The purposes of cell division
growth and development
tissue regeneration/repair
reproduction
asexual reproduction
one organism makes 2 (or more) copies of itself
accomplished through → mitosis and binary fission
sexual reproduction
two different individuals contribute genetic info to make new genetically unique organism(s)
accomplished through → meiosis
Why do cells need to divide eventually? Explain surface-to-volume ratio & genome-to-volume ratio explanation (also diffusion problems as it grows)
surface to volume ratio: as a cell grows in size its contents (volume) can become too much to hold inside it
SA = s^2*6
vol = s^3
genome to volume ratio: as a cell grows and its cytoplasm and organelles increase, it becomes harder and harder for the nuclear instructions to control it
diffusion rates are affected by cells that keep growing in size
It is ideal for the time diffusion takes to be as little as possible so it doesn’t take too long for particles to make their way through the cell
origin of the name chromosomes
chromo_somes~colored body
some guy stained some cells and the dye stuck to the DNA → colored bodies
Chromosome
packages of DNA that gets condensed when cell division is about to start
during interphase it is more loosely arranged in the nucleus
during mitosis chromosomes exist as condensed chromatin
chromatid
A chromatid (Greek khrōmat- 'color' + -id) is one half of a duplicated chromosome. Before replication, one chromosome is composed of one DNA molecule
sister chromatids
refers to the identical copies formed by the DNA replication of a chromosome, with both copies joined together by a common centromere. In other words, a sister chromatid may also be said to be 'one-half' of the duplicated chromosome. A pair of sister chromatids is called a dyad.
chromatin
Chromatin is a complex of nucleic acids (e.g. DNA or RNA) and proteins (e.g. histones).
condensed chromatin
DNA+protein and looks like the “X“ shape which is technically a duplicated condensed chromosome.
centromere
it links the sister chromatids together
the region of a chromosome to which the microtubules of the spindle attach, via the kinetochore, during cell division
telomere
molecular protective caps of repetitive DNA at the ends of chromosomes
kinetochore
the intersection of microtubules (from the spindle fiber) and centromere
non-kinetochore
the part of the microtubule that does not intersect with the centromere
What is the Cell Cycle?
a means of regulating often how quickly a cell divides
has multiple phases that can most generally be divided into interphase and M-phase
why are checkpoints important in the cell cycle
checkpoints have to occur before one phase leads to the next
G1 Checkpoint
Checks for:
cell size
nutrients
growth factor
DNA damage
G2 Checkpoint
Checks for:
cell size
DNA replication
Spindle Assembly Checkpoint
Checks for:
chromosome attachment to spindle
Interphase
the in-between divisions phase → a cell does its regular cellular activities and prepares for division in the future
Tends to be the majority of the cell cycle
has 3 subphases:
G1, S, G2
G1 (growth phase 1)
increases in size + G1 Checkpoint
S Phase
DNA is synthesised
G2 (Growth Phase 2)
cells grows some more G2 checkpoint
G0 (resting state)
the cell is “resting“ and not participating in the cell cycle
it is a cellular state that is not in the cell cycle
Mitosis (M Phase)
cell division in which 1 parent cell divides once to make 2 genetically identical daughter cells
There are 6 phases of mitosis:
prophase
prometaphase
metaphase
anaphase
telophase
cytokinesis
What are somatic cells?
body cells
Prophase
the chromosomes condense
nuclear envelope breaks down
nucleolus disappears
spindle apparatus begins to form between the centriole pairs
Prometaphase
spindle fibers (microtubules) attach to the centromeres of the chromosomes to begin to move them where they need to be in the next phase of mitosis
Metaphase
chromosomes are moved to the equator (metaphase plate) by the spindle
Anaphase
shortening of the microtubules pulls apart the sister chromatids, moving them towards the poles
non-kinetochore microtubules elongate relative to the shortening of the kinetochore ones
this elongates the cell as well
Telophase
opposite of prophase
chromosomes begin to unravel/decondense
nuclear envelopes reform
spindle apparatus breaks down
nucleoli reform
Cytokinesis
separation of cytoplasms to complete the mitosis process
-Cytokinesis → how does it differ in animal vs. plant cells?
animal cells: (cell membrane)
a cleavage furrow exists at the pinching in point. Microfilaments pull in on the plasma membrane to accomplish this.
plant cells: (cell wall)
vesicles filled with cellulose align along the equator and fuse together to make a new cell wall
-Binary fission → which cells do this? Why don’t we use the phases (Prophase, Metaphase, etc.) for this?
how bacteria divides
no phases of mitosis because bacteria doesn’t have multiple chromosomes and no need to segregate the chromatids etc.
