Unit 8

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Cell Cycle

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Cell Cycle

The sequence of growth & division in a cell

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Cell Cycle Interphase

  • The cell most of its life in interphase

    • During this phase, the cell carries out normal functions

    • DNA also replicates in preparation for division

  • Not apart of mitosis but it helps get the cell ready

    • DNA in this phase is called chromatid

  • Divided into 3 phases

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Cell Cycle Interphase Phases

G1 Phase, S Phase, G2 Phase

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G1 Phase

  • The cell grows/increases in size

  • New proteins & organelles are made

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S Phase

  • The cells DNA is copied (DNA replication occurs)

  • By the end, each chromosome will consist of 2 sister chromatids

  • Where cell spends majority of its life in

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G2 Phase

  • Shortest part of interphase

  • Cell continues to grow and prepares for nucleus to divide

  • New proteins are made, including microtubules used during division

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Cell Cycle Mitosis (M phase)

  • The nucleus divides

  • Duplicated chromosomes split and are distributed to daughter cells

  • Includes several sub phases

    • Prophase, Metaphase, Anaphase and Telophase

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Cytokinesis

  • The cytoplasm divides

  • Usually begins before mitosis is complete

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Control of Cell Cycle

and Mitosis Cells have a “control system” consisting of certain proteins that trigger & coordinate the key events in the cell cycle, like a stoplight system

  • The cell cycle has key checkpoints

  • Signals from the cell at these checkpoints can either…

    • Trigger the next phase of the cell cycle

    • Delay the next phase to allow for completion of the current phase

  • There are 3 main checkpoints

    • G1 checkpoint, G2 checkpoint and Mitosis checkpoint

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G1 Checkpoint

  • Decides if cell divides

  • If conditions are favorable & cell is healthy & large enough, proteins signal the cell to move into the S phase & copy the DNA

  • If not, cells stop here until they are ready or to rest

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G2 Checkpoint

  • Checks DNA replication

  • If passed, proteins help to trigger mitosis & start that process

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Mitosis Checkpoint

  • Triggers exit from mitosis

  • Signals beginning of G1 phase & cell cycle starts over

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If Control is Lost

  • Certain genes contain the info to make the proteins that control the cell cycle

  • If one of these genes mutates, the control protein may not function & regulation of the cell cycle can be disrupted

  • This can result in cancer

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Cell Division Allows For

  • Replacement of old cells

  • Repair to damaged tissue

  • Growth within an organism

  • Maintenance of small size for sa : v ratio

  • Creation of entirely new organisms

  • When a cell divides the DNA is copied through DNA replication

  • Cell division/mitosis is how each new cell ends up with a copy of the DNA

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Asexual Reproduction

One cell divides to produce genetically identical offspring

  • Simplest & most primitive method of reproducing

  • Allows organisms to produce many offspring a short period of time without using energy to produce gametes or find a mate

  • Went through by prokaryotic cells

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Binary Fission

A form of asexual reproduction

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Steps of Binary Fission

  • DNA replicates

  • Chromosomes separates

  • Cell splits (cytokinesis)

  • Results in 2 genetically identical daughter cells

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Sexual Reproduction

Genetic material from 2 separate organisms combine to produce genetically unique offspring

  • Genetic variations allows for adaptations to happen to help survive the enviorment

  • Eukaryotic cells go through this

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All organisms that go through sexual reproduction have…

An even number of chromosomes, which is true for all body cells

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Somatic Cells

Body cells

  • Diploid

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Human Chromosomes

  • Humans have 46

  • Each human body cell has 2 copies of 23 different chromosomes

  • The 23 pairs are called “homologous chromosomes”

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Homologous Chromosomes

Chromosomes that have the same size, shape, and genes

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Diploid

A cell containing 2 sets of chromosomes ( a homologous pair)

  • Usually abbreviated as 2n

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Gametes

Reproductive cells (egg & sperm)

-Haploid

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Haploid

Contains only 1 set of chromosomes

  • Usually abbreviated as n

  • Human haploid number is 23

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Autosomes

Chromosomes that code for most traits

  • 22 out of 23 pairs of chromosomes are this

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Sex Chromosomes

Chromosomes that determine genetic sex of an individual (x & y chromosomes)

  • 1 of the 23 pairs of chromosomes are this

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Genes

A section of a chromosome that codes for a particular trait

  • What chromosomes divide into

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The term chromosomes can be used to refer to..

