How do cells reproduce?

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How do cells reproduce?

• Cell division is at the heart of reproduction
• Multicellular organisms originate from a rapidly
  dividing fertilized egg (cell) eggs and sperm
  are themselves created from a special type of
  cell division
• Cell division replaces worn-out or damaged cells,
  keeping the total number of cells relatively
  constant
• There are two types of cellular division mitosis
  and meiosis

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Cell division and reproduction

• Asexual reproduction involves the creation of
  genetically-identical offspring from a single
  parent no eggs or sperm are involved
• Involves replication of chromosomes, the
  structures containing the organisms DNA
• Bacteria, yeast, protists, and certain
  plants and animals

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Asexual Reproduction

• Asexual reproduction is a very efficient means of
  reproduction
• Faster than sexual reproduction
• Increases numbers of organisms quickly
• Ability to reproduce in absence of mate (male
  doesnt need female and vice versa)

• Genetic diversity, however, is sacrificed

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Sexual Reproduction

• The ability for an organism to form gametes, or
  sex cells (eggs and sperm), results in the
  formation of similar, but not identical,
  offspring
• In sexual reproduction, the resulting offspring
  are genetically similar, but not
  identical to either parent
  offspring inherits a combination
  of genes from each parent

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Cells arise from pre-existing cells

• Cell division allows an embryo to develop into an
  adult, and is the basis of egg and sperm
  formation
• It also ensures the continuity of life from one
  generation to the next
• In the case of unicellular organisms, cell
  division can reproduce an entire organism

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Binary fission

• Prokaryotes (Bacteria and Archaea) reproduce by a
  type of cell division called binary fission
  dividing in half
• These cells possess a single chromosome, which is
  replicated prior to the cell dividing into 2

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What about eukaryotic cells?

• A bacteria contains 3,000 genes human cells
  contain 25,000 which are grouped into multiple
  chromosomes located in the nucleus
• Each chromosome consists of 1 long DNA strand,
  with hundreds or thousands of genes
• Integrated into this chromosome are proteins!,
  which help maintain its structure and control the
  activity of its genes

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Chromosomes

• Human cells have 46 chromosomes
• Before a eukaryotic cell can divide, it must
  replicate its chromosomes
• The DNA molecule of each chromosome is copied and
  new proteins attach as needed

A duplicated chromosome
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Chromosome duplication
Sister chromatids
Centromere
Chromosome distribution to daughter cells
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The cell cycle

• The process of cell division is a key component
  of the cell cycle, an ordered sequence of events
  beginning with the birth of the cell from a
  dividing parent and ending with its own division
  into 2 cells
• The cell cycle consists of a growing stage called
  interphase, and the actual cell division, called
  the mitotic phase

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The Cell Cycle

• Most of the cell cycle in spent in interphase
• During this time, the cell performs its various
  functions within the organism
• Additionally, the cell acquires a rich supply of
  proteins, creates more organelles such as
  mitochondria and ribosomes, and grows during this
  time
• Chromosomes are replicated during interphase

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The Cell Cycle

• Interphase is divided into 3 stooges, er, stages
• The G1 phase cell grows
• The S phase cell grows, chromosomes replicated
• The G2 phase cell grows
• G stands for gap (first and second gap)
• S stands for synthesis (DNA)

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INTERPHASE
S (DNA synthesis)
G1
G2
Cytokinesis
Mitosis
MITOTIC PHASE (M)
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The Cell Cycle

• During interphase, the cell grows (G1), continues
  to grow while DNA is replicated (S), and then
  grows more as it completes preparations for cell
  division (G2)
• Cell division occurs in the mitotic phase (also
  called the M phase)
• Accounts for only 10 of the total time required
  for the cell cycle

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The Cell Cycle

• Like interphase, the mitotic phase is divided
  into (2) stages
• Mitosis the nucleus (and all its contents,
  including the duplicated chromomes) divide and
  are evenly distributed to the daughter cells
• Cytokinesis the cytoplasm is divided into 2
• Mitosis and cytokinesis produces 2 genetically
  identical cells, each with a single nucleus,
  surrounding cytoplasm and plasma membrane

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Mitosis

• Mitosis (the division of nuclear material) is
  subdivided into 5 main stages
• Prophase
• Prometaphase
• Metaphase
• Anaphase
• Telophase

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Mitosis

• During mitosis, chromosome movement is dependent
  on the mitotic spindle, a football (go Giants! Go
  Jets!) shaped structure of microtubules that
  guides the separation of the 2 sets of separating
  chromosomes
• During interphase, chromosomes are not
  distinguishable because they exist as loose
  fibers of chromatin chromatin becomes more
  tightly packed and visible as mitosis ensues,
  allowing easy tracking of each step of mitosis

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Prophase

• The mitotic spindle forms during the first stage,
  prophase
• The chromatin fibers containing DNA become more
  tightly coiled and folded forming discrete
  chromosomes that can be seen with a light
  microscope
• Remember, there are 2 pairs of chromosomes at
  this stage as they were replicated during the S
  phase of interphase

