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Cell Division and Genetics – Mechanisms for a Knit of Identity and Thread of Distinction
DNA and Its Faithful Replication – The Knit of Identity
Because DNA stores genetic information and is faithfully replicated, information is passed largely unaltered from cell-to-cell, generation-to- generation.
Proteins and Their Production – The Primary Reason for DNA
Cell Division DemandsCoordination of DNA Replication, Mitosis and Cytokinesis
What’s so important about cell division?
Cell division requires coordinated division of chromosomes (mitosis) …..
…… and division of the cytoplasm (cytokinesis).
DNA Replication – Simple in Principle, Complicated in Practice
DNA is Packaged into Chromosomes
DNA in the cell is virtually always associated with proteins.
The packaging is impressive – 2 meters of human DNA fit into a sphere about 0.000005 meters in diameter.
chromatin
duplicatedchromosome
The Link Between DNA Replication and Chromosome Duplication
DNA is Condensed into Visible Chromosomes Only For Brief Periods in the Life of a Cell
95% of the time, chromosomes are like this.
Easily visible chromosomes are apparent perhaps 5% of the time in an actively growing cell and less in a non-growing cell.
A Karyotype is an Arranged Picture of Chromosomes At Their Most Condensed State
A normal human karyotype
Boy or girl?
Note that almost all chromosomes come in homologous pairs.
From Birth to Rebirth, a Cell Progresses Through Characteristic Stages That Constitute the Cell Cycle
In multicellular organisms like us, progress through the cell cycle is carefully regulated.
Cancer Is One Outcome of A Runaway Cell Cycle
Licentious division - prostate cancer cells during division.
The Knit of Identity - Mitosis Precisely and Evenly Divides Duplicated Chromosomes
Precisely dividing the duplicated chromosomes has the consequence of providing each new cell with an identical and complete set of genetic instructions.
interphase prophase metaphase
Mitosis Precisely and Evenly Divides Duplicated Chromosomes
Cytokinesis is the process of cell division and it is distinct and separable from mitosis.
Mitosis in Action
Blue shows DNA, green shows spindle fibers.
In Animal Cells, a Cleavage Furrow Forms and Separates Daughter Cells
Cleave furrow in a dividing frog cell.
The Plant Cell Wall Forces Cytokinesis to Play by Different Rules
Part 1: Cell Division
• Cell division is the cornerstone of life
• Genome: an organisms complete set of genetic material (DNA)
Prokaryotic Chromosomes
Bacteria & viruses
A single DNA (or RNA in some virusis) moleucle that contains all the genetic information for the individual.
Chromosomes (colored bodies)
Prokaryotes have a single, circular chromosome
– Prokaryotes (bacteria) reproduce through cell division called Binary fission
– Circular chromosome (DNA) is replicated
– Replicated chromosomes are attached to the cell membrane at nearby sites
– As membrane expands, the copies separate
– New cell wall forms between copies, cell splits
Eukaryotic Chromosomes
DNA & protein structures with only part of the DNA information. All of the chromosome together make up the genome.
Mitosis
– Eukaryotes use cell division to create new cells
– MITOSIS: division of the nucleus. Responsible for Growth, Maintenance and Repair
Type of Asexual cell division
Chromosomes (colored bodies) • Chromosome made of a DNA and protein complex:
chromatin. Following DNA replication, a chromosome contains two sister chromatids attached by a centromere
Human Chromosomes: 46 – 46 chromosomes (2n = 46)
• 2 sets of 23 chromosomes (diploid)
– n = number of chromosomes in a set
– Fundamental number, haploid number
– n = contribution from each parent
– 23 = 1 sex chromosome, 22 autosomes
Mitosis and Interphase Alternate
• The cell cycle
– Cells are in interphase from 75-90% of the time
– G1 = Gap 1 (cell growth)
– S = chromosome synthesis (duplication)
– G2 = Gap 2 (cell growth)
– M = Mitosis
Chromosomes cont.
