Cellular Reproduction

In eukaryotes the DNA strand is wound up in a tight coil called chromosome

• Chromosomes are condensed structures that contain DNA visible during mitosis.

• Chromatin is a mixture of proteins and DNA that forms chromosomes

• Nucleosomes are basic packing unit of DNA built from histone proteins around it and DNA coiled and are grouped together into chromatin fibers

  • important for how proteins gain access to DNA-binding sites located within the nucleosomes

  • fundamental unit of chromatin

    • HUMAN CELLS HAVE 46 CHROMOSOMES

Properties of Chromosomes

• contains cell’s genetic code

• It has a matching pair called homologous pairs

The Cell Cycle

• A cycle that cells reproduce by growing and dividing

• Importance of the cell cycle

  • allows your body to grow and heal certain injuries

• Duration of the cell cycle: 8 minutes in eukaryotic cells while other cells take one year

• Three main stages of the cell cycle

  • Interphase

  • Mitosis

    • the cells divide into two identical cells

    • somatic cell division

  • Cytokinesis

• Types of Cell Division

  • Bacteria(Prokaryotes)

    • Binary fission

Interphase: the stage during which the cell grows, develops into a mature, functioning cell, duplicates DNA in its nucleus and prepares for division.

• each chromosome copies itself

Divided into three stages:

1. Gap 1(G1)

2. Synthesis(S)

3. Gap 2(G2)

G1 Phase:

• known as the growth phase

• occurs after the cell divides

• chromosomes uncoils into U chromatin which is used to produce RNA

• produce the materials needed by the cell such as protein

• increases in size; doubles in size

• Restriction Point: some cells such as muscle and nerve cells exit the cell cycle (G not phase, organelles that does not wish to divide, some cells spend majority of their time in the resting phase - nerve cell)

Synthesis(S)

• Replication Phase

• produces a small amount of protein and organelles

• focuses on DNA replication

• Replicated chromosome and original chromosome are joined together at the centromere

  • the centromere is important because it ensures that a copy of the replicated DNA will become part of the daughter cells

• 46 original chromatids and 46 replicated chromatids to make a total of 92 chromatids

G2 Phase

• Makes sure the cell is fully prepared for mitosis

• Continues producing organelles and proteins to make sure it’s enough

• Contains a checkpoint at the end: the cell checks for the level of protein known as Mitosis Promoting Factor/Maturation promoting factor

• If the concentration level is high enough, the cell enters Mitosis

Mitosis: the 2nd stage of the cell cycle where a single cell divides into two identical daughter cells

• mistakes made during mitosis can lead to changes in DNA and genetical disorders

Importance of Mitosis:

1. to replace worn out cells

2. New cells are produced when an injury occurs

3. important for maintaining chromosome number in organisms that undergo asexual reproduction.

4 stages of mitosis

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

Prophase

• The cell’s chromatin condenses to form chromosomes (shaped like an X)

• Half of the X is called a sister chromatid

• 

• Spindle Fiber+Centrioles+Aster Fibers= Spindle apparatus

  • Important in moving and organizing the chromosomes before cell division

• Centrioles begin to move towards opposite end of the cells, the spindle fibers began to grow

• The nuclear membrane starts to disintegrates and nucleolus disappears

  • allows the spindle fiber to make way to the nucleus region of the cell to attach to special region of the centromere

Metaphase:

• Spindle fibers move the chromosomes to the center of the cell; they align them on an equatorial line

• it ensures that the cell is ready to divide

Anaphase:

• known as disjunction

• Chromosomes break at the centromere

• The chromatids are pulled apart by the spindle fiber by breaking the centromere

• Each of the chromatid has a unique centromere region, begin to move to opposite ends

• Results in separation of chromosomes.

