Exam: Cycle 4

Saranya Varakunan@saranya.v02

Genetic recombination

Many types of reproduction
Octopi: reproduction is occasional, preceded by courtship involving intermingling tentacles
Slipper limpet: reproduction is a lifelong group activity
Both genetic alikeness and difference are necessary for evolution
Mutation is the main source of genetic diversity: usually from errors in DNA replication, need more diversity so other mechanisms (e.g. genetic recombination) exist
Genetic recombination: widespread in nature; part of meiosis
Needs two double DNA helices, and a mechanism for bringing the strands close, enzymes to cut/exchange/paste DNA back together
Recombination usually occurs between homologous regions
Enzymes break hydrogen bonds of one double helix and allow bases to reassociate with complementary bases in a homologous chromosome. DNA backbone is also cut, and then rearranged by nuclease enzymes

Recombination

List of mechanisms to generate genomic diversity
Meiosis: independent assortment, homologous recombination (not covered yet)
Transposons, slippage, tautomerization (during repair process), DNA fidelity (Polymerase adds a wrong base and fails to proofread), non-homologous end joining
Radiation exposure: radon, medical, internal, cosmic, terrestrial, consumer products
Ionizing radiation creates ROS (reactive oxygen species) from water that damages DNA
Oxygen is unstable because e- was taken from it so it’s very reactive (e.g. hydrogen peroxide, hydroxyl radical, etc)
Comes from decay of radioactive iodine (t1/2 = 1 week) and cesium (t1/2 = 30 yrs)
Repairing double stranded breaks can create rearrangements (e.g. non-homologous end joining)
Causes deletion, duplication, inversion, translocation (one chromosome to a different one)
Example: translocation of c-myc one gene (involved in cell cycling) from chromosome 8 → 14 results in Burkitt lymphoma (deregulates a cancer gene)
Promoter for this gene is usually turned off, but when translocation occurs, c-myc ends up next to a promoter for antibody production (very active)
De novo mutations are only passed on to the next generation if they affect germ cells
Unequal recombination can generate copy number variations (CNV)
Crossing over normally occurs between alleles of homologous chromosomes during prophase I at the same point on each homolog
Unequal crossing over occurs when chromosomes aren’t aligned properly (one chromatid will have more genes and one will have less)
Copy number variations contributes to organism complexity
We have way more CNVs than simpler organisms
Zygotes bring DNA from two different parents into the same cell to contribute to diversity of population (is an organism’s purpose of life, from a biological perspective)
Homologous chromosomes carry the same genes but different alleles
Variation between humans’ genomes is about 14%
Recombination during meiosis cuts and pastes DNA backbones
Technically, recombination is a mutation bc it’s a double stranded change in the sequence
Meiosis (important):
Ploidy change (2n → n) at the end of meiosis I
Homologous pairs (not centromeres) split up
No ploidy change in meiosis II; equational phase
Homologous recombination is in prophase I
After meiosis II, we get one genome per haploid cell (gamete)
Won’t survive if you get more than one set
Life cycles (diploid, haploid phases; how are gametes produced and what are they called’ n and C values) of animals, plants, and fungi (NEED TO KNOW)
Animal life cycle: animal (2n) produces gametes (n) → fertilize to form animal
Plant life cycle: produce spores (n) which undergo mitosis to produce a gametophyte (n) and male/female gametes (n), which fertilize to make a zygote (2n) and a sporophyte (2n) which undergoes meiosis to produce spores
Some fungi/algae: gametophyte (n) undergoes mitosis to produce male and female gametes (n), which fertilize to form zygote (2n), which undergoes meiosis to form a spore (n), which divides to produce gametophyte

Meiosis

Life cycles of animals, plants, fungi: what process produces gametes (mitosis/meiosis), what are the gametes called
Animals: gametes formed by meiosis
Plants: gametes formed by mitosis, spores are formed by meiosis
Alteration of generations (multicellular organisms exist both in diploid and haploid)
Fern: sporophyte (2n) does photosynthesis; is dominant form for fern. Spores are in sporangium.
Gametophyte have male and female organs (where male and female gametes are made, respectively): hermaphroditic
In flowering plants, female is carpal (includes ovary) and male is stamens (includes anther)
Some fungi/algae: gametes formed by mitosis, zygote divides by meiosis to form spore
Majority of life is as a gametophyte (n)
Diversity generated during meiosis: independent assortment in meiosis I, homologous recombination in prophase I
Increases randomness + possibilities of distribution of alleles
Differences between alleles of same genes: SNPs, InDel mutations, etc
Cytoskeleton: centromeres are part of chromosomes, kinetochores are attached to chromosomes and attach to spindle fibers to pull sister chromatids apart towards spindle pole, microtubules made of a polymer of tubulin proteins
Higher chance of recombination between farther genes bc there’s more possible space for recombination to occur (genes located close to each other are linked)
Aneuploidy: abnormal number of chromosomes; results from a single partitioning error
Occurs more when regulation is bad; spindle fibers don’t attach in right direction
Some chromosomes/sister chromatids end up in wrong cell
Nondisjunction during meiosis I: two final cells are n+1, two are n-1
Nondisjunction during meiosis II: two normal (n) cells, one is n+1, one is n-1
Examples of aneuploidy
Trisomy 21: Down’s syndrome
As age of mother increases, risk of kid having Down syndrome increases (different for men: they produce sperm every day so spindle fibers are fresh)
Trisomy 13: Patau Syndrome
Trisomy 18: Edwards Syndrome
Trisomy 8
Turner syndrome: OX (1 in 5000)
Klinefelter syndrome: XXY (1 in 2000)