous pair lie side by side.
Gametes
- Most species have different male + female gametes.
- Female gametes are usually larger w/ more cytoplasm and food reserves than male gametes.
- Male gametes are usually produced in considerably larger numbers than female gametes.
- Male gametes are mobile, often swimming to the female gamete using a ‘tail’.
- Human sperm contain many mitochondria to provide the energy required for swimming to the ovum.
Taxonomic classification by Phylogenetic groups
Groups ordered according to their evolutionary history. Organisms are grouped together into progressively larger groups, creating a hierarchy. There is no overlap between groups.
Species
Organisms which can interbreed to produce fertile offspring.
Genus
Species which share similar features
Characteristics used to define a phylogenetic group are called homologous features. Give an example of these features.
- Similar biological structures.
- Details of embryological development.
- Shared biochemistry
- DNA sequences
Kingdom - Animalia
- Includes all the 5 classes of vertebrates and invertebrates.
- Animals are eukaryotic and multicellular.
- Their cells have no walls.
- Are heterotrophs
- Have muscles and nervous systems
5 classes of vertebrates
Mammals Birds Reptiles Amphibians Fish
Kingdom - plantae
- Includes flowering plants, conifers and ferns along w/ simpler plants like mosses and liverworts.
- Eukaryotic and multicellular.
- Cells have walls made from cellulose. Have a vacuole.
- Most plants have leaves, stems and roots.
- Most plants possess chlorophyll and are autotrophs.
- A few are modified to a parasitic way of life
Kingdom - fungi
- Includes filamentous fungi and yeasts.
- Single celled fungi are yeasts.
- Eukaryotic, but multicellular fungi don’t have separate cells. Nuclei are dotted around in tissue.
- Walls of fungal cells contain chitin.
- Feed by extracellular digestion, so are hetertrophs.
- Most fare saprophytes. Some are parasitic causing damage to their host
Saprophyte
Lives on dead/dying matter.
Kingdom - Protoctista
- Includes single cells protozoans, plasmodium, algae and sponges.
- Members of this kingdom have very little in common. Due to their evolutionary relationships.
- Eukaryotic, but show a diversity of cell types.
- Feed in a variety of ways
Plasmodium
Malaria parasite
Protozoa
Single celled eukaryotic organisms, e.g. amoeba.
Kingdom - Prokaryotea
- Includes bacteria and single celled organisms.
- Cells of prokaryotic organisms are smaller than eukaryotes.
- Most feed by extracellular digestion, so are heterotrophs.
- Most are saprophytes. Some are parasitic.
- Cyanobacteria are photosynthetic.
- Prokaryotes reproduce mainly by binary fission, but can reproduce sexually (e.g. conjugation)
Continuous variation
- Alleles are usually polygenic.
- Polygenic traits are not discrete, but continuous and quantitative
- Frequencies of these form a normal distribution
- Environmental factors often majorly impact phenotype
Polygenic
Allele controlled by more than one gene.
Examples of polygenic alleles
Height, Skin Colour, Eye Colour, Many forms of behaviour, Intelligence
Discontinuous variation
- Characteristics that show separate/discrete categories or classes. e.g. blood group
- Normally shown on a bar chart or pie graph.
- There are no intermediate types.
- Qualitative
- Strong genetic factor; environment having no effect.
- Normally coded for by one or two genes
Process of natural selection
- Selection pressure in environment changes
- Variation of phenotypes will be present due to random mutations, and other factors.
- Organisms possessing advantageous alleles or genotypes may be more likely to survive.
- Surviving organisms reproduce and pass on their alleles to offspring.
- Increase in allele frequency for beneficial trait
How may another species form?
Reproductive isolation, if enough genetic changes occur to a group of organisms.
Allopatric speciation
Occurs when a population is separated geographically. The different populations may be exposed to different selection pressures. These may be abiotic or biotic. Different alleles are selected in each environment, leading to variation and development new species.
Abiotic factors
Non-living factors
Biotic factors
Living factors
Sympatric speciation
Occurs when a population forms a new species in the same area as the parent. No geographical isolation occurs. May result from a change in habitat or a partitioning of resources that are essential and may limit survival, or for animals a host preference may change. In plants, changes in ploidy may cause instant speciation as gametes are incompatible.
Ploidy
Chromosome number
Which processes result in genetic variation and how?
Crossing over, independent assortment of chromosomes and random fusion of gametes all result in new allele combinations.
Mutations produce new alleles.