Telophase:

• chromosomes are moved to opposite end, containing an identical set of chromosomes

• the nuclear membrane begins to reform on both sides, developing the nuclear membrane and nucleolus

  • these encloses the two sets of identical chromosomes

• Spindle fibers deteriorate and chromosomes decondenses

Cytokinesis:

• the breaking of the cytoplasm

• the cell membrane begins to divide

• organelles are distributed between two cells equally

• Chromosomes condenses into chromatin

• The daughter cells are smaller than the original cell

Animal Cells:

1. microfilaments are used to pinch the cytoplasm; occurs in the furrow

Plant Cells:

1. a new structure called a cell plate is formed between the two daughter nuclei

2. Cell walls form on either side of the cell plate.

Cell Cycle Regulation

Three Major Checkpoints

1. End of G1 : Is the DNA damaged?

2. End of G2: Has all DNA been replicated?

3. End of S: Is the DNA replicating correctly

4. Mitosis: Are chromosomes properly attached to spindles?

The checkpoints are regulated by:

Cyclin/CDK combinations control different activities at different stages in the cell cycle

1. Cyclones

   1. proteins that bind enzymes called Cyclin-dependent kinases

2. CDK

   1. phosphorylate proteins either activating or deactivating them

   2. are kinases

3. Tumor suppressors

   1. P53

   2. can activate several pathways including cell cycle arrest when an DNA is damaged
      

Meiosis & Sexual Reproduction

Gene- The DNA on chromosomes arranged in regions, codes the information for proteins

Homologous chromosomes- one chromosome from each parent, pair of chromosomes that are similar length and have a similar gene positioning

Gametes- sex cells that have half the number of chromosomes

Fertilization- process by which one haploid gamete combines with another haploid gamete to form a zygote

Haploid- a cell with n number of chromosomes

• the symbol n can be used to represent the number of chromosomes in a gamete

Diploid - a cell that contains 2n number of chromosomes

Sex chromosomes- determines the individual’s sex, one pair of chromosome

Autosomes- the other 22 pairs of chromosomes

Meiosis- a type of cell division that reduces the number of chromosomes to form gametes, referred to as reduction division

Meiosis

• occurs in the reproductive structures of organisms that reproduce sexually

• forms haploid gametes or spores

• reduces the chromosome number by half through the separation of homologous chromosomes

• Gametocytes

  • Males- Spermatocyte

  • Females - Oocytes

• Forms four haploid cells

• Tetrad - group of sister chromatids

Meiosis I

Before Meiosis takes place: DNA is replicated through homologous pair of chromosome that codes for the gene of a certain aspect.

Diploid cell to two haploid cells

Prophase I

• Spindle fibers are formed by two centrioles

  • thy bind to sister chromatids at the centromere

• Pairing of homologous chromosome occurs

  • they begin to condense in a process called synapsis

• The nuclear envelope breaks down

• Cross over occurs to increase genetic variation

  • they exchange fragments

Metaphase I

• Chromosomes line up at the equatorial line

  • forms the metaphase plate

• Centromeres attach to the spindle fibers

Anaphase I

• The chromosomes start getting pulled apart

  • the sister chromatids stay together

• The homologous pairs get pulled apart

  • guided by the spindle fibers to move towards opposite poles of the cell

• The chromosome number is reduced from 2n to n

Telophase I

• Homologous pairs are fully split apart

• The nuclear membrane begins to form again

• The spindles break down

• Chromosomes uncoil and form two nuclei

• Separates into two cells and is divided into a cleavage furrow

Result of Meiosis I: Two haploid cells that contain one duplicated copy of each homologous chromosome pair.

Meiosis II

Before Meiosis II, the cells undergo interkinesis(a period of rest)

• no DNA is replicated

Prophase II

• The two haploid cells produced undergo their own processes

• Spindle fibers start to form, extends toward the chromosomes

• Chromosomes condense

• nuclear membrane starts to break apart

Metaphase II

• Sister chromatids line up on the metaphase plate.

• The chromosomes are connected to the centriole poles at the kinetochores of sister chromatids through the microtubules.

Anaphase II

• The spindle fibers pull on both ends, separating the chromatids

• Splitting of the centromere of the chromosome

• The kinetochore microtubules shorten

Telophase II

• Chromosomes decondenses after arriving at opposite poles

• Nuclear Envelope Forms around the chromosomes

Cytokinesis

• Separates the two cell into four unique haploid cells after dividing the cytoplasm