BIO Honors U5

Unit 5: DNA, Protein Synthesis and Cell Division

DNA: a molecule that carries the genetic instructions

• Double helix

• Composed of nucleotides

• Each nucleotides contains a deoxyribose sugar, a phosphate group and a nitrogen base

  • Nitrogen bases can be adenine (A), thymine (T), cytosine © or guanine (g). Adenine only bonds to Thymine, Cytosine only bonds to Guanine

Bonds

Nucleotides: Covalent bonds

Base pairs: Hydrogen bonds

Chargaff’s rules:

• The amount of guanine should be equal to the amount of cytosine and the amount of adenine should be equal to the amount of thymine

• Therefore… A binds with T, and C binds with G

Purine: Adenine and Guanine

Pyrimidines: Cytosine and Thymine

Purine only binds with Pyrimidines because the diameter of DNA should be constant (3 rings)

DNA Replication

Why/When?

New cell must have complete genetic informations so DNA must be replicated

Steps

1. Unwind/unzip/separate ⇒ break H bonds between Nitrogen bases

2. Add complementary bases to each side  ⇒  form H bonds between Nitrogen bases

3. Connect the backbone  ⇒ form covalent bonds between sugars and phosphates

DNA Replication is considered semiconservative because half of the DNA is conserved.

All these steps are possible due to enzymes!

1. For split/unzip/separating -> Helicase

1. For adding complementary bases -> DNA Polymerase

2. For connecting the backbone -> Ligase

Making Proteins

DNA contains genes

Gene: a segment of DNA that code for a particular trait.

Each gene contains the directions to make one protein that gives you a particular trait.

Ex. Melanin: protein that determines your eye color.

DNA is located in the nucleus and cannot leave.

Proteins are assembled by free ribosomes in the cytoplasm or on the endoplasmic reticulum.

Because DNA is too large to go directly to ribosomes for protein production, it must be changed to RNA (Ribonucleic Acid) before it becomes a protein.

DNA vs. RNA

RNA is different from DNA in that:

• It has one strand instead of 2

• It has nitrogen base Uracil (U) instead of Thymine (T)

• It has sugar Ribose instead of Deoxyribose

3 Types of RNA

1. mRNA (Messenger RNA)

• Carries message from nucleus

• DNA cannot leave the nucleus so a temporary simplified copy of a gene is made

• Codon: 3 base pairs of mRNA

2. tRNA (transfer RNA)

• Transfers an amino acid to growing protein

• One end holds an amino acid

• One end has the anticodon

3. rRNA (ribosomal RNA)

• Makes up ribosomes

Steps of Protein Synthesis:

1. Transcription - a gene in DNA is “transcribed” into mRNA in the nucleus

Steps of Transcription

1. RNA polymerase “unzips” a gene in DNA

2. RNA polymerase attaches RNA nucleotides to the exposed DNA strand using the base pairing rules

3. RNA polymerase detaches at the end of the gene.

2. Translation - the segment of mRNA is “translated” into a amino chain (aka protein) on a ribosome

Steps of Translation

1. The mRNA (made by transcription) exits the nucleus and binds together with a ribosome.

2. A tRNA carrying methionine binds to the start codon

   1. Codon: set of 3 mRNA nucleotide bases codes for 1 amino acid

   2. Anticodon: the part of the tRNA that binds to the codon.

   3. AUG = start codon (methionine)

3. A tRNA carrying the amino acid specified by the next codon binds to the mRNA strand. The amino acids bind together with a peptide bond.

4. The process continues until one of the three “stop” codons appear.

How is RNA used to make a protein?

• Each set of 3 mRNA nucleotides bases (codon) codes for 1 amino acid

• To determine which amino acid is coded for, we use the genetic code.

Mutations

Gene Mutations

• A problem at the nucleotide level. 

• One gene involved

• One gene = one enzyme/protein

Ex. The boy ate pie 

Point Mutations: mutations that occur at ONE point/base pair. Usually only one messed up amino acid. 

Substitution

Ex. Substitution: The boy ate pie -> The boy ate pee

Some substitutions are not a big problem because of wobble.

Wobble: several codons code for the same Amnio Acid

Frameshift mutation: usually mutations that result in many incorrect amino acids from point of mutation on. 

