DNA Replication, Mitosis and the Cell Cycle

Why Do Cells Need to Divide?

• Growth– Growth in size of individual cell– Growth in size of multicellular organism

• Repair• Replace• Reproduction– Asexual– Sexual

Reproduction

• Asexual – only needs 1 parent– Usually done by bacteria, protists (single celled

organisms)– Binary Fission– Makes EXACT copy of parent– Mitosis

• Sexual – needs more than 1 parent– 1 cell from each parent– Sex cells produced by meiosis– Further embryonic development - Mitosis

Asexual vs. Sexual Reproduction

Development and Apoptosis

• Development – Eukaryotic– After fertilization of sex cells– Done by mitosis– Creates trillions of cells that are you!

• Apoptosis – programmed cell death– In multicellular organisms, helps shape organs and

body parts– Webbing between toes, tadpole tails, peeling sunburn

• Cell division = cell death

6 weeks pregnant 10 weeks pregnant 20 weeks pregnant 40 weeks pregnant

DNA Replication and Cell Cycles

DNA Replication

• Before any cell can divide, must make a copy of WHOLE genome

• Semiconservative – each new DNA molecule conserves half of the original DNA

• Done using various enzymes (proteins) to attract and bond new nucleotides

Steps in DNA Replication

1. DNA Helicase unzips DNA double helix2. Binding proteins stabilize DNA strands so do not

bind back together3. RNA primase makes short stretch of RNA on DNA

template4. DNA polymerase adds nucleotides to create new

strand of DNA1. Addition starts at 5’ end to 3’ end

5. Ligase binds together fragments of DNA

DNA Replication

Preparation for Cell Division

• Replicated DNA condenses into CHROMOSOMES• Chromosome made of chromatin– DNA, proteins (enzymes that help DNA replicate,

transcribe, translate)– Nucleosome – 1 stretch of DNA + 8 proteins

• Makes a “necklace” of DNA

• 1 Replicated chromosome– 2 sister chromtids (contains DNA, proteins, etc)– 1 centromere – attaches sister chromatids together

Chromosomes

Replicated Chromosome

Chromosome TermsTerm Definition

Chromatin Complex of macromolecules found in cells, consisting of DNA, protein and RNA

Chromosome

Single piece of coiled double-stranded DNA, containing many genes, regulatory elements and other non-coding DNA

Prokaryotes—bacteria and archaea—have single circular chromosome, but many variations exist. Eukaryotes - nuclear chromosomes packaged by proteins into a condensed structure called chromatin.

Chromatid One half of duplicated chromosome, joined to the other copy by a centromere

Centromere Part of chromosome that links sister chromatids

Mitosis

• Cell Cycle – Events that occur from 1 cell division to the next– Occurs 300 million times/minute

• Interphase – No cell division– Protein synthesis, DNA replication

• Cytokinesis – Splitting of the cell (organelles, cytoplasm, and

membranes)

Cell Cycle

Interphase• G1 – normal cell function– Nerve cells permanently in G0 (non-dividing phase) –

no growth of brain or repair after adulthood• S Phase – DNA replication, repair damaged DNA– End of S phase have replicated chromosome with 2

sister chromatids– In animal cells – centrosomes move chromosomes

around for division phase• G2 – continued growth and prep for division– Chromosomes wind more tightly, proteins produced– End of interphase

Cell Cycle: Interphase (before Mitosis begins)

Interphase

Division Phase

• Prophase – DNA coils tightly into condensed chromosomes– Random arrangement in nucleus– Centrosomes migrate to poles (animals)– Nucleolus disappears

• Prometaphase– Immediately after formation of mitotic spindle– Nuclear envelop disappears– Kinetochores assemble on centromere

Cell Cycle: Prophase

Prophase

Division Phase

• Metaphase – Mitotic spindle aligns chromosomes on equator of cell

• Anaphase– Mitotic spindle splits chromosome at centromere– Pulls sister chromatids to opposite ends of cell

• Telophase– Reverse of prophase and prometaphase– Mitotic spindle breaks down– Chromosomes unwind

• Nuclear envelope and nucleolus reappear

Cell Cycle: Metaphase

Metaphase

Cell Cycle: Anaphase

Anaphase

Cell Cycle: Telophase

Telophase

Division Phase

• Cytokinesis– Division of organelles, cytoplasm, macromolecules

evenly divide in to daughter cells– Animal Cells – cleavage furrow – indentation at

middle of cell, signals beginning of cytokinesis– Plant Cells – Cell plate begins to form between

new cells, signals beginning of cytokinesis

Cell Cycle: Cytokinesis

Out of Control Cell Division• Proteins regulate cell cycle– Cyclins – fluctuate depending on cell phase– Interactions of all cyclins make checkpoints during

cycle• Cell will not go past a check point until cyclin concentrations

change– Checkpoints

• G1 – screens for DNA damage, if damaged, cell dies• S Phase – Lots of checkpoints during DNA replication• G2 – makes sure 2 full sets of chromosomes• Metaphase – ensures all chromosomes are aligned correctly

Cell Division Checkpoints

DNA damaged?