DNA replication happens like in eukaryotes but the splitting of the parent cell happens pretty quickly
tumor suppressor genes
suppress the likelihood of a tumor developing
oncogenes
correlated with the incidence of cancer
some tumor suppressor genes can mutate into an oncogenes
Meiosis
cell division that involves a diploid cell dividing 2 times to produce 4 genetically unique daughter cells that have half the number of chromosomes (haploid)
What are gametes?
female egg cell and male sperm cell
Why would this be referred to as “reduction division”?
because it results in the cells have half the number of chromosomes as the parent cell
Diploid
2 sets of chromosomes
somatic cells are diploid cells
in humans the 2n = 46
Haploid
1 set of chromosomes
gametes = sperm, egg, pollen, etc.
in humans the n = 23
tetrads
Two pairs of sister chromatids (a dyad pair) aligned in a certain way and often on the equatorial plane during the meiosis process.
Homologous Chromosomes
the pairs of chromosome you have (one from each bio parent)
different pairs are different sizes (pair 1 is the largest… pair 22 is very small)
What is genetic recombination (aka “crossing over”) and when does it happen?
genetic recombination/crossing over is when chromatids from the homologous chromosomes overlap randomly during Prophase 1 and exchange parts - this create much more genetic variation in the resulting gamates
What is independent assortment?
randomly lining up and segregation of chromosomes from each parent during the formation of sex cells
How do they contribute to genetic variation in the resulting cells?
Meiosis I involves crossing over and independent assortment. Crossing over occurs in prophase I and this results in the exchange of DNA between homologous chromosomes. This creates new combinations of alleles. Independent assortment describes the different ways in which chromosomes can assemble along the metaphase plate. This occurs in metaphase I. Meiosis II involves independent assortment but not crossing over.
What happens in Meiosis I?
Prophase 1 → tetrads and crossing over
Metaphase 1 → they line up homologous chromosomes pairs etc. ie. diploid 6, haploid 3 3 lined up next to 3
Anaphase 1 → homologous chromosomes split up
Telophase 1 they split with 3 haploids in each
Meiosis II
Prophase II → the exact same thing
Metaphase II → they line up 3 in a line along the equator
Anaphase II → they split like they usually do
Telophase II they split into the like chromatid things
chiasmata
chiasm → singular
the crossover points between homologous chromosomes
exact number that will be made is not yet predicatable
gonads
sexual organs
-What are similarities and differences between mitosis and meiosis?
differences:
mitosis:
involves 1 cell division to make 2 cells and meiosis involves 2 cell division to make 4 cells
clones of parent cells
takes place everywhere where cells are dividing in an organism (somatic cells with diploid # of chromosomes
meiosis:
genetic recombination written tetrads of homologous chromosomes to make genetically unique cells
takes place in gonads to produce gametes (haploid #)
similarities:
cell division
cell cycle basic phases (prophase, metaphase, anaphase, telophase)
synthesis of DNA
Gametogenesis: Oogenesis and Spermatogenesis
Oogenesis and Spermatogenesis: Similarities and Differences
Similarities:
both start with diploid cells and make genetically unique haploid cells through meiosis
both require fertilization to occur to create a new individual (haploid gamete + haploid gamete = diploid cell, the 1st cell of new life)
Differences:
oogenesis begins much earlier (oocytes are paused meiosis I and the “play“ button isn’t pressed in a chemical sense until ovulation/fertilization by sperm)
oogenesis produces only viable ovum per meiosis event, spermatogenesis produces 4 viable sperm each time. Polar bodies are not viable ova
ova are MUCH larger and have all the important organelles of the cytoplasm - sperm are very small. Besides the haploid nucleus mitochondria in the mid-piece, and the flagellum they have hardly any other major organelles