Unit 4

• DNA stands for

  Deoxyribonucleic Acid
• DNA is packaged by special proteins called histones
• Where is DNA found in the cell?

  • DNA id found in the mitochondria present in most plant and animal cells, as well as in the chloroplasts of cell walls
  • DNA molecules found in mitochondria and chloroplasts are small and circular, much like the DNA of typical bacterium. There are usually many copies of DNA in a single mitochondrion's or chloroplasts
• DNA structure and function

  • shape of a double helix
  • Codes for your genes (traits)
  • Made of repeating subunits call nucleotides
• Nucleotide structure

  has three parts:

  1. Phosphate
  2. Deoxyribose (sugar) 5 carbon
  3. Nitrogenous base (A,T,G,C)
• Nitrogenous bases

  Pyrimidine bases

  • cytosine (C)
  • Thymine (T)

  Purine bases

  • Adenine (A)
  • Guanine (G)
• Base - Pair rule

  Hydrogen bonds hold the pairs of nucleotides together

  • Adenine only pairs with Thymine (A-T)
  • Guanine only pairs with Cytosine (G-C)

  Why is this?

  • Two hydrogen between adenine (A) and Thymine (T)
  • Three hydrogen bonds between Cytosine (C) and Guanine (G)
• 5' and 3' strands of DNA

  • 5' and 3' mean "Five prime" and "Three prime"
  • Indicates the carbon numbers in the DNA's sugar backbone
  • This asymmetry gives a DNA strand a "direction"
• What is DNA replication?

  • The process by which DNA makes a copy of itself
  • It is semi-conservative which means that half of the parent DNA is used
• Where, When, Why

  Where? in the nucleus of eukaryotes

  When? during interphase specifically S-phase

  Why? To copy genetic material prior to cell division (mitosis). Daughter cells are identical to parent cell
• Step 1 - Unzipping

  The cell unwinds the DNA double helix by breaking the hydrogen bonds using the enzyme helicase. This exposes the nitrogenous bases, creating a fork replication.
• Step 2 - Complementary Base pairing

  • in every nucleus, there are free nucleotides floating around
  • Nucleotides line up according to the base pairing rules
  • The enzyme DNA polymerase links the nucleotides to form a new DNA strand
  • Two daughter strands are created
• Step 3 - Recoiling DNA Strands

  • The cell checks for errors
  • The sugar phosphate backbone is finished
  • Two DNA strands are recoiled to form two double helixes
• Direction of DNA replication

  • DNA polymerase moves along the parent DNA strand in a 3' to 5' direction and therefore adds nucleotides in a '5 to 3' direction
  • One stand can be replicated directly as it unzips (the leading strand) - in the same direction as the replication fork
  • The other strand needs to wait until a certain amount is unzipped (the lagging strand). Enzyme exonuclease removes primers and DNA polymerase fills in the gaps
• What is cell division?

  • New cells are formed when existing cells divide
  • Eukaryotes have two types of cell division - mitosis and meiosis
  • Mitosis is used for growth and repair and produces two identical daughter cells
  • Meiosis produces our sex cells (sperm and ovum) for sexual reproduction
• Mitosis - brief overview

  One parent cell two identical daughter cells -> Growth and repair
• Steps of Mitosis

  Interphase: resting

  Prophase: Chromosomes visible, spindle forms as centrioles move

  Metaphase: Chromosomes line up along equator

  Anaphase: Chromatids separate

  Telophase: Nuclear membrane forms on each side, cytokinesis begins
• Meiosis - brief overview

  One parent cell, 4 unique haploid cells -> reproduction
• What happens during interphase?

  During interphase DNA replication occurs. This happens before cell division begins
• Overview

  Meiosis 1 - PMAT

  Meiosis 2 - PMAT

  • The process of cell division that produces gametes (eggs and sperm)

  Involves two divisions:

  • First division divides the chromosomes number
  • Second division divides the remaining chromosomes (much like mitosis)

  
  

  • Four haploid cells are produced from one diploid cell
  • Each cell is unique with half the number of chromosomes of the original parent cell
• Meiosis 1

  Prophase 1: The Chromosomes condense and the nuclear envelope breaks down. Crossing over occurs

  Metaphase 1: pairs of homologous chromosome move to the equator of the cell

  Anaphase 1: Homologous chromosomes move to the opposite poles of the cell

  Telophase 1 and Cytokinesis: Chromosomes gather at the poles of the cells. The cytoplasm divides
• Meiosis 2

  Prophase 2: a new spindle forms around the chromosomes

  Metaphase 2: Chromosomes line up at the equator

  Anaphase 2: centromeres divide. Chromatids move to the opposite poles of the cell

  Telophase 2 and cytokinesis: a nuclear envelope forms around each set of chromosomes. The cytoplasm divides
• What are homologous pairs?

