Cell Division!

Subtopics are highlighted, terms are bolded, and definitions are underlined :)

Stem Cells:

A stem cell is a relatively unspecialized cell that can divide during a single division into one identical stem cell and one more specialized daughter cell, which can undergo further differentiation. It forms new cells for growth, replacement, or repair. They can theoretically divide without limit to replenish other cells.

Asymmetrical Division - this is when one cell remains a stem cell (self-renewal) and the other becomes another type of cell with a more specialized function

Asymmetrical Division is still mitosis. The difference occurs in the expression of the genes. The self renewed cell expresses the genes as that particular stem cell would, The differentiated cell turns on and off different genes to become more specialized.

Cell Differentiation - process that leads to cell specialization

All body cells in a person have the same genome (same genetic information). What makes cells different is which genes are on and off. Cells receive chemical messages from other cells which influences gene expression (which genes are on and off) and this is what causes different cell types to form.

Mitosis:

Why do cells divide?

For growth, repair, and renewal.

The Cell Cycle - an ordered series of events that occurs in a cell leading to its division into two cells

The Cell Cycle consists of two main phases... 

1. Interphase - the periods when the cell is not dividing; G1, S, G2

2. Mitotic Phase - Cell Division;  Mitosis & Cytokinesis

Interphase - non-dividing phase of the cell cycle where the cell is doing a series of steps to prepare for division

During interphase, cell metabolic activity is high, chromosomes are duplicated and cell size may increase. This accounts for about 90% of the time in the cell. Consists of the stages G1, S, G2. The nucleus is well-defined and DNA is loosely packed in long chromatin fibers.

G1 = 1st Gap (Growth) - cell grows

S = DNA Synthesis - DNA is duplicated

G2 = 2nd Gap (Growth) - prepares for division, cell grows (more),  produces organelles

Mitotic Phase - the phase where Cell Division occurs

It consists of mitosis and cytokinesis.

Mitosis - nuclear division, giving each new cell a copy of the DNA

Cytokinesis - splitting of the cytoplasm, which leads to separation of the cell into two cells

Cytokinesis may or may not occur. If it doesn't occur it will lead to multi nucleated cells.

Genome - the entire collection of an organism’s DNA

There are two main types of chromosomes... 

Sex Chromosomes - chromosomes that determine an individual’s sex; ex. X & Y

Autosomes - any chromosome that is not a sex chromosome; ex. chromosomes 1-22 in humans

Sex-Determination Systems:

Females are organized to receive genetic material and males are organized to transmit genetic material. The sex-determination system varies based upon the species.

Types of sex-determination systems... 

Hermaphrodites – have both functional male and female gonads (testes and ovaries); ex. earthworms

Environmental Variables like... 

Alligators - temperature during development within the egg

Clown Fish - the dominant fish in a group becomes female (sequential hermaphrodite)

Chromosomal Differences like... 

Honey Bees – XX (fertilized egg - 2 x chromosomes) – females; XO (unfertilized egg – 1 x chromosome) – males

Marsupial & Placental Mammals - (ex. humans) Females – XX Males – XY

Sex-determining region Y (SRY) gene - ordinarily found on the Y-chromosome in placental and marsupial mammals

It encodes a protein that is involved in activating genes that leads to the development of male gonads (testes) and prevents the development of female reproductive features (uterus and fallopian tubes). Lack of this gene leads to the formation of a female.

DNA Organization:

Chromosome - a threadlike, gene-carrying structure found in the nucleus

Each eukaryotic chromosome is made of chromatin and is condensed further during mitosis.

Chromatin - a complex of DNA and its associated histone proteins

During interphase the DNA is found in the loose Chromatin state so the genes can be accessed easily. During prophase of mitosis, the chromatin coils and folds tightly into condensed chromosomes so the sister chromatids can be separated easily during mitosis.

Sister Chromatids - two identical copies of DNA making up a replicated chromosome

The additional copy of DNA is formed during s-phase of the cell cycle. They are joined by the centromere. When the sister chromatids are separated during anaphase of mitosis, we no longer call them chromatids. A chromatid is one among the two identical copies of DNA making up a duplicated chromosome, which are joined at their centromeres. The term chromatid is used so long as the centromeres remain in contact. When the chromatids separate (during anaphase of mitosis and anaphase 2 of meiosis), the individual strands are called now called chromosomes.

G0 - non dividing stage where the cell has left the cell cycle

G0 is not part of interphase. Cell doing its normal daily job. Most cells in the body are in this stage.

