Topic 2: DNA & Cell Division

Structure and Function

• Inside the nucleus of almost all the cells in our body contain chromosomes

• Chromosomes are made of DNA - our genetic information

  • DNA stands for DeoxyriboNucleicAcid

DNA function - 2 functions

1. Pass on information to offspring

2. Production of proteins

DNA structure → Spiral Staircase = Double Helix

• Structure: 2 Complementary strands(2 sides)

• Backbone: Alternating deoxyribose sugar and phosphates

• Rungs: 2 Nitrogen bases

  • Adenine - Thymine

  • Guanine - Cytosine

  • Nitrogen bases are like the instructions in a recipe, therefore different instructions = different products

• Nucleotide: basic building block of DNA

  • made up of:

    • One sugar

    • One phosphate

    • One nitrogen base

  • DNA is made up of many nucleotide monomers

Protein Synthesis

Gene - small sections of a chromosomes(DNA) that carries instructions for a specific protein

• A gene is like a recipe in a cookbook

Composition of Proteins

• Proteins are made up of amino acids linked together in a chain

  • There are 20 different amino acids

• Different proteins are made of different amino acids

Protein Synthesis

• Proteins make up 50% of body mass and play an important role in the body

DNA → Amino Acid

• Each amino acid is coded for by 3 nitrogen bases on a DNA molecule called a triplet

• To find the amino acid chain produced, use the strand that is given

  • It is not necessary to find the complementary strand

Genetic Engineering

• Genetic engineering - Modification of genetic material where scientists isolate genes from one organism and insert it into the DNA of another organism using a virus(vector)

Application of Genetic Engineering

Pros and cons of genetic engineering

• Pros

  • Treat genetic disorders

  • Create crop resistant to pesticide and certain weather

  • Therapeutic cloning - decrease of organ donor rejections

• Cons

  • Is it ethical?

    • GMO deformities

    • Reproductive cloning

Introduction to Cell Division

• Cells divide for 2 reasons:

1. Mitosis: 2n→2n

• Growth, maintenance, and repair

2. Meiosis: 2n→n

• Gamete formation

• Diversity

Chromosomes

• Our DNA is stored in strands called chromosomes

• Humans have 46 individual chromosomes or 23 homologous pairs

  • In each pair, one chromosome comes from each parent(mom and dad)

• The first 22 pairs are known as Autosomes

• The last pair are sex chromosomes

  • Basic females have xx chromosomes

  • Basic males have xy chromosomes

Diploid and Haploid

• Diploid cell - 2n

  • Total number of chromosomes in every body(somatic) cell

  • A diploid cell has both pairs of homologous chromosomes

  • In humans: diploid cells have 46 chromosomes

• Haploid cell - n

  • Total number of chromosomes in every gametes(sperm and egg)

  • A haploid cell has only one chromosome from a homologous pair

  • In humans: haploid cells have 23 chromosomes

DNA replication

• DNA replication necessary for cell division because it creates 2 identical copies

  • ensures there is a correct number of chromosomes when the cell divides

• DNA replication is know as semiconservative replication

• Every chromosomes must be replicated so that they look like X’s

Process of DNA replication

1. The DNA double helix shape untwist and unzip

• H bonds break

2. Free nucleotides come and pair up with exposed nitrogen bases

• Mutations are most prone to happen here

3. Retwisting of DNA

• Old strands are paired with new strands creating 2 new double helix shapes that are semi-conservative(half new, half old)

4. Duplicated/replicated chromosomes

Mitosis

IPMAT → I Pee on a MAT

• Interphase

  • DNA replication occurs

  • DNA is not visible

    • In chromatin form(looks like noodles)

  • Centrioles, nucleus, and nucleolus are present

• Prophase

  • Replicated chromosome are present

    • visible as an X

  • Nuclear walls are disappearing

  • Spindle fibers form

• Metaphase → “middle”

  • Replicated chromosomes line up in the middle of the cell(equatorial plate)

  • Spindle fibers attach to centromere

• Anaphase → “Away”

  • Centromeres split and daughter chromosomes are pulled to opposite poles

• Telophase → “Two”

  • Division of the cytoplasm

    • 2 cells form

  • Chromosomes lengthen to form chromatin

  • Nuclear walls reappear

Asexual reproduction - cloning

• When a organism is able to reproduce through mitosis

• Cloning usually occurs when environmental conditions are stable/favorable as offspring are identical to their parent

• Pros

  • Quick

  • No partner needed

• Cons

  • No genetic variation = limited gene pool

  • Mutations are pass on

Meiosis

• A type of cell division that makes haploid cells called gametes

  • 2n → n

  • Gametes include

    • Sperm(formed in the testes)

    • Egg(formed in the ovaries)

• Haploid cells are necessary for sexual reproduction because it ensures that the embryo has the correct number of chromosomes when 2 haploid gametes fertilize

• Synapsis(joining of homologous pairs) and crossing over allow pieces of DNA to be exchanged between chromatids

  • Allows for genetic variations

    • Therefore, daughter cells are not genetically identical in meiosis

  • Significant pro for crossing over is that it increases genetic diversity where a population can be less prone to diseases.

Gametogenesis → meiosis in humans

• Spermatogenesis - Sperm productions

  • occurs in testes

  • produces 4 haploid sperm

• Oogenesis - egg production

  • Occurs in ovaries

  • Produced 1 haploid egg and 3 polar bodies

Mutations

• Mutations is a mistake in replication that happens naturally or are caused by outside factors

  • outside factors ex)chemicals or radiation

• Mutations can cause genes to stop working or to function differently

• Mutations can be passed on

Types of Mutations

1. Chromosomal Mutations

• When there are changes in structure or number of chromosomes

• Can be diagnosed by a karyotype

  • Organized by size, banding pattern, and centromere position

  • Cells are collected by amniocentesis or chorionic villus sampling

2. Point/Gene Mutations

• When there are changes in part of a DNA strand    

  • occurs in DNA replication

• Can affect the amino acids and proteins produced

• Types of point mutations

  • Addition or Deletion of 1 or 2 nitrogen bases to a DNA sequence

    • Often has more sever consequences as it changes the rest of the sequence

  • Substitution of 1 or 2 nitrogen bases for another

Antibiotic Resistance

• Bacterial infections are usually treated with antibiotics

  • However, over time bacteria may become resistant

• There are 3 main ways this occurs

1. Mutation and Natural Selection

• Random mutations can occur in genes that make bacteria resistant to antibiotics

• When bacteria reproduce, their mutated gene is passed on

• Increases bacteria’s survival

2. Transformations of DNA Fragments

• Process of transformation

  • When bacteria picks up pieces of DNA from their environment

  • The new DNA is then added to the bacterial chromosomes

  • This allows the bacteria to make the proteins from the new DNA

3. Transformations of Plasmids

• Plasmids are special small circular pieces of DNA in bacteria

• They have genes that are not necessary for regular function of the cell but often have extra genes like antibiotic resistance

• When bacteria come in contact with each other, they can exchange plasmids.

• This means that genes for antibacterial resistance can be shared