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Prophase
Visible chromosomes nuclear envelope still
present
Early mitotic spindle present
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Prometaphase

• During the second stage of mitosis, prometaphase,
  the nuclear envelope breaks away
• Proteins embedded in the chromatin attach to
  microtubules of the spindle, and move the
  chromosomes towards the center of the cell

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Prometaphase
Dissolution of nuclear envelope chromosomes
moved towards the center of the cell
Mitotic spindle extend pole to pole
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Metaphase and Anaphase

• During metaphase, the mitotic spindle spreads
  across the entire cell, with the chromosomes
  aligned perpendicularly at its center (remember
  each chromosome has been replicated into 2 prior
  to mitosis)
• In Anaphase, the sister chromatids of each
  chromosome separate and move away from each other
  (toward opposing poles)

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During mitosis, each chromosome has been
replicated consisting of 2 sister chromatids
these chromosomes align and separate during
metaphase and anaphase, respectively
METAPHASE
ANAPHASE
Metaphase plate
Daughter chromosomes
Spindle
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Telophase

• During the fifth (and final) stage of mitosis
  called telophase, nuclear envelopes form around
  the 2 copies of separated chromosomes the
  chromatin fiber uncoils and the mitotic spindle
  disappears
• Sort of a reverse prophase!
• Cytokinesis follows this final stage of mitosis,
  pinching the cell into 2

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Telophase and Cytokinesis
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Got all that?

• The eukaryotic cell cycle consists of
• Interphase (G1, S, G2) growth DNA replication
• Mitosis
• Prophase mitotic spindle forms, chromatin
  condenses
• Prometaphase nuclear envelope dissolves,
  chromosomes attach to spindle
• Metaphase mitotic spindle spreads pole to pole
  with chromosomes aligned at center
• Anaphase each sister chromatid of replicatec
  chromosome separates
• Telophase nuclear envelope reforms, chromatin
  uncoils
• Cytokinesis cell divides into 2

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Cell Division

• The timing of cell division must be regulated in
  order to grow and develop normally
• Skin cells and stomach cells are replaced
  regularly as they are constantly abraded and
  sloughed off
• Other cells, such as liver cells, do not divide
  unless damaged In this way, cell division
  repairs wounds and heals

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Cell Division

• Proteins regulate cell division by stimulating
  cells to divide in their presence
• For example, injury to the skin causes blood
  platelets to release a protein which promotes
  rapid growth of connective tissue cells that help
  seal the wound
• Proteins control each cycle of mitosis and each
  stage does not occur until triggered to do so by
  these proteins

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Cell Division

• Proteins serve as a control system for each stage
  of the cell cycle
• Want a job? Research on controls over the cell
  cycle is one of the hottest areas in biology
  today. Why?
• Without check points, cells will continue to
  divide unregulated.. cancer

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Cancer

• Cancer is a disease of the cell cycle
• Cancer cells divide uncontrollably and do not
  respond normally to the cell cycle control system
• Cancer begins when a single cell undergoes
  transformation from a normal cell to a cancer
  cell
• Cancer cells may proliferate into a tumor, an
  abnormally growing mass of body cells

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Cancer

• Benign tumors remain at the site and can usually
  be removed easily with surgery
• Malignant tumors spread into neighboring tissues
  and other parts of the body, interrupting organ
  function as it goes
• Cancer cells may secrete molecules that cause
  blood vessels to spread toward the tumor, and
  allow proliferation of the cancer cells via the
  circulatory system (metastasis)

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Cancer

• Radiation damages DNA in cancer cells moreso than
  it does in normal cells and can be used as a
  cancer treatment
• Chemotherapy is used to treat metastatic or
  widespread tumors involves the use of drugs that
  disrupt cell division (some drugs prevent the
  mitotic spindle from forming in the first place)
  however side effects are seen in normal,
  rapidly-dividing cells

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Meiosis

• Meiosis is the process of cell division in which
  the number of chromosome is cut in half
• Unlike mitosis, which results in a daughter
  cell containing the exact number of chromosomes
  as the parent cell
• Meiosis takes place in reproductive organs and
  produces gametes, sex cells, such as eggs, sperm,
  and pollen (plants)

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Meiosis

• Human cells have 46 chromosomes, made up of 23
  pairs of homologous chromosomes
• Cells with 2 sets of chromosomes are considered
  diploid

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Meiosis

• The two chromosomes composing a pair are called
  homologous because they both carry genes
  controlling the same inherited characteristics
• One exception are the sex chromosomes, X and Y
• Females have a homologous pair (XX), while males
  have 1 X and 1 Y
• The other 22 chromosomes are called autosomes

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Meiosis

• For both sex chromosomes and autosomes, we
  inherit one chromosome of each pair from our
  mother and the other from our father

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Meiosis

• The 46 chromosomes in the human cell consists of
  23 pairs of homologous chromosomes
• Homologous chromosomes are similar, but not
  identical they may carry different versions of
  the same genetic information
• For example, one chromosome may code for blond
  hair, while the other codes for dark hair or
  both may contain the same gene (ex. Blue eyes)

