Why do Cells Divide?

Growth/Differentiation: Mitosis enables organisms to grow from a single-celled zygote into a mature organism that might contain hundreds of trillions of specialized cells

Maintenance: new cells produced to replace worn out/dead cells

Repair: They can regenerate damaged tissues (finger cut → new skin). Some organisms can regenerate entire body parts.

Single Cell Reproduction (asexual)

• No new combination of cellular material occurs (all new cells contain same DNA as original cell)

• Occurs in all somatic (body) cells, unicellular organisms, and simple multicellular organisms (budding, runners)

DNA Organization

1. DNA Molecule: two sugar-phosphate backbones with nitrogen base “rungs”

2. Histones: DNA molecule wraps around histones, forming a bead-like structure

3. Chromatin Strands: The bead-like structure is packed tightly, producing chromatin strands

4. Chromatin Fibres: Stands from loops which are attached to a supporting protein scaffold

5. Chromosomes: Protein scaffold folds further to condense the genetic material into chromosomes (duplicate during replication)

Chromosomes

• Small, sausage-like structure of nucleic acids

• May be found as individual chromatids (during late stages of cell division) or as paired/sister chromatids (connected at the centromere)

• Sister chromatids are identical to each other (exact copies)

• All somatic cells contain homologous pairs of chromosomes

  • one from the mother’s egg (maternal)

  • one from the father’s sperm (paternal)

• Humans have 23 pairs of chromosomes (46 chromosomes total)

• Each homologous pair is similar in shape and length and is responsible for the same types of characteristics

• The last “pair” of chromosomes (#23), determines gender (sex chromosomes)

  • Homologous pair (XX chromosomes) = female

  • Heterologous pair (XY chromosomes) = male

Homologs

• Homologous chromosomes carry the same genes, in the same order

• Despite this, homologous chromosomes often have slightly different DNA sequences resulting in different alleles (different form of the same gene)

• Share several other characteristics, including:

  • Length

  • Centromere location

  • Banding pattern

Spontaneous Generation

• Spontaneous generation was the theory that living organisms could arise from non-living matter

• Most scientists accepted this theory until the mid-1800’s when advances in mircoscope maganification allowed for obervations of cell division

• These oberservations led scientists to propose an alternative; new cells arise only from the division of other cells

The Cell Cycle

• Does not start and stop, but continues → different cells may cycle at different pace

Stages of the Cycle

Interphase: growth stage consists of G1, S & G2

• G1 (first cell growth stage): organelle replication

• S (synthesis phase): DNA is replicated

• G2 (second cell growth stage): rebuilds energy reserves and prepares for mitosis

Cell Division (Mitosis): division of genetic material & nucleus

• Prophase

• Metaphase

• Anaphase

• Telophase

• Cytokinesis

Mitosis

• Cell division in somatic cells

• All the cells produced by mitosis are IDENTICAL in genetic makeup to the original cells (particularly important that the chromosome # doesn’t change)

• The unique appearance and functionality found in different cells of the body (except the sex cells) is NOT due to difference in cellular content, but a difference in the way that content is expressed (differentiation)

Prophase (Step 1)

• Nuclear envelope breaks down; contents of nucleus become visible

• DNA strands shorten and thicken, causing chromatin to condense into visible chromosomes

• Centrioles separate and move to opposite poles of cell

• Centrioles start growing spindle fibres

• Nucleolus becomes invisible

Metaphase (Step 2)

• Chromosomes move to centre of cell

• Centromeres align across equator

• Spindle fibres attach to the centromeres

Anaphase (Step 3)

• Spindle fibres shorten and start pulling the sister chromatids apart

• Chromatids separate at centromeres

• Chromatids move to opposite poles of cell (same number of single-copy chromosomes should be at each pole)

Telophase (Step 4)

• Chromosomes at opposite ends of cell

• Chromosomes un-condense to form chromatin

• Nuclear envelope and nucleolus reappears

Cytokinesis (Step 5)