An unduplicated piece of DNA or a duplicated one

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What form are chromosomes in most of the time

Unduplicated form

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What happens before cell division

The coil duplicates all the chromosomes (DNA Replication)

  • Each chromosomes now consist of sister chromatids, which are attached at the centromere

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Sister Chromatids

A pair of identical chromosomes

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Centromere

The region of the chromosome that holds 2 sister chromatids

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Chromatid Separation

  1. Chromatids physically move to opposite sides of the dividing cell

  2. Microtubules attach to the centromeres of the sister chromatids & the centrosomes at the poles, the chromatids can be separated

    1. Once chromatids separate they are called chromosomes

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Chromatid movement is controlled by what ?

Spindles

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Spindles

A framework of microtubules & centrioles involved in moving the chromosomes

  • Grows from 2 centrosomes

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Centrosomes

Regions of cytoplasmic material that organize the assembly of the spindles

  • Contain a pair of centrioles

    • Only found in animal cells

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Chromatin

Combo of DNA & proteins in long, thin fibers

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Phases of Mitosis

  1. Prophase

  2. Metaphase

  3. Anaphase

  4. Telophase

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Prophase

  • Chromatin condenses into chromosomes & become visible

  • Nucleolus & nuclear envelope disappear

  • Spindles form & centrioles move to opposite ends of the cell (poles)

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Metaphase

  • Chromosomes move to the middle of the cell & line up on the metaphase plate/equator

  • Chromosomes attach to spindle fibers at the centromeres

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Anaphase

  • Sister chromatids separate (now considered chromosomes)

  • Spindle fiber shorten & move chromosomes to opposite ends of the cell

  • Centrioles divide

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Telophase

  • Chromosomes reach opposite ends of the cell & start to uncondense (reform into chromatin)

  • Nuclear envelope reforms around each group of chromosomes

  • Spindle fibers break down

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Cytokinesis

  • Cytoplasm is divided in 1/2

  • Cell membrane forms to enclose around the cell

  • Results in 2 daughter cells

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Cytokinesis in Animal Cells

  • Has flexible membrane that can pinch cytoplasm off

    • Forms a cleavage furrow

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Cytokinesis in Plant Cells

  • Forms a cell plate

Cell plate = Disk controlling cell wall material, eventually thickens & divides the 2 daughter cells

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Cancer

Uncontrollable growth of cells

  • Begins when a single cell undergoes changes that convert it to a cancer cell

    • Most of the time these cells are destroyed by the bodies immune system

  • If the cell avoids destruction, it can multiply and form a tumor

  • If cancer cell spreads they can form secondary tumars

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2 Types of Tumors

Benign Tumor and Malignant Tumor

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Benign Tumor

A mass of abnormally growing cells that remain at their original site in the body

  • Can cause problems if they grow and disrupt certain organs, but can be removed with surgery

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Malignant Tumor

A mass of abnormally growing cells that spread into neighboring tissue

  • People with this are said to have cancer

  • Cancer cells can split off from the tumor & travel to other parts of the body, this is known as metastasis

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Metastasis

The spread of cancer cells beyond their original site

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Reproduction

The process of producing offspring

  • Can be asexual or sexual

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Clones

Organisms that are genetically identical to the parent

  • Product of asexual reproduction

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Downside to Asexual Reproduction

A genetically identical population may not be able to adapt to new environments

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How Is The Diploid Number Restored

The nucleus of a haploid gamete from the father joins with the nucleus of a haploid gamete from the mother