Centrosomes - release microtubules to form the mitotic spindle

Mitotic Spindle - begins to form during prophase

Consists of fibers of microtubules called spindle fibers that are involved in the movement of chromosomes.

Phases of Mitosis (Remember: PMAT)... 

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

Prophase - chromatin condenses into visible chromosomes

Centrosomes move to opposite poles of cell mitotic spindle forms spindle fibers attach to centromeres on chromosomes. The nucleolus disappears and nuclear membrane breaks down.

Metaphase - chromosomes align along middle of cell metaphase plate (meta = middle)

The spindle fibers coordinate movement and helps to ensure chromosomes separate properly so each new nucleus receives only 1 copy of each chromosome.

Anaphase - sister chromatids of the duplicated chromosome separate into 2 chromosomes, which are exact copies of each other

Chromosomes move to the opposite poles.

Telophase - chromosomes arrive at opposite poles

The daughter nuclei form nucleoli form chromosomes uncoil into the loose chromatin form

The spindle fibers breakdown, cytokinesis begins and cell division occurs.

Cytokinesis is the division of the cytoplasm, which leads to the separation of the cell into two daughter cells.

Animal Cell Cytokinesis:

Contractile Ring - of actin microfilaments around equator of cell

It splits cell in two like tightening a draw string.

Plant Cell Cytokinesis:

Cell Plate - forms vesicles line up at equator containing cell membrane & cell wall material

It is derived from Golgi and vesicles then fuse to form the cell membrane and cell wall between the two cells.

Meiosis:

Asexual Reproduction - offspring that inherit all their genetic material from just one parent

Sexual Reproduction - offspring that differ genetically from their parents because the genetic material of both parents was combined

Types of Asexual Reproduction...

Fragmentation - when a single parent breaks into parts that regenerate into new individuals (Sponges, starfish)

Budding - splitting off of new individuals from the parent form (Hydra)

Parthenogenesis - females produce offspring from unfertilized eggs (honeybee drones, some lizards)

Vegetative Propagation - the process of producing new plants from existing plant tissue; ex. runners, stem/root cuttings, etc.

Advantages of Asexual Reproduction - beneficial in stable environments, it eliminates the energy cost of finding a mate, it is successful at low population density, it eliminates the energy cost of fertilization/pollination, and it eliminates the need for pollinators in plants

Advantages of Sexual Reproduction - generation of offspring arising from the fusion of two gametes, offspring receive half of their genetic material from each parent, generates genetic variation in the offspring, children are genetically different from their parents and each other, and is beneficial in changing/unstable environments

Karyotype - a display of an individual’s chromosomes

Homologous Chromosomes (homologs) - paired chromosomes with the same chromosome structure

Same genes, but may have different versions (alleles) of the genes. One inherited from Mom (egg cell) and the other from Dad (sperm cell); ex.. – the maternal and paternal version of chromosome 1 that you received from your parents.

Diploid - two sets of chromosomes (2N)

One set of chromosomes from mom and one set of chromosomes from dad.

Haploid - one set of chromosomes (N)

Combination of chromosomes from mom and dad.

Somatic Cells - body cells; diploid

Gametes - sex cells; haploid

Fertilization - a sperm (N) fuses with an egg cell (N) to form a zygote

Twins:

Dizygotic (Fraternal) Twins - twins who develop from two eggs fertilized by two different sperm

Monozygotic (Identical) Twins - soon after one sperm fertilized one egg to form the zygote, the embryo splits to form two identical embryos

Conjoined Twins - a form of monozygotic twins

Occurs when the embryo fails to completely separate into two distinct embryos.

Meiosis - starts with a diploid cell, produces 4 non-identical haploid gametes, and consists of 2 divisions

Division 1 - splits the homologous chromosomes, reducing the chromosome number in half

Division 2 - splits the sister chromatids like mitosis

Sexual reproduction introduces genetic variation three ways...

1. Independent assortment of chromosomes - random alignment of homologous chromosomes in Metaphase 1 of meiosis

Based upon the way in which the homologous chromosomes line up during metaphase I and then separate will lead to a unique combination of chromosomes in the gametes.

2. Crossing-over - mixing of alleles across homologous chromosomes during prophase 1 of meiosis

3. Random fertilization - which sperm fertilizes which egg

Mitosis vs. Meiosis -

Non-disjunction - the failure of the chromosomes to properly segregate during meiotic or mitotic anaphase, resulting in daughter cells with abnormal numbers of chromosomes; ex. Down Syndrome - Trisomy 21, Turner Syndrome - One X Chromosome, Klinefelter Syndrome - XXY or XXXY