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Meiosis

• Human cells contain 22 pairs of autosomes, and 1
  pair of sex chromosomes (X and/or Y)

Chromosome 1 is the largest containing 8000 genes
Chromosome 21 is the smallest containing only
300 genes
Sex chromosomes
http//www.sciencemuseum.org.uk/exhibitions/genes/
153.asp
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Meiosis

• Meiosis is a special type of cell division that
  will produce cells containing half the number of
  chromosomes
• Cells containing half the number of chromosomes
  are sex cells, or gametes
• Gametes contain a single set of chromosomes and
  are considered haploid (half)
• All other cells containing 2 homologous sets of
  chromosomes is said to be diploid

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Meiosis

• For humans, the diploid number is 46
• Nearly all of our cells are diploid the
  exceptions are the gametes!
• Sexual reproduction allows a haploid sperm cell
  to fuse with a haploid egg cell during the
  process of fertilization producing a zygote
• The resulting zygote is diploid it has 2 sets of
  homologous chromosomes 1 from Mom, and 1 from
  Dad

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Meiosis

• Meiosis occurs only in reproductive organs
• During meiosis, a mother cell divides and
  produces 4 genetically distinct daughter cells
  which contain half the number of chromosomes as
  the other cell
• Why 4? This is because meiosis begins with
  mitosis! (insert UGH!!!s here.)

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Meiosis reduces the chromosome number from
diploid to haploid

• Just as a cell entering mitosis has duplicated
  its chromosomes, so too, does a cell entering
  meiosis (resulting in 92 chromatids)
• During prophase 1 (so called because it is the
  first cycle occuring during Meiosis 1 (out of 2))
  the process of crossing over occurs

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Crossing over

• Crossing over is the process by which aligned
  chromatids of homologous chromosomes exchange
  genetic segments resulting in a genetically-new
  chromatid
• The driving force of genetic diversity and
  evolution!
• Independent orientation of chromosomes in meiosis
  and random fertilization lead to varied offspring

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Crossing over
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Cell division

• In both mitosis and meiosis, the chromosomes
  duplicate only once, in the preceding interphase
• Mitosis replicates cells for growth, tissue
  repair and asexual reproduction and produces
  daughter cells genetically identical to the
  parent cell (diploid)
• Meiosis produces haploid cells that are
  genetically distinct from the parent cell

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Genetic diversity

• Changes in an organisms DNA create different
  versions of genes (and resulting characteristics)
• Reshuffling of these different versions during
  sexual
  reproduction
  produces
  genetic
  
  variation

http//www.duggarfamily.com/
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Genetic diversity

• For a human there are 23 chromosomes and 223
  combinations of chromosomes that meiosis can
  package into gametes
• 223 equals 8 million (possible combinations)!!!
• Each gamete you produce contains 1 of 8 million
  possible combinations inherited from your father
  and your mother
• The random fusion of egg and sperm will produce a
  zygote with any of 64 trillion (8 mil x 8 mil)
  combinations of chromosomes!!!

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• Occurs at 1 or more
  points along adjacent
  chromatids

• Points contact each other
• DNA is exchanged

http//www.accessexcellence.org/AB/GG/crossing.htm
l
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Homologous chromosomes carry different versions
of genes

• A pair of homologous chromosomes can bear 2
  different kinds of genetic information for the
  same characteristic

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Brown coat (C) black eyes (E)
Coat-color genes
Eye-color genes
C
E
Brown
Black
C
E
C
E
Meiosis
e
c
e
c
e
c
Pink
White
White coat (c) pink eyes (e)
Chromosomes of the four gametes
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Alterations of chromosome number and structure

• With 64 trillion possible combinations of
  chromosomes, what could possibly go wrong??!!??
• Chromosome number abnormalities do occur and are
  often fatal
• An additional copy of chromosome 21 (the short
  one) results in Downs syndrome (also known as
  trisomy 21)

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Alterations of chromosome number and structure

• Individuals with Downs syndrome exhibit
  distinctive features flattened nose bridge,
  short stature, heart defects, and a shortened
  life span
• The additional chromosome usually comes from the
  mother (with a risk of 1 with pregnancy after
  age 40)
• Why?

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Alterations of chromosome number and structure
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Abnormal numbers of sex chromosomes

• Alteration in the number of copies of sex
  chromosomes are not lethal

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Abnormal numbers of sex chromosomes

• The Y chromosome is very small and carries
  relatively few genes
• What about the X chromosome? Its big and it does
  carry a lot of genes
• In females, the extra X chromosome is inactivated
  since the presence of this additional chromosome
  would otherwise be fatal!

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Abnormal numbers of sex chromosomes

• The inactivation of X chromosomes is random
  results in a random expression of genes!

www.flickr.com/photos/mayason/3194660130/
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Alterations of chromosome structure can cause
birth defects

• Other errors can occur involving deletion or
  duplication of chromosome structure
• Such chromosomal changes present in sperm and egg
  can cause congenital disorders
• Such changes in a somatic cell may contribute to
  cancer (not inheritable) why damage to our DNA
  may cause cancer (radiation, UV, etc.)