DNA replicatingright?

• All DNA replicated?• Damaged DNA reparable?• Spindle making good to go?

After Metaphase• Spindle built?• Chromosomes

attached?• Chromosomes

aligned?

Cancer• No balance between cell division and cell death• Cell gets through checkpoints– Tumor – abnormal mass of tissue formed from

uncontrolled cell division• Benign – slow growing harmless, capsule enclosed• Malignant – invades surrounding tissue• Metastasis – Cells from break away from original tumor

and travel blood or lymphatic system– Cancer – class of diseases characterized by

malignant cells

Cancer• How are cells different?– Uncontrolled cell division– Different shape – rounder, more fluid– Some have multiple nuclei– Immortal – internal clock does not work (50ish cell divisions)

• Telomeres – internal clock, ends of chromosomes that degrade with each cell division

• High amount of telomerase– Growth Factors – divide even if no factors are present– Contact Inhibition – Normal cells stop dividing when touch

each other in 1 cell layer. • Cancer cells just keep dividing

Cancer

• What causes cancer?– Mutations in 2 classes of cancer-related genes– Oncogenes – Mutated genes that stimulate cell

division• Accelerator of cell division

– Tumor Suppressor Genes – normal genes code for proteins that promote apoptosis• Mutating these genes stop cell death and promote

more cell division– BRAC 1 – Breast cancer gene

Cancer

• Stages of Cancer– Stage I – start to invade surrounding tissue– Stage II – Spread to tissue around original affected

tissue– Stage III – Spread to organs and lymph nodes near

cancer’s origin– Stage IV – Spread to distant areas throughout the

bode

Cancer

• Diagnosis– X-ray, CAT scans, MRI’s PET scans, ultrasound– Endoscope to inspect inside of organs– Biopsy – cell sample– Microscopy to id cells from biopsy– Blood tests – white cell count, tumor markers,

proteins

Cancer

• Treatment– Surgical removal of tumor– Chemotherapy – intravenous medicine to stop cell

division– Radiation Therapy – directed streams of

radioactive isotopes to kill tumor cells

Cell Division TermsTerm DefinitionInterphase the phase of the cell cycle in which the cell spends the majority of its time and prepares for

cellular divisionG0 phase the cell functions normally; occurs in cells that do not divide often (or ever)

G1 phase the cell grows and functions normally

G2 phase the cell resumes its growth in preparation for division

S phase the cell duplicates its DNAMitosis the process by which cells replicateProphase a stage of mitosis in which the chromatin condenses into chromosomesPrometaphase the nuclear membrane breaks apart into numerous "membrane vesicles,” and the chromosomes

inside form protein structures called kinetochoresMetaphase chromosomes align in the equator of the cell before being separated into the two daughter cellsAnaphase chromosomes are split move to opposite poles of the cellTelophase the final stage in both meiosis and mitosisCentrosome an organelle that serves as a regulator of cell-cycle progressionMitoic spindle the subcellular structure that segregates chromosomes between daughter cellsKinetochore the protein structure on chromatids where the spindle fibers attach during cell division

Cytokinesis the process in which the single eukaryotic cell is divided to form two daughter cellsCleavage furrow the indentation of the cell's surface that begins the progression of cleavage, by which some cells

undergo cytokinesisCell plate the plate that causes cytokinesis in plant cells

Evolution of Sexual Reproduction

• Combining genes from two individuals• First form of gene exchange – conjugation– Outgrowth of bacterial cell – Sex pilus to transfer

gene material to another bacteria– Still used today

• Paramecium – uses bridge of cytoplasm to exchange nuclei

Sexual Reproduction and Chromosomes

• Diploid Cell (2n) – 2 full sets of chromosomes– 1 set of chromosomes from each parent– You have 46 chromosomes

• 23 from Mom, 23 from Dad

• Karyotype – size ordered chart of all chromosomes in a cell

• Autosomes – body chromosomes, same for male and female, 22 pairs

• Sex Chromosomes – determine an individual’s sex, different for male and female– Male – XY– Female – XX

Human Karyotype

Chromosomes

• Pairs of chromosomes are HOMOLOGOUS– Matching pair that look alike and have same sequence

of genes– NOT identical– Have different versions of a gene – ALLELES

• Can be brown eyes/blue eyes• Can be codes for melanin/does not code for melanin

• Sex Chromosomes – NOT homologous – different sizes and number of genes

• X• Y

Homologous Pair of Chromosomes

Gametes

• Gametes = Sex Cells (egg and sperm)– Haploid (n) – half the amount of genetic material• ONLY 1 set of chromosome• For you – 23

• Fertilization – fusion of 2 HAPLOID gametes to make DIPLOID zygote– Zygote divides by MITOSIS for make new individual

Meiosis

• Meiosis – process by which gametes are produced– Makes genetically DIFFERENT gametes– Occurs in testes or ovaries• Occurs in germ cells

– Divides the amount of genetic material in HALF• Start with 46, after meiosis have 23

