Week 7 - Cytoskeleton. Cell division of eukaryotic cells - mitosis and meiosis Flashcards

Question

Telophase

Answer 1

The chromosomes arrive at the cell poles. ◼ The mitotic spindle disassembles. ◼ The vesicles that contain fragments of the original nuclear membrane assemble around the two sets of chromosomes. ◼ Phosphatases dephosphorylate the lamins at each end of the cell. This dephosphorylation results in the formation of a new nuclear membrane around each group of chromosomes.

Answer 2

◼ The physical process that finally splits the parent cell into two identical daughter cells. ◼ The signal for the start of cytokinesis is dephosphorylation of proteins, which are targets of Cdks.

Answer 3

The cell membrane pinches in at the cell equator, forming a cleft called the cleavage furrow. The position of the furrow depends on the position of the astral and interpolar microtubules during anaphase. The action of a contractile ring of overlapping actin and myosin filaments forms cleavage furrow.

Answer 4

We have cells (e.g. epithelial cells) which are joined together with the cell junctions. In cytokinesis → not complete separation but leave cell junction open between 2 newly divided daughter cells

Answer 5

◼ Growth of the organism. ◼ Repair. ◼ Replacement. ◼ Asexual reproduction (in plants vegetative multiplication).

Answer 6

◼ Meiosis (in Greek: «reduction»). ◼ Process where diploid cells give rise to haploid gametes. Accomplished by combining two rounds of division with only one round of DNA replication. ◼ A type of cell division that is unique to germ cells. ◼ Within the gonads, the germ cells proliferate by mitosis until they receive the right signals to enter meiosis. ◼ Meiosis involves two cell divisions - Meiosis I and Meiosis II.

Answer 7

◼ Meiosis I consist of four stages: ➢Prophase I; ➢Metaphase I; ➢Anaphase I; ➢Telophase I.

Answer 8

➢**Leptotene** (Greek for "thin threads") - chromosomes begin to **condense**. ➢**Zygotene** (Greek for "paired threads") - chromosomes become **closely paired**. Homologous chromosomes begin to align along their entire length - pairing or forming **synapsis**. Chromosomes are held together by a **synaptonemal** complex. ➢**Pachytene** (Greek for "thick threads") - synapsis completed, and each pair of homologues appears as a bivalent. Crossing over occurs. ➢**Diplotene** (Greek for "two threads") - after recombination, the synaptonemal complex begins to break down, homologous chromosomes begin to separate but remain attached by the chiasmata. ➢**Diakinesis** (Greek for "moving through") - chromosomes condense and separate until terminal chiasmata only connect the two chromosomes.

Answer 9

◼ Crossing over is the **exchange of genetic material** between **non-sister chromatids of homologous chromosomes** during **meiosis**. ◼ Result: new allelic combinations in the daughter cells. Crossing over (Genetic recombination) generates genetic variability of the chromosomes Crossing over is ensured by **homologous recombination**: process in which DNA molecules are broken and the fragments are re-joined in new combinations.

Answer 10

◼ Involves DSB (double strand breakes) followed by homologous reparation (re-joining the strands based on sequence of homologous chromosome). Many proteins are involved - recombination complex. ◼ The formation of DSBs is catalysed by highly conserved proteins with topoisomerase activity. ◼ In the absence of recombination, chromosomes often fail to align properly for the first meiotic division – as the result there is high incidence of chromosomal loss, called nondisjunction. ◼ A failure in homologous recombination is often reflected in poor fertility.

Answer 11

◼ Nuclear membrane disappears. ◼ A spindle forms. ◼ The paired chromosomes align on the equatorial plane with their centromeres oriented toward different poles

Answer 12

◼ The two members of each bivalent move apart, and their respective centromeres with the attached sister chromatids are drawn to opposite poles of the cell. ◼ A process termed disjunction. ◼ Each maternal and paternal chromosome in a homologous pair segregates randomly into a daughter cell in meiosis I. ◼ The original paternal and maternal chromosome sets are sorted into random combinations. *The possible number of combinations of the 23 chromosome pairs that can be present in the gametes is 2²³ (more than 8 million).*

Answer 13

◼ Telophase I - the two haploid sets of chromosomes have normally grouped at opposite poles of the cell. ◼ Cytokinesis - the cell divides into two haploid daughter cells and enters meiotic interphase. ◼ In contrast to mitosis, interphase is brief - interkinesis, and meiosis II begins.

Answer 14

◼ The second meiotic division is similar to an ordinary mitosis except that the chromosome number of the cell entering meiosis II is haploid (n). There is no DNA replication before the next division.

Answer 15

◼ The most common form of **polyploidy** in human is triploidy **(3n)**, caused by the fertilization of an egg by two sperm (dispermy). ◼ **Meiotic failure** - a diploid sperm or egg cell is produced, can also produce a triploid zygote. ◼ The most common cause of **aneuploidy** **(2n +1/-1)** is nondisjunction, the failure of chromosomes to disjoin normally during meiosis.

Answer 16

Down syndrome ➢A particular combination of phenotypic features that includes mental retardation and characteristic face features. Trisomy 21 Most cases are caused by **nondisjunction**, and most of the remainder are caused by chromosome **translocations**. Extra chromosome often given by mother Mosaicism?

Answer 17

◼ **Somatic mosaicism** - refers to the occurrence of two genetically distinct populations of cells within an individual, derived from a postzygotic mutation. ◼ **Germ line mosaicism** - refer to the presence of genetically distinct groups of cells within germ line tissues.

Answer 18

◼ Mitochondrial populations: often heterogeneous: higher mutation rate for the mt genome. ◼ When a cell divides, its mitochondria are distributed to the two daughter cells. However, mitochondrial segregation occurs randomly and is not nearly as organized as the highly regulated process of mitotic chromosome segregation. ◼ Cells will receive similar, but not identical, mitochondrial DNA populations.

Answer 19

◼ Tumour cells typically are characterized by widespread alteration in DNA, chromosome breaks, and aneuploidy. This condition is termed **genomic instability.** ◼ **Aneuploidy** can contribute to tumorigenesis by - creating extra copies of oncogenes or by - deleting tumour suppressor genes

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Week 7 - Cytoskeleton. Cell division of eukaryotic cells - mitosis and meiosis Flashcards

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