Homologous pair (2n, diploid) in parent cell
•Chromosomes are replicated during S-phase of cell cycle. Chromosomes and copies are separated during mitosis.
One of each pair to daughter cell
Copies
Mitosis is a Continuum
Prophase Chromatin coils into visible chromosomes
Under a light microscope, only the nuclear envelope (with nucleoli) and a tangle of chromatin are visible
Pro-metaphase (late prophase)• Nuclear envelope breaks
• Microtubules attach centromere to centrioles
Metaphase Chromatids align on a plane at cell’s equator
Anaphase • Chromatids separate simultanously
• Sister chromatids become daughter chromosomes
Telophase Daughter chromosomes stop moving
Chromosomes uncoil, nucleus and nucleoli reformresulting in two new, identical cells.
Cytokinesis • Division of cell cytoplasm after mitosis is known as
cytokinesis
Phases of the Cell Cycle
• The cell cycle consists of– Interphase– Mitosis– Cytokinesis
INTERPHASE
G1
S(DNA synthesis)
G2Cyt
okin
esis
Mito
sis
MITOTIC(M) PHASE
Figure 12.5
• Heredity– Is the transmission of traits from one generation to the
next
• Variation– Shows that offspring differ somewhat in appearance
from parents and siblings
Figure 13.1
• Genetics– Is the scientific study of heredity and hereditary
variation– Offspring acquire genes from parents by inheriting
chromosomes
Inheritance of Genes• Genes
– Are the units of heredity– Are segments of DNA
• Each gene in an organism’s DNA– Has a specific locus (location) on a certain
chromosome
• We inherit– One set of chromosomes from our mother and one
set from our father
Comparison of Asexual and Sexual Reproduction
• In asexual reproduction (mitosis)– One parent produces
genetically identical offspring by mitosis
Figure 13.2
Parent
Bud
0.5 mm
• In sexual reproduction– Two parents give rise to offspring that have unique
combinations of genes inherited from the two parents
• Genes are located on Chromosomes
• Individuals inherit two sets of genes (chromosome) – one from each parent
• These “Matching” chromosomes are called Homologous chromosomes, because they carry genes for the “same” (homo) traits– Have genes for the same characteristics– Are also be called autosomes
• Sex chromosomes– Are distinct from each other and carry genes for
different triats– We all inherit two sex chromosomes, one from each
parent– represented as X and Y – Determine the sex of the individual
• XX = female,
• XY = male
• A diploid cell (2n)– Has two sets of each of its chromosomes– Human have 46 chromosomes (2n = 46)
• We have a “Problem” in sexual reproduction
– How can we have 46 chromosomes in our cells, combine a cell from a father and a mother, and still have only 46 chromosomes in the offspring’s cells?