– Start with 1 cell end up with 4

Meiosis NO INTERPHASE

Meiosis• DNA replicates once, nucleus divides twice

– 2 rounds of cell division – only 1 interphase• Prophase I

– REPLICATED chromosomes condense, spindle forms, centrosomes, form, nuclear envelop breaks down, HOMOLOGOUS chromosomes line up next to each other

• Metaphase I– Paired homologous chromosomes line up on equator of cell

• Anaphase I, Telophase I, Cytokinesis– Homologous pairs pull apart (NOT chromatids), separate

into 2 daughter cells and have opposite of prophase

Meiosis

• Meiosis II uses daughter cells from Meiosis I to form 4 haploid gametes

• NO INTERPHASE = NO DNA REPLICATION• Phases now resemble mitosis– Prophase II – chromosomes condense, spindle and

centrosomes form, nuclear envelop breaks down– Metaphase II – chromosomes line up on equator– Anaphase II – Sister chromatids pull apart– Telophase/Cytokinesis – opposite of prophase and

splitting of cells

Meiosis End Game

• 1 diploid germ cell (spermatocyte/oocyte)• 1 round of DNA replication (Interphase I)• 2 rounds of cell division– Meiosis I and Meiosis II

• 4 haploid gametes– 4 viable sperm– 1 viable egg and 3 polar bodies

Spermatogenesis

Oogenesis

Mitosis and Meiosis

Mitosis• Done in somatic cells• 1 round of cell division• Chromosomes line up on

equator• Pull apart chromosomes• Makes 2 DIPLOID cells• Used for growth,

development, repair• Asexual reproduction

Meiosis• Done in germ cells• 2 rounds of cell division• Homologous pairs line up

on equator• Pull apart homologous pairs• Makes 4 HAPLOID cells• Used for formation of

gametes • Sexual reproduction

Mitosis and Meiosis

Genetic Variability

• Crossing Over – 2 homologous chromosomes exchange genetic material– Occurs during prophase I– Get new combinations of genes on a chromosome– Result = 4 genetically different sister chromatids

Genetic Variability—Crossing Over

Genetic Variability

• Independent Assortment – chromosomes RANDOMLY align on the equator during meiosis I– Get different combinations of chromosomes– Number of possible combinations = 2n

• Where n is the number of homologous pairs

223 = 8,388,608

Genetic Variability Independent Assortment

Genetic Variability

• Random Fertilization – any of the female’s 8,388,608 possible combinations combine with the male’s 8,388,608 combinations– That give you approximately 70 trillion genetically

different possibilities!– BUT, can have identical twins….hmmmm……

Errors in Meiosis

• Polyploidy – extra 1 or more sets of chromosomes– Ex. Normal sperm (23) + Oops egg (46) = 69– Normally human polyploids do not survive– 30% flowering plants are polyploids and survive

• Nondisjunction – extra or one missing chromosome– 50% spontaneous miscarriages– Ex. Trisomy 21 – Down Syndrome– Ex. Trisomy 18 – Edwards Syndrome– Ex. Trisomy 13 – Patau Syndrome

Errors in Meiosis• Extra or Missing Sex Chromosomes

Chromosomes Name of Condition Likelihood Symptoms

XXX Triplo-X about once in every 1,000 female births

Symptoms may include tall stature; small head; vertical skinfolds that may cover the inner corners of the eyes; speech and language learning disabilities, such as dyslexia; or weak muscle tone. Increased risk of motor coordination problems and auditory processing disorders, and scoliosis. Because the vast majority of Triple X females are never diagnosed, it may be very difficult to make generalizations about the effects of this syndrome.

XXY Klinefelter Syndrome

exists in 1:500 to 1:1000 male live births

Some degree of language learning or reading impairment may be present, although these deficits can often be overcome through early intervention. There may also be delays in motor development which can be addressed through therapy. As they grow older, they tend to become taller than average. Because these boys do not produce as much testosterone as other boys, they have a less muscular body, less facial and body hair, and broader hips. They may develop breast tissue and also have weaker bones, and a lower energy level than other males. Affected males are often infertile, or may have reduced fertility.

XYY Jacobs Syndrome occurs in 1 in 1,000 male births

Average final height approximately 3" above expected final height. Approximately half of XYY boys identified by a newborn screening program had learning difficulties—a higher proportion than found among siblings and above-average-IQ control groups. However, they often have no outstanding difference from XY boys.

XO Turner Syndrome occurs in 1:2000 to 1:5000 females

Symptoms include short stature, broad chest, reproductive sterility, increased weight, small fingers, webbed neck, swelling of hands and feet, nonverbal learning disabilities, ADHD, visual impairments, ear infections or hearing loss, heart (aorta) problems, and characteristic facial features.

Chromosomal Abnormalities

• Chromosomal Deletion – Loss of one or more genes– Ex. Cri du chat - several missing genes at chrom. 5

• Chromosomal Duplication – makes multiple copies of parts of chromosome– Ex. Fragile X – extra CGG sequence (10 – 200)– Not always harmful

• Inversion – Part of chromosome flips and reinserts at a different location

• Translocation – nonhomologous chromosomes exchange parts– Usually results in leukemia or other cancers