  • This known as fertilization

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Fertilization

The fusion of nuclei & cytoplasm from gametes

  • produces a zygote

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Zygote

The fertilized egg; it is diploid

  • Begins to divide by mitosis

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Meiosis

A form of cell division that halves the number of chromosomes when forming reproductive cells like gametes

  • Involves 2 rounds

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2 Rounds of Meiosis

  1. Meiosis I

  2. Meiosis II

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Interphase in Meiosis

  • Same as in mitosis

  • Cells grow

  • DNA replicates

    • This is the only time in meiosis that DNA replication occurs

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Prophase I

  • The first stage of meiosis

  • Chromosomes condense

  • Nuclear membrane breaks down

  • Crossing over occurs

  • Homologous chromosomes form tetrads

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Metaphase I

  • Tetrads move to the center of the cell & lineup on the metaphase plate

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Anaphase I

  • The tetrads / homologous chromosomes separate & move to opposite ends of the cell

  • ** Sister chromatids stay together

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Telophase I & Cytokinesis

  • Chromosomes arrive at the poles

  • Each side is now haploid because it only has 1 set of chromosomes, even if they are duplicated / sister chromatids

  • Cytoplasm splits & forms 2 haploid daughter cells

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Tetrads

The pair of homologous chromosomes made up of 4 chromatids

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Crossing Over/Synapsis

When portions of the chromatids are exchanged

  • (Segments of mom's chromosomes break off &

swap with the matching portions of dad's

chromosome)

  • This is how we get genetic variation / diversity

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Prophase II

  • Spindles form & attach to centromeres of sister chromatids

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Metaphase II

  • Sister chromatids line up in the middle of the cell

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Anaphase II

  • Sister chromatids separate(now called chromosomes.unduplicated)

  • Move to opposite sides of the cells

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Telophase II & Cytokinesis

  • Haploid daughter cells

  • Chromosomes reach the opposite sides of the cells

  • New nuclear membranes form & spindles break down

  • Cytoplasm splits

  • Forms 4 genetically different haploid daughter cells

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2 Types of Gamete Formation

  • Spermatogenesis

  • Oogenesis

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Spermatogenesis

The process of creating sperm - the male gamete

  • Results in 4 equally sized haploid gametes

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Oogenesis

  • The process of creating eggs - the female gamete (Eggs are AKA: ovum)

  • As meiosis & cytokinesis occur, the cytoplasm does not divide evenly

    • This results in 1 large egg cells & 3 small cells known as polar bodies (the polar bodies are neverused; the larger egg is what is stored in the ovaries & can be fertilized)

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Ways Genetic Variation is Created

  1. Independent Assortment

  2. Crossing Over

  3. Random Fertilization

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  1. Independent Assortment

  • The random distribution of homologous chromosomes during meiosis

  • Which of the 2 chromosomes an offspring receives from the 23 pairs is a matter of chance, like the flip of a coin

  • Each of the 23 pairs separate independently, producing about 223 combos (about 8 million!)

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  1. Crossing Over

Exchange of genetic material can produce a chromosome that contains a new combo, totally different from either parent

  • Because we have hundreds of different genes, a single crossover can affect many genes at once

  • More than 1 cross over can occur per tetrad,

    so the variation is practically endless!

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  1. Random Fertilization

  • Fertilization of an egg by any given sperm is totally random

  • The number of possible outcomes is squared

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Mutations

Any change in the nucleotide sequence of DNA

  • Mutations may or may not lead to changes in the proteins coded for by the affected genes

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Beneficial Mutations

They are the source of genetic variability in species and can make those organisms better suited to their environment

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Examples of Benefical Mutations

Bone density, seeing color in females, sickle cell anemia

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Harmful Mutations (Deleterious)

They can cause drastic changes in protein structure

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Examples of Harmful Mutations

Cystic fibrosis, hemophilia, Down’s Syndrome, sickle cell anemia

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Neutral Mutations

They do not cause change in the protein or chromosomes or result in any negative effects

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Somatic Cell Mutations

When a mutation occurs in the somatic cells and affects the organism in which they occur

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