14 - Mitosis, Meiosis and Errors

CELS191 lecture 14, 15 and 16 - includes description of all stages of the cycle, terminology around chromosomes/genes, differences between mitosis and meiosis, and errors that can occur

Karyotype

An ordered, visual representation of the chromosomes in a cell stopped at metaphase.

Cell cycle

interphase & mitotic phase

Interphase

G1 (growth), S (DNA duplication), G2 (preparation for cell divison)

Mitotic (M) phase

telophase and cytokinesis

G2 phase

two centrosomes form
nuclear envelope intact
nucleolus visible
DNA not yet condensed

Mitosis phases

prophase, prometaphase, metaphase, anaphase, telophase & cytokinesis

Prophase

nucleoli disappear
duplicated DNA condenses
mitotic spindle forms
microtubules lengthen
centrosomes move to opposite poles

Kinetochore microtubules

microtubules that attach to kinetochores

Nonkinetochore microtubules

lengthen the cell by interacting with those from the opposite pole of the spindle

Prometaphase

nuclear envelope breaks down
kinetochores form

Mitotic spindle

structure composed microtubules that separates duplicated chromosomes

Kinetochore

a protein structure that forms on the centromere of a chromosome. Point of attachment for microtubules.

Metaphase

centrosomes at opposite poles
kinetochore microtubules attach
chromosomes line up at metaphase plate (homologous pairs do not interact)

Anaphase

ends when two poles of cell contain identical and complete collections of chromosomes
sister chromatids disjoin:
kinetochore microtubules shorten, moving daughter chromosomes to opposite poles
nonkinetochore microtubules lengthen, elongating cell

Telophase

chromosomes become less condensed
mitotic spindle breaks down
two daughter nuclei form
nucleoli reappear
mitosis (the division of one nucleus into two genetically identical nuclei) is complete

cytokinesis

cytoplasm divides
animals = formation of cleavage furrow which pinches off to give two daughter cells
plants = formation of a cell plate

each daugher cell has…

one copy of each duplicated chromosome
(sister chromatids separate during anaphase and each chromatid becomes a daughter chromosome)

where do homologous pairs disjoin?

chiasmus

where do sister chromatids disjoin?

centromere

Meiosis

production of 4 genetically distinct haploid daughter gametes used for reproduction from diploid parent cell.

Mitosis

cell division that produces two daughter cells identical to each other and to parent cell.

Mitosis function

to produce somatic cells used for growth and repair

Prophase I

homolgous chromosomes undergo synapsis and crossing over

Metaphase I

homologous pairs/chiasmata align at metaphase plate

Anaphase I

homologous pairs are separated

Telophase I

duplicated chromsomes (pairs of sister chromatids) reach the poles at opposite ends of the cell

Cytokinesis I

two genetically distinct haploid cells (only one of each pair of the homologous chromosomes)

Prophase II

spindles form, kinetochore microtubules attach

Metaphase II

duplicated chromosomes/pairs of sister chromatids align at metaphase plate

Anaphase II

sister chromatids disjoin

Telophase II & Cytokinesis II

four genetically distinct haploid cells, each with a single set of of unreplicated chromosomes

chiasmata present in…

meiosis I

disjunction occurs between…

chromatids in mitosis and meiosis II, homologous chismata in meiosis I

chromosomal number of resultant cells and genetic make up of resultant cells in mitosis, meiosis I and meiosis II

mitosis (2n → 2n), genetically identical
meiosis I (2n → n), genetically distinct
meiosis II (n→n), genetically distinct

genetic diversity occurs through

independent assortment (meiosis II), crossing over (meiosis I), random fertilization

alignment in mitosis, meiosis I and meiosis II

chromosomes align independently in mitosis and meiosis II), homologous chromosomes synapse (meiosis I)

genetic diversity allows

variable environments, changing environments, reduced sibling competition

number of possible gametes (with different chromosomal combinations)

2^n

Meiotic nondisjunction

failure of chromosomes to separate properly during meiosis

effect of nondisjunction during meiosis I

all 4 gametes affected (n+1, n+1, n-1, n-1), sister chromatids not identical at centromeres.

effect of nondisjunction during meiosis II

only 2 gametes affected (n+1, n-1, n, n)
sister chromatids identical at centromeres

Aneuploidy

zygote has a missing or extra copy of a chromosome

how does aneuploidy arise

gamete with abnormal number of chromosomes fuses with a normal gamete.

monosomy

n-1 gamete fuses with n gamete. zygote is 2n-1, missing one chromosome

trisomy

n+1 gamete fuses with n gamete. zygote is 2n +1, one chromosome is present in triplicate (3 copies)

down syndrome

trisomy 21 - 3 copies of chromosome 21

Klinefelter syndrome

XXY - two copies of X chromosome

Turner syndrome

XO — only one sex chromosome (X)

Polyploidy

possession of two or more complete chromosome sets

polyploidy caused by…

nondisjunction of all chromosomes in one gamete, or the failure of a diploid zygote to divide after replicating its chromosome in the G2 phase.

autopolyploid

self fertilization of a zygote with two incorrect gametes

Deletion

chromosome breaks in one or more places and a segment (deletion) is lost — sister chromatid forms deletion loop as nothing to pair with.

Duplication

when a segment (duplication) is present more than once — can arise when a broken fragment of one chromosome reattaches as an extra segment to a sister or non-sister chromatid

Inversion

segment turned 180 degrees — requires breaks at two points and joining of segments to opposite breakage points — ABCDEF → AEDCBF

Translocations

segment of a chromosome attaches to a non-homologous chromosome — e.g. translocation 14/21