DNA Replication lecture CC-BY, Leslie Orzetti-Gollhofer

1. DNA Replication, Mitosis
and the Cell Cycle

2. Why Do Cells Need to Divide?
• Growth
– Growth in size of individual cell
– Growth in size of multicellular organism
• Repair
• Replace
• Reproduction
– Asexual
– Sexual

3. 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

4. Asexual vs. Sexual Reproduction

6. 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

7. 6 weeks pregnant 10 weeks pregnant 20 weeks pregnant 40 weeks pregnant

8. DNA Replication and Cell Cycles

9. 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

10. Steps in DNA Replication
1. DNA Helicase unzips DNA double helix
2. Binding proteins stabilize DNA strands so do
not bind back together
3. RNA primase makes short stretch of RNA on
DNA template
4. DNA polymerase adds nucleotides to create
new strand of DNA
1. Addition starts at 5’ end to 3’ end
5. Ligase binds together fragments of DNA

11. DNA Replication

12. 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

13. Chromosomes

14. Replicated Chromosome

15. Chromosome Terms
Term 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

16. 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)

17. Cell Cycle

18. 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

19. Cell Cycle: Interphase
(before Mitosis begins)
Interphase

20. 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

21. Cell Cycle: Prophase
Prophase

22. 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

23. Cell Cycle: Metaphase
Metaphase

24. Cell Cycle: Anaphase
Anaphase

25. Cell Cycle: Telophase
Telophase

26. 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

27. Cell Cycle: Cytokinesis

28. 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

29. Cell Division Checkpoints
DNA damaged?
DNA replicating
right?
• All DNA replicated?
• Damaged DNA reparable?
• Spindle making good to go?
After Metaphase
• Spindle built?
• Chromosomes
attached?
• Chromosomes
aligned?

30. 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

31. 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

32. 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

33. 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

34. 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

35. Cancer
• Treatment
– Surgical removal of tumor
– Chemotherapy – intravenous medicine to stop cell
division
– Radiation Therapy – directed streams of
radioactive isotopes to kill tumor cells

36. Cell Division Terms
Term Definition
Interphase the phase of the cell cycle in which the cell spends the majority of its time and prepares for
cellular division
G0 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 DNA
Mitosis the process by which cells replicate
Prophase a stage of mitosis in which the chromatin condenses into chromosomes
Prometaphase the nuclear membrane breaks apart into numerous "membrane vesicles,” and the chromosomes
inside form protein structures called kinetochores
Metaphase chromosomes align in the equator of the cell before being separated into the two daughter cells
Anaphase chromosomes are split move to opposite poles of the cell
Telophase the final stage in both meiosis and mitosis
Centrosome an organelle that serves as a regulator of cell-cycle progression
Mitoic spindle the subcellular structure that segregates chromosomes between daughter cells
Kinetochore 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 cells
Cleavage furrow the indentation of the cell's surface that begins the progression of cleavage, by which some cells
undergo cytokinesis
Cell plate the plate that causes cytokinesis in plant cells

37. 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

38. 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

39. Human Karyotype

40. 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

41. Homologous Pair of Chromosomes

42. 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

43. 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

44. Meiosis NO
INTERPHASE

45. 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

46. 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

47. 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

48. Spermatogenesis

49. Oogenesis

50. 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

51. Mitosis and Meiosis

52. 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

53. Genetic Variability—Crossing Over

54. 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

55. Genetic
Variability
Independent
Assortment

56. 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……

57. 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

58. 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.

59. 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