Additions or deletions

Cell Cycle

DNA exists in two forms: Chromatin and Chromosomes

Binary Fission: Cell Division in Prokaryotes (Bacteria and Archaea); Asexual Reproduction; Results in 2 genetically identical daughter cells (clones)

For eukaryotes…

Cell Cycle

1. Cell undergo Interphase where they grow and replicate their DNA.

2. Mitosis: division of the nucleus or Meiosis

3. Cytokinesis: division of the cytoplasm

C words:

Chromatin: Not condensed DNA

Chromosome: condensed DNA

Chromatid:

Centromere:

We count # of chromosomes by # of centromere so the number of chromatid doesn’t matter.

The duplicated chromosomes are called sister chromatids.

Body/Somatic Cells

• Diploid (2n)

• 46 Chromosomes

• Made by mitosis

Gametes/sex cells

• Haploid (n)

• 23 Chromosomes

• Made by meiosis

Autosomes: 22 pairs of chromosomes that are common between egg and sperm producers.

Sex chromosomes: 23rd pair of chromosome that determines the biological sex.

Mitosis steps

1. Prophase

• Chromosomes condense

• Spindle fibers form

• Nucleus and nucleolus disappear

2. Metaphase

• Chromosomes line up in the middle

3. Anaphase

• Centromeres split

• Sister chromatids separate to opposite sides of the cell.

4. Telophase

• New nuclei form

• Chromosomes begin to uncoil.

• If Cytokinesis happens, it begins here.

After cytokinesis…

We end up with 2 daughter cells that are clones.

Karyotypes: visual display of an organism’s chromosomes

Why does Karyotypes matter?

• Chromosome abnormalities (NOT DNA mutations)

  • Too few, too many, too short, too long

• Biological sex

Credit:Matthew Meng

Meiosis: sex cell reproduction

• Leads to variety in organisms that can be inherited

• Occurs only in sex cells (gametes)

  • Sperm cell (male) and egg cell (female)

• Goes through interphase just like mitosis

  • DNA replication occurs →Chromosomes become sister chromatids

• Contains 2 steps: meiosis I and meiosis II

  • Meiosis I: leads to a haploid cell

    • Same steps as mitosis: PMAT

    • Prophase I

      • Nuclear membrane disappears

      • Chromatin → chromosomes

      • Centrioles go to opposite sides

      • Synapsis: homologous chromosomes pair up

        • Same chromosome from mother and father pair up

        • This combination is a tetrad (XX shaped)

        • Consists of 4 chromatids (I shaped)

      • Crossing over: the 1st form of genetic variation

        • Tetrads exchange sections of genes at the chiasmata

        • Forms recombinant chromatids

    • Metaphase I

      • Tetrads line up on metaphase plate

      • Independent assortment: the 2nd form of genetic variation

        • Tetrads line up randomly, so when they get split, end up in different gametes

    • Anaphase I

      • Homologous chromosomes of the tetrad split apart (X and X)

      • Moves to their centrioles

    • Telophase I

      • Nuclear membrane reforms

      • Nucleolus reforms around isolated sister chromatids

    • Result: 2 cells with 23 replicated chromosomes (X shaped)

  • Meiosis II: literally just mitosis

    • Starts with 2 haploid cells

    • Separates sister chromatids

    • Maintains the same haploid number of chromosomes

    • Result: 4 cells 23 unreplicated chromosomes (I shaped)

• Result of meiosis: 4 genetically different haploid daughter cells

  • Haploid is half the number of a diploid cell (2n)

    • Human cells usually have 46 chromosomes diploid

    • Becomes 23 after meiosis

  • Diploid is usually what the normal cells are (chromosomes from both dad and mom)

  • Haploid is either dad or mom

    • When haploid gametes combine into a zygote (fertilized egg) their chromosomes will combine to make diploid again

    • Random fertilization: the 3rd form of genetic variation

      • Any random sperm can fertilize any random egg

        • Crossing over and independent assortment leads to variety of gametes, random fertilization creates a random offspring out of it

  • 4 daughter cells are genetically different due to the 3 forms of genetic variation

  • Gametogenesis: the production of gametes that uses meiosis

    • Oogenesis: production of egg cells

      • Cytoplasm is not evenly split after meiosis

        • 3 polar bodies are nonfunctional, leaving 1 functional egg

    • Spermatogenesis: production of sperm cells