46 (father) + 46 (mother) = 46 (offspring)
– There must be a way to reduce the number of chromosomes in the parents to half the number (23 for humans)
Behavior of Chromosome Sets in the Human Life Cycle
• At sexual maturity– The ovaries and testes produce special haploid (n)
cells by meiosis– The cells are called gametes– Gametes, sperm and egg cells are haploid cells,
containing only one set of chromosomes– In humans, the haploid (n) number is = 23, ½ of our
diploid number (2n)
• Meiosis reduces the number of chromosome sets from diploid to haploid
• Meiosis– Takes place in two sets of divisions, meiosis I and
meiosis II
• Meiosis I– Reduces the number of chromosomes from diploid to
haploid
• Meiosis II– Produces four haploid daughter cells
An Overview of Meiosis
Figure 13.7
Interphase
Homologous pairof chromosomesin diploid parent cell
Chromosomesreplicate
Homologous pair of replicated chromosomes
Sisterchromatids Diploid cell with
replicatedchromosomes
1
2
Homologous chromosomes separate
Haploid cells withreplicated chromosomes
Sister chromatids separate
Haploid cells with unreplicated chromosomes
Meiosis I
Meiosis II
• Meiosis I
– Reduces the number of chromosomes from diploid to haploid
• Meiosis II
– Produces four haploid daughter cells
• The haploid reproductive cells are the gametes
• Meiosis only occurs in the sex organs – Males testes to produce sperm– Females ovaries to produce eggs
• Gametes are not produced until the individual reaches puberty and special hormones “kick” in and start the process– Males testosterone– Females estrogen & progesterone
Newborn (2n)
Growth Development Maintenance
Repair
Gametogenesis
Adult (2n)
Growth Development Maintenance
Repair
Mitosis
GametesEgg (n)Sperm (n)
Meiosis
Zygote (2n)
Embryo (2n)
MitosisGrowth
Development Maintenance
Repair
MitosisGrowth
Mitosis
• Reshuffling of genetic material in meiosis– Produces genetic variation
Origins of Genetic Variation Among Offspring
• In species that produce sexually– The behavior of chromosomes during meiosis and
fertilization is responsible for most of the variation that arises each generation
Independent Assortment of Chromosomes
• Homologous pairs of chromosomes– Orient randomly at metaphase I of meiosis
• In independent assortment– Each pair of chromosomes sorts its maternal and paternal homologues
into daughter cells independently of the other pairs
Figure 13.10
KeyMaternal set ofchromosomes
Paternal set ofchromosomes
Possibility 1
Two equally probable arrangements ofchromosomes at
metaphase I
Possibility 2
Metaphase II
Daughtercells
Combination 1 Combination 2 Combination 3 Combination 4
Centrosomes(with centriole pairs)
Sisterchromatids
Chiasmata
Spindle
Tetrad
Nuclearenvelope
Chromatin
Centromere(with kinetochore)
Microtubule attached tokinetochore
Tetrads line up
Metaphaseplate
Homologouschromosomesseparate
Sister chromatidsremain attached
Pairs of homologouschromosomes split up
Chromosomes duplicate
Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example
INTERPHASE MEIOSIS I: Separates homologous chromosomes
PROPHASE I METAPHASE I ANAPHASE I
• Interphase and meiosis I
Figure 13.8
TELOPHASE I ANDCYTOKINESIS
PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II ANDCYTOKINESIS
MEIOSIS II: Separates sister chromatids
Cleavagefurrow Sister chromatids
separate
During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing single chromosomes
Two haploid cellsform; chromosomesare still double
Haploid daughter cellsforming
Figure 13.8
Telophase I, cytokinesis, and meiosis II
Figure 13.9
MITOSIS MEIOSIS
Prophase
Duplicated chromosome(two sister chromatids)
Chromosomereplication
Chromosomereplication
Parent cell(before chromosome replication)
Chiasma (site ofcrossing over)
Prophase I
Tetrad formed bysynapsis of homologouschromosomes
Metaphase
Chromosomes positioned at themetaphase plate
Tetradspositioned at themetaphase plate
Metaphase I
Anaphase ITelophase I
Haploidn = 3
MEIOSIS II
Daughtercells of
meiosis I
Homologuesseparateduringanaphase I;sisterchromatidsremain together
Daughter cells of meiosis II
n n n n
Sister chromatids separate during anaphase II
AnaphaseTelophase
Sister chromatidsseparate duringanaphase
2n 2n
Daughter cellsof mitosis
2n = 6
MEIOSIS I
• A comparison of mitosis and meiosis
A Comparison of Mitosis and Meiosis
• Meiosis and mitosis can be distinguished from mitosis by three events in Meiosis l– Crossing over: homologous chromosomes physically connect
and exchange genetic information
– paired homologous chromosomes (tetrads) are positioned in the middle of the cell
– At anaphase I of meiosis, homologous pairs move toward opposite poles of the cell
In anaphase II of meiosis, the sister chromatids separate
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