• Cytoplasm division

• In plant cells, a cell-plate forms first, separating two cells by forming cell wall

• In animal cells, cell membrane pinches in at the cleavage furrow to form two distinct daughter cells

Asexual vs. Sexual Reproduction

Asexual

• Single individual is the sole parent

• Single parent passes on all genes to its offspring

• Offspring are genetically identical to the parent, resulting in a clone

• Genetic differences rarely occur, but are the result of a mutation

Sexual Reproduction

• Two parents (mother and father)

• Each parent passes on half of their genes, resulting in the offspring having a unique combination of genes

• Increases genetic variation

Haploid vs. Diploid Cells

• The life cycle of all sexually reproducing organisms alternates between haploid and diploid cells

Somatic Cells

• Somatic cells are diploid cells

• They have DNA from maternal and paternal sides combined

Gamete Cells

• Gametes (sperm or eggs) are haploid cells

• They only hold half the DNA from somatic cells from which they came

• When an ovum is fertilized by a sperm, the original number of chromosomes (46 = 2n) is restored, forming a zygote

Zygote

• A diploid cell that results from the fusion of two haploids gametes

Meiosis

• Creates gamete cells by reducing the number of chromosomes from 46 to 23 by copying chromosomes once, but dividing twice

• Meiosis I: separates homologous chromosomes (first division)

• Meiosis II: separates sister chromatids (second division)

Step 1: Prophase I

• Same as prophase of mitosis:

• Chromatin condenses into chromosomes

• Centrioles move to opposite poles

• Spindle fibres appear

• Nuclear envelope begins to disappear

• Nucleolus becomes invisible

• Homologous chromosomes pair up side by side (synapsis) by corresponding genes forms tetrad (4 chromatids)

• Homologous chromosomes overlap and occasionally break, exchanging identical sized segments (crossing over)

• Crossing over leads to more genetic variation!

Step 2: Metaphase I

• Homologous pairs move to centre → centromeres on either side of equator

• Spindle fibres attach to centromeres only on exposed sides

Step 3: Anaphase I

• Homologous pairs separate (not sister chromatids) at the centromere

• Chromosomes move to opposite poles = segregation

• There should be 23 double chromosomes at each pole (sister chromatids remain intact)

Step 4: Telophase I

• Chromosomes at opposite poles

• Chromosomes do not uncoil to form chromatin

• Nuclear envelope occasionally reappears (in some cells)

Step 5: Cytokinesis

• Division of the cytoplasm & organelles

Step 6: Prophase II

• Centrioles move to opposite poles

• New spindle fibres form

• Note: Meiosis II is very similar to Mitosis

Step 7: Metaphase II

• Cell moves directly to metaphase → no DNA replication and no formal organization of nucleus

• Chromosomes move to centre, centromeres align on equator (metaphase plate)

• Spindle fibres attach to outside of centromeres

Step 8: Anaphase II

• Spindle fibres shorten → chromatids separate at centromeres

• Chromatids move to opposite poles

• There should be 23 single stranded chromosomes at each pole

Step 9: Telophase II

• Chromosomes at opposite ends un-condense to form chromatin

• Nuclear envelope reappears

Step 10: Cytokinesis

• Division of the cytoplasm & organelles

Meiosis in Oocytes

• In oocytes (female germ cell involved in reproduction; an immature ovum/egg cell) meiosis I is put on hold at the end of prophase I

• Once the female reaches puberty, meiosis I is completed

• Meiosis II is completed if/when the oocyte is fertilized

Gametogenesis

• The formation of ova and sperm follow the process of meiosis, specializations dependent on their function

• Sperm are designed for movement (little cytoplasm), lots of cell division, produce 4 small sperm

• Eggs are designed to nourish the zygote – only one ovum is produced per oocyte → the other 3 polar bodies sacrifice their cytoplasm to produce one large egg

