Unit 8

Cell Cycle

The sequence of growth & division in a cell

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

Cell Cycle Interphase Phases

G1 Phase, S Phase, G2 Phase

G1 Phase

* The cell grows/increases in size
* New proteins & organelles are made

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

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

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

Cytokinesis

* The cytoplasm divides
* Usually begins before mitosis is complete

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

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

G2 Checkpoint

* Checks DNA replication
* If passed, proteins help to trigger mitosis & start that process

Mitosis Checkpoint

* Triggers exit from mitosis


* Signals beginning of G1 phase & cell cycle starts over

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

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 

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

Binary Fission

A form of asexual reproduction

Steps of Binary Fission

* DNA replicates
* Chromosomes separates
* Cell splits (cytokinesis)
* Results in 2 genetically identical daughter cells

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

All organisms that go through sexual reproduction have…

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

Somatic Cells

Body cells

* Diploid

Human Chromosomes

* Humans have 46
* Each human body cell has 2 copies of 23 different chromosomes
* The 23 pairs are called “homologous chromosomes”

Homologous Chromosomes

Chromosomes that have the same size, shape, and genes

Diploid

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

* Usually abbreviated as 2n

Gametes

Reproductive cells (egg & sperm)

\-Haploid

Haploid

Contains only 1 set of chromosomes

* Usually abbreviated as n
* Human haploid number is 23

Autosomes

Chromosomes that code for most traits

* 22 out of 23 pairs of chromosomes are this

Sex Chromosomes

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

* 1 of the 23 pairs of chromosomes are this

Genes

A section of a chromosome that codes for a particular trait

* What chromosomes divide into

The term chromosomes can be used to refer to..

An unduplicated piece of DNA or a duplicated one

What form are chromosomes in most of the time

Unduplicated form

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

Sister Chromatids

A pair of identical chromosomes

Centromere

The region of the chromosome that holds 2 sister chromatids

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

Chromatid movement is controlled by what ?

Spindles

Spindles

A framework of microtubules & centrioles involved in moving the chromosomes

* Grows from 2 centrosomes

Centrosomes

Regions of cytoplasmic material that organize the assembly of the spindles

* Contain a pair of centrioles
* Only found in animal cells

Chromatin

Combo of DNA & proteins in long, thin fibers

Phases of Mitosis

1. Prophase
2. Metaphase
3. Anaphase
4. Telophase

Prophase

* Chromatin condenses into chromosomes & become visible
* Nucleolus & nuclear envelope disappear
* Spindles form & centrioles move to opposite ends of the cell (poles)

Metaphase

* Chromosomes move to the middle of the cell & line up on the metaphase plate/equator
* Chromosomes attach to spindle fibers at the centromeres

Anaphase

* Sister chromatids separate (now considered chromosomes)


* Spindle fiber shorten & move chromosomes to opposite ends of the cell
* Centrioles divide

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

Cytokinesis

* Cytoplasm is divided in 1/2


* Cell membrane forms to enclose around the cell
* Results in 2 daughter cells

Cytokinesis in Animal Cells

* Has flexible membrane that can pinch cytoplasm off
* Forms a cleavage furrow

Cytokinesis in Plant Cells

* Forms a cell plate

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

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

2 Types of Tumors

Benign Tumor and Malignant Tumor

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

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

Metastasis

The spread of cancer cells beyond their original site

Reproduction

The process of producing offspring

* Can be asexual or sexual

Clones

Organisms that are genetically identical to the parent

* Product of asexual reproduction

Downside to Asexual Reproduction

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

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

Fertilization

The fusion of nuclei & cytoplasm from gametes

* produces a zygote

Zygote

The fertilized egg; it is diploid

* Begins to divide by mitosis

Meiosis

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

* Involves 2 rounds

2 Rounds of Meiosis

1. Meiosis I
2. Meiosis II

Interphase in Meiosis

* Same as in mitosis
* Cells grow
* DNA replicates
* This is the only time in meiosis that DNA replication occurs

Prophase I

* The first stage of meiosis


* Chromosomes condense
* Nuclear membrane breaks down
* Crossing over occurs
* Homologous chromosomes form tetrads

Metaphase I

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

Anaphase I

* The tetrads / homologous chromosomes separate & move to opposite ends of the cell
* \*\* Sister chromatids stay together

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

Tetrads

The pair of homologous chromosomes made up of 4 chromatids

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

Prophase II

* Spindles form & attach to centromeres of sister chromatids

Metaphase II

* Sister chromatids line up in the middle of the cell

Anaphase II

* Sister chromatids separate(now called chromosomes.unduplicated)


* Move to opposite sides of the cells

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

2 Types of Gamete Formation

* Spermatogenesis
* Oogenesis

Spermatogenesis

The process of creating sperm - the male gamete

* Results in 4 equally sized haploid gametes

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)

Ways Genetic Variation is Created

1. Independent Assortment 
2. Crossing Over
3. Random Fertilization

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!)

2. 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!

3. Random Fertilization

* Fertilization of an egg by any given sperm is totally random
* The number of possible outcomes is squared

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

Beneficial Mutations

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

Examples of Benefical Mutations

Bone density, seeing color in females, sickle cell anemia

Harmful Mutations (Deleterious)

They can cause drastic changes in protein structure

Examples of Harmful Mutations

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

Neutral Mutations

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

Somatic Cell Mutations

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

Germ Cell Mutations

When a mutation is present in an organisms gametes

* Can be passed on to offspring

Mutagens

Causes mutations

* A chemical or physical agent that changes DNA
* Examples: UV rays, X-rays, chemicals in cigs, viruses or bacteria

Genetic (Gene) Mutations

Mistakes in the nucleotide sequence/bases during DNA replication or protein synthesis

Chromosomal Mutations

Mistakes in the structure and number of chromosomes caused during mitosis or meiosis

* Can involve lots of genes

Genetic Alterations

Results from mutations that change a gene

* Usually results in the placement of the wrong amino acids during protein synthesis
* Includes point mutations

Point Mutations

A change in a single nucleotide

* Generally involve a substitution

Substitution

One base is changed to another usually affects a single amino acid

* Generally the same thing as point mutations

Types of Point Mutations

Missense Mutation, Nonsense Mutation, Silent Mutation, Frameshift Mutations

Missense Mutation

Changes one amino acid into another

Nonsense Mutation

Changes one amino acid codon to a stop codon

* This will prematurely end the protein and it will probably not function properly

Silent Mutation

Changes one nucleotide, but does not result in a change in the amino acid

* These mutations don’t matter because the resulting protein does not change

Why Are Frameshift Mutations a Problem

Because mRNA is read as a series of triplets during translation, adding or subtracting nucleotides can alter the group

* All nucleotides after the insertion or deletion will be regrouped into different codons
* These will produce non-functional proteins & often have disastrous effects

Frameshift Mutations

Occurs when the number of nucleotides inserted or deleted is not a multiple of 3

2 Types of Frameshift Mutations

* Insertion
* Deletion

Insertion

Nucleotides are added

Deletion

Nucleotides are removed