Bacterial Division & Evolution
A bacterial cell has one circular chromosome. After replication, this must happen before binary fission can produce two viable daughter cells.
What is the replicated DNA molecules must segregate, and inward growth of the plasma membrane and new cell wall must separate the cell into two?
A diploid cell contains 12 chromosomes before S phase. After DNA replication, it contains this many chromosomes and chromatids.
What is 12 chromosomes and 24 sister chromatids?
- This is because DNA replication doubles DNA content and not chromosome number.
Two homologous chromosomes pair closely during prophase I. This pairing is called what, and this four-chromatid structure is produced by it.
What is synapsis; a tetrad containing four chromatids?
Fertilization restores diploidy rather than produces a cell with twice the normal diploid chromosome number because of this.
What is a haploid egg and a haploid sperm each contribute one chromosome set, restoring 2n in the zygote?
Two homologous chromosomes carry different alleles of the same gene. This feature of homologous chromosomes allows them to be compared at that gene's location.
What is they contain the same genes in the same order, although their alleles may differ?
E. coli can complete a division cycle in 20 minutes, with DNA replication occupying 19 minutes. This reveals what about the timing of replication relative to cell division.
What is DNA replication occupies nearly the entire cycle, leaving only about one minute for the remaining interval?
A cell enters meiosis I with 16 chromosomes, each consisting of two sister chromatids. Immediately after meiosis I, this many chromosomes and chromatids are present in each daughter cell.
What is 8 chromosomes and 16 chromatids?
- This is because homologous chromosomes separate, but sister chromatids remain joined.
A microscope image shows homologous chromosomes aligned at the equator, with sister chromatids still attached. This meiotic stage is visible, and this separates next.
What is metaphase I; homologous chromosomes separate during anaphase I?
A diploid animal cell undergoes one round of DNA replication followed by two meiotic divisions. This many daughter cells are produced as a result, and this is their chromosome-set number.
What is four haploid cells, each with half the parental cell's chromosome number?
Crossing-over occurs between two nonsister chromatids of homologous chromosomes. This exchange generates a new allele combination without creating a new gene.
What is it rearranges existing DNA segments between homologs, producing recombinant chromosomes with new combinations of alleles?
Replication begins at the bacterial chromosome's origin (ori) and proceeds in opposite directions. This happens to the replicated origins as replication continues.
What is the two replicated origins migrate toward opposite cell poles?
A cell has two homologous chromosome pairs. After replication, this many DNA molecules are present, and this many homologous pairs exist.
What is 8 DNA molecules and 2 homologous pairs?
- Each of the four chromosomes contains two chromatids, but replication does not create additional homologous pairs.
During prometaphase I, the maternal and paternal homologs attach to kinetochore microtubules from opposite spindle poles. This happens to the sister chromatids at this stage.
What is they remain attached to one another; the homologous chromosomes, not the sister chromatids, are oriented for separation?
A diploid cell undergoes mitosis instead of meiosis. These two outcomes distinguish its daughter cells from normal meiotic products.
What is mitosis produces two genetically identical daughter cells with the parental chromosome number, rather than four genetically varied haploid cells.
An organism has six homologous chromosome pairs. Ignoring crossing-over, gametes can receive this many different maternal–paternal chromosome combinations through independent assortment.
What is 26 = 64 combinations?
A bacterial cell finishes replicating its chromosome but cannot synthesize a new cell wall. This critical step of binary fission is compromised, and this is why DNA replication alone insufficient.
What is physical cell separation is compromised, and replicated chromosomes must be partitioned into separate daughter cells by completion of division?
A human cell completes meiosis I. This is why each resulting cell is haploid even though every chromosome still has two chromatids.
What is each cell has only one chromosome from each of the 23 homologous pairs, so it has 23 chromosomes rather than 46?
A mutation prevents crossing-over during prophase I but leaves homologous pairing and segregation intact. This source of genetic variability is directly disrupted.
What is genetic recombination through crossing-over?
A cell completes meiosis I but is mistakenly treated as though it has completed a mitotic division. This crucial chromosome-separation event is still missing.
What is separation of sister chromatids during meiosis II; each chromosome still contains two chromatids after meiosis I?
Nondisjunction occurs in meiosis I when both homologous chromosomes move to the same pole. This chromosome-distribution error results immediately from this event.
What is one daughter cell receives both homologs, while the other receives neither; both cells have an abnormal chromosome complement?
The evolution of mitosis is especially advantageous for an ancestral eukaryotic cell with several different chromosomes rather than a single chromosome because of this.
What is mitosis coordinates the precise segregation of multiple chromosome types so that each daughter cell receives a complete chromosome complement?
You count 10 chromosomes in a cell immediately before meiosis II. Assuming normal meiosis and no chromosome loss, this is how many chromosomes each final daughter cell contains and this is why.
What is 5 chromosomes; meiosis II separates sister chromatids but does not restore the homologous chromosome pairs lost in meiosis I?
A cell completes anaphase I normally but cannot form the new spindles during interkinesis. This subsequent division process is directly impaired, and those spindles must accomplish this.
What is meiosis II; the new spindles must attach to sister chromatids and separate them into different daughter cells?
A human zygote has 46 chromosomes. Explain how chromosome number is maintained in its ordinary body cells after fertilization without repeating meiosis.
What is DNA replication followed by mitosis maintains 46 chromosomes per body cell as the zygote develops?
Two offspring have the same parents but are not identical twins. Explain three independent mechanisms that make them unlikely to inherit the same genetic combination.
What is crossing-over reshuffles alleles, independent assortment mixes maternal and paternal chromosomes, and random fertilization combines different egg and sperm genotypes?