Karyotyping

• A method of identification of chromosomes

• Pictures of chromosomes are taken as cell undergoes mitosis → picture enlarged

• Individual chromosomes are cut out

• Chromosomes are matched up based on:

  • Size (largest to smallest)

  • Centromere position

  • G-banding

Fetal Tests

Amniocentesis

• Procedure where a small amount of amniotic fluid is removed from the sac surrounding the fetus

Chorionic Villi Sampling

• A sample of chorionic villi is removed from the placenta for testing

Nondisjunction of Autosomes

• Chromosomes don’t separate properly during anaphase I or II

• One daughter cell produced during separation will be lacking information, the other will have too much

  • If one too many chromosomes, one pair will be a triplet (trisomy)

  • if one too few chromosomes, one pair will be a singlet (monosomy)

• Non-disjunction occurs quite often among humans

• The impact is usually so severe to zygote that miscarriage occurs very early in pregnancy

• If the baby survives, the set of traits is called a syndrome

• Trisomy 21, 13 and 18 are the only known trisonomic autosomal genetic disorders that result in offspring surviving for a short period of time

Down’s Syndrome (Trisomy 21)

• Most commonly known trisomy

• 1 out of 700 births

• Life expectancy has raised in recent decades (from 25 in 1983 to 60 today)

• Short stature, fingers, and toes

• Large tongue – makes speech difficult

• Mental Disability Prone to heart defects, respiratory problems & leukemia

Risk of Down Syndrome

• Odds of having a Down’s child increases with the age of the mother

  • At age 25, the risk is 1 in 1250

  • At age 30, the risk is 1 in 1000

  • At age 35, the risk is 1 in 400

  • At age 40, the risk is 1 in 100

  • At age 45, the risk is 1 in 30

Patau’s Syndrome (Trisomy 13)

• 1:15,000 births as most fetuses die before term

• Of those that survive, 5% live to age 3; 45% die within the first month

• Serious eye, brain, and circulatory defects, malformations, kidney/heart defects

Edward’s Syndrome (Trisomy 18)

• Only 10% survive past one year

• All die early in infancy

• Many complications

• (Babies are small, small heads, intellectual disabilities)

Nondisjunction of the Sex Chromosomes

• These can be fatal, but most survive just fine

Klinefelter’s Syndrome (XXY Chromosomes)

• Affects 1:500 males

• Tall, sterile males

• Normal intelligence

• Has female characteristics

Jacob’s Syndrome (XYY super male)

• Genetic conditions where a male has an extra male chromosome (Y)

• 1 out of 1000 males

• Physical features:

  • Somewhat taller than average

  • Slightly below normal intelligence

    • Delayed emotional development

    • Learning problems in school

  • Extra testosterone

• Other common symptoms include immaturity, acne, swollen joints, arthritis, and many more

Super Female (XXX)

• 1:1000 live births

• Normal intelligence

• Fertile

• No physical problems

• There are some women who are XXXX and XXXXX – each increasing X results in lesser intelligence and fertility

Turner’s Syndrome (X)

• The ONLY surviving monosomy

• 1:2700 births

• Live normal lives but do not mature sexually at puberty

• Sterile

• Physical features:

  • Short stature

  • Short broad neck

  • Broad chest

Other Chromosomeal Issues

Deletion

• A segment of the chromosome is missing

• Example:  Cri-du-chat (1:1,000,000)

  • Improperly developed larynx

  • Severely mentally handicapped

Duplication

• Ex. Fragile X 1:1500 males, 2500 females

• Most common form of mentally handicapped offspring

Prader-Willi Syndrome

• Part of chromosome 15 is missing

• Obese

• Reduced muscle tone

• Reduced mental ability produce little or no sex hormones

Polyploidy

• nondisjunction is actually a desired characteristic in the development of large luscious fruit – big strawberries might be 4n or even 6n (polyploidy)

• An estimated 30-80% of living plant species are polyploid