L12 - It's all about sex Flashcards
What is the cellular basis of sex?
Mitosis, before prophase 1 of meiosis 1, DNA replication takes place in the same manner as during mitotic DNA synthesis. Meotic cell division consists of two cell divisions: meiosis 1 and meiosis 2. During meiosis 1, homologous chromosomes separate. During meiosis 2, sister chromatids separate.
What occurs in Meiosis 1: Prophase 1?
Chromosomes first become visible as thin threads. DNA replication is already complete. Homologous chromosomes continue to condense and undergo synapsis (gene-for-gene pairing). When synapsis is complete, each pair of homologous chromosomes forms a bivalent. Each chromosome consists of two sister chromatids.
What occurs in Meiosis 1: Prophase 1: Crossing over?
Through the process of crossing over, meiosis allows homologous chromosomes of maternal and paternal origin to undergo an exchange of DNA segments. This process is random, resulting in completely unique chromosomes when meiosis is complete. No nucleotides are (usually) gained or lost in this process. Crossing over also helps hold together the bivalents for metaphase 1.
What occurs in the later stages of Meiosis 1: Prophase 1?
Chromosomes continue to shorten and thicken and the chiasmata between non-sister chromatids become apparent. The nuclear envelope begins to breakdown.
What has occurs by the end of Meiosis 1: Prophase 1?
The chromosomes are fully condensed and have formed chiasmata. The nuclear envelope has begun to disappear. The meiotic spindle is forming.
What occurs in Prometaphase 1: Metaphase 1?
Prometaphase 1, the nuclear envelope breaks down and the meiotic spindles attach to kinetochores on chromosomes. During metaphase 1, the meiotic spindle is complete and the bivalents move to the center plane of the nucleus so that they come to lie on an imaginary plane cutting transversely across the spindle. Each bivalent lines up so that its two centromeres lie on opposite sides of this plane, pointing toward opposite poles of the cell. Importantly, the orientation of these bivalents is random with respect to each other.
What occurs in Anaphase 1: Telophase 1?
Anaphase 1 - Homologous chromosomes separate, but sister chromatids do not separate (centromeres do not split). Telophase 1 and cytokinesis - Daughter cells are ready to move into prophase 2. In telophase 1, the chromosomes may uncoil slightly and a nuclear envelope can briefly reappear. This process is referred to as reductional division because it reduces the number of chromosomes in daughter cells by half.
What occurs in Meiosis 2?
No DNA synthesis between meiosis 1 and 2. Results in haploid cells. Critically, meiosis leads to genetically unique daughter cells.
What occurs in Meiosis: cytoplasmic division?
Females: most cytoplasm is retained in one daughter cell - this will develop into the egg cell (oocyte). Males: cytoplasm divides equally. All daughter cells become sperm. During this process, most cytoplasm is lost, and a ‘tail’ flagellum develops. During fertilisation, a haploid egg fuses with a haploid sperm, producing a diploid single cell zygote.
What is the evolution of Meiosis?
The similarity between meiosis 2 and mitosis suggests that meiosis likely evolved from mitosis. Mitosis occurs in all eukaryotes and was certainly present in the common ancestor all living eukaryotes. Meiosis is present in most, but not all, eukaryotes. Because the steps of meiosis are the same in all eukaryotes, meiosis is thought to have evolved in the common ancestor of all eukaryotes and has been subsequently lost in some groups.
What can happen to chromosomes?
They can get misplaced, nondisjunction results in missing and extra chromosomes. For autosomes (non-sex chromosomes), this usually results in death of the foetus, but some don’t e.g. trisomy of chromosome 21 causes down syndrome. Extra or missing sex chromosomes are relatively common. In mammals, females usually have XX and males usually have XY. Presence of Y chromosome (usually) leads to male development. XXX and XYY : often symptomless and go undetected. X and XXY - more problematic, causing infertility and can cause cognitive issues. Sex is not always a simple binary trait, either in the chromosomes or phenotypically: influence on gender perceptions.
Why reproduce sexually?
Many organisms can reproduce asexually. This is the ancestral state and sexual reproduction evolved from it. Asexual reproduction doesn’t require multiple individuals or individuals of different sexes.
What are the key features of sexual reproduction?
Sex is not required for reproduction. Recombination creates the variety of genetic combinations within each chromosomes. Meiosis creates genetically unique gametes. Gametes from different parents are combined during fertilisation - increases diversity.
What are the disadvantages to sexual reproduction?
Destroys adaptative combinations of alleles e.g. if heterozygotes are fittest - sickle cell haemoglobin. Requires two individuals (two hermaphrodites or a male and female). Leads to costs of seeking, choosing and competing for males. Production of males (that can’t bear offspring) reduces reproductive success.
What are the disadvantages to sexual reproduction? pt2
Evolution of meiotic drivers: genetic variants that selfishly manipulate the production of gametes to increase their own rate of transmission…. often to the detriment of the rest of the genome and the individual that carries them. Cause intra-genomic conflict. Sex is an evolutionary paradox.
What are the advantages to sexual reproduction?
Can facilitate adaptation to new environments by combining beneficial mutations (under directional selection). May be particularly important for adaptation to varying environments. Can separate beneficial from harmful mutations.
How do prokaryotes benefit from gene exchange in sexual reproduction?
Occurs through conjugation, transformation and transduction. Conjugation: here, a donor with a plasmid to be copied to another bacteria. This is sometimes integrated into the genome.
Why reproduce sexually - transformation?
Bacteria takes DNA from the surrounding environment. E.g. used in cell biology e.g. to express a gene or to replicate DNA with an unknown sequence
Why reproduce sexually - transduction?
Bacterial viruses (bacteriophages) carry DNA from one bacterium to another.
What is the relationship status?
It’s complicated. Many species reproduce both sexually and asexually e.g. many plants & fungi reproduce via asexual budding (clonal) and seed production. Many species are hermaphrodites, producing gametes of both sexes (both can therefore bear offspring). Some species reproduce sexually but don’t have male and female gametes (e.g. yeast).
What is the relationship status? pt2
Some species evolve asexual lineages from lineages e.g. dandelions (Taraxacum officinale). Around mediterranean - diploid sexual. Northern europe - triploid asexual (produce seeds with clones). Asexual lineages tend to be relatively short-lived, supporting the idea that sexual reproduction may reduce extinction probability.
Relationship status - Parthenogenesis?
Females produce eggs that are not fertilised by males. They undergo mitosis and develop into new individuals. Adults can be haploid or diploid (if the DNA content doubles), depending on the organism.
How did the sexes evolve?
The evolution of sperm and eggs. Isogamy: same-sized gametes - many unicellular organisms e.g. yeast. Larger gametes are more likely to survive, produced in smaller numbers, promotes selection for smaller gametes to parasitize on their resources, produced in large numbers. Divergent selection in gamete size. Anisogamy: different sized gametes - virtually all multicellular plants and animals.
What are the mechanisms by which biological sex is determined - 1?
1) Chromosomes, Females - XX and Males - XY (All mammals, some plants, some insects, some reptiles). Heterogametic sex - two different sex chromosomes. Homogametic sex - two copies of the same sex chromosome. Exceptions exist e.g. platypus has 10 sex chromosomes: females have tens Xs, males have XYXYXYXYXY.