  • Sexually reproducing organisms inherit their genetic sequences from both parents
  • This means that these organisms will possess two copies of each chromosome (one of maternal origin; one of paternal origin)
  • These maternal and paternal chromosome pairs are called homologous chromosomes

  Homologous chromosomes are chromosomes that share:

  • The same structural features
  • The same genes at the same loci positions
• Chromosome structure

  Chromosomes can be distinguished by:

  1. Their relative size
  2. The position of the centromere
  3. The patterns of the light and dark bands
• Variation: Why are we all different?

  • Variation is achieved by crossing over (recombination) and independent assortment
• Independent Assortment

  • Gregor Mendel's law of independent assortment states that the alleles of two (or more) different genes get sorted into gametes independently of one another
  • This occurs during metaphase 1
  • Chromosomes line up at the equator of the cell in a random arrangement
• Crossing Over

  • During meiosis, genetic information can be exchanged between homologous chromosomes in a process called crossing over
  • Crossing over results in the recombination of alleles, ensuring every sperm and egg is unique
• Gametogenesis

  • Gametogenesis is the process by which diploid precursor cells undergo meiotic division to become haploid gametes (sex cells)
  • In males, this process is called spermatogenesis and produces spermatozoa (sperms)
  • In females, this process is called oogenesis and produces ova (eggs)
• Location of Spermatogenesis

  • spermatogenesis describes the production of spermatozoa (sperm) in the seminiferous tubules of the testes
• Process of Spermatogenesis

  • The process begins at puberty when the diploid cells called spermatogonia undergo mitosis and cell growth to form spermatocytes
  • The spermatocytes undergo two meiotic divisions to form four haploid daughter cells (spermatids) with equal amounts of cytoplasm
  • The spermatids then undertake a process of differentiation to become functional sperm cells called spermatozoa
  • The whole process takes approximately 70 days and is continuous
• Location of Oogenesis

  • only 300-400 secondary oocytes will be produced during a females reproductive lifetime
  • At birth, female ovaries contain approx 1 million precursors called oogonia which are able to develop into oocytes
• Oogenesis


  1. Diploid germ cells (oogonia) formed during foetal development

  2. Oogonia enter meiosis 1 and prophase 1 to become primary oocytes

  3. Primary oocytes pause at this step until puberty

  4. After mensuration, FSH triggers a few primary oocytes to complete meiosis 1

  5. Meiosis 1 results in one large secondary oocyte and one small polar body (which usually dies)

  6. Secondary oocyte becomes locked at metaphase 2 until ovulation

  7. If fertilized, the secondary oocyte will complete meiosis 2 and a secondary polar body is also produced
• Random fertilization

  • Random fertilization refers to the fact that if two individuals mate, and each is capable of producing over 8 million potential gametes, the random chance of any one sperm and egg coming together is a product of these two possibilities - some 70 trillion different combinations of chromosomes in a potential offspring

  Therefore genetic variation is achieved by -

  • Independent assortment
  • Crossing over
  • Random fertilization
• What are the similarities?

  • Both processes start with a precursor cell e.g., primary oocyte or a spermatocyte
  • Both processes take place in the reproductive organs
  • Both use meiosis 1 and 2 as a method of cell division
  • Both processes produce gametes/sex cells that are haploid
• Gene expression - Summary

  • The set of proteins expressed in a cell determine its phenotype and that of an organism.
• Gene Expression

  • the process by which the information encoded in a gene is used to direct the assembly of a protein molecule
  • Gene expression is controlled to only produce the proteins needed at a particular protein
• Genome and Gene

  Genome

  • All the genetic material in the chromosomes of an organism, including its genes and DNA sequences

  Gene

  • Region/s of DNA that are made up of nucleotides, the molecular unit of heredity
• Polypeptides and proteins

  • A polypeptide is a single linear chain of many amino acids held together by a peptide bands.
  • A protein consists of one or more polypeptides folded to form a 3- dimensional structure
• Making Proteins

  • Gene expression is the process by which the information in a gene is used to synthesis a protein
  • It involves two key processes called transcription and translation
• It is the exons which code for our proteins and hence determine our traits