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 organism’s 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
doesn’t 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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Centromere
Chromosomeduplication
Sister chromatids
Chromosomedistribution
todaughter
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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S(DNA synthesis)G1
G2
Cytokinesis
Mito
sis
INTERPHASE
MITOTICPHASE (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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Metaphaseplate
METAPHASE ANAPHASE
DaughterchromosomesSpindle
During mitosis, each chromosome has been replicated consisting of 2 sister chromatids; these chromosomes align and separate during metaphase and anaphase, respectively
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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
Sister chromatids One duplicatedchromosome
Homologous pair ofchromosomes • 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
From Mother From Father
Homologous pair ofchromosomes • 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)
http://www.sciencemuseum.org.uk/exhibitions/genes/153.asp
Chromosome 1 is the largest;
containing 8000 genes
Chromosome 21 is the smallest;
containing only 300
genes
Sex chromosomes
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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 organism’s 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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http://www.accessexcellence.org/AB/GG/crossing.html
• Points contact each other
• DNA is exchanged
• Occurs at 1 or more points along adjacent chromatids
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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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Coat-colorgenes
Chromosomes ofthe four gametes
Meiosis
PinkWhite
BlackBrown
Eye-colorgenes
C
e
E
c
C
e
E
c
C
e
E
c
White coat (c); pink eyes (e)
Brown coat (C); black eyes (E)
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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 Down’s syndrome (also known as trisomy 21)
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Alterations of chromosome number and structure
• Individuals with Down’s 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? It’s 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.)