Unit 6: Molecular Genetics

Slogan: Genetic Inheritance vs. Expression

Critical Unit notes

DNA is only synthesized/read/EVERYTHING in the 5’ to 3’ direction

DNA → RNA → PROTEIN

ALL LIFE FORMS USE THE SAME UNIVERSAL CODE

6.1: DNA Synthesis

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Facts on DNA

• Rosalind Franklin and her Co-worker used X-ray diffraction to capture a photo of DNA

• Watson and Crick later used this photo to determine the structure of DNA w/o her knowledge

• DNA is a double helix

• DNA is Antiparallel

Replication

• DNA is replicated during the S phase

• Chargaff’s rule:

  • Adenine pairs with Thymine

  • Cytosine pairs with Guanine

• Semi-conservative replication

  • The original strand replicates TWO new strands of DNA

  • Each new strand of DNA is made up of one original and one replicated

Key players in DNA Replication

Because DNA is antiparallel:

• DNA polymerase 3 can only synthesize in the 5’ → 3’ direction

• One strand will be leading

  • Will follow DNA helicase and replicate continuously

• The other will be the lagging strand

  • Will go in the opposite direction of DNA helicase and create Okazaki fragments

  • * DNA ligase fixes “Glitches“ (gaps) in the lagging strand*

Proofreading and Repairing DNA

• DNA Polymerase proofreads newly made DNA

  • Replacing incorrect nucleotides

• In nucleotide excision repair, a nuclease cuts out and replaces damaged stretches of DNA

  • Damage caused by UV Rays, cigarette smoke, X-rays

What’s the significance of altered DNA?

• Some sequence changes may become permanent and passed onto the next generation

• Adds genetic variation and connects to natural selection

  • Chromatin mutations are NOT permanent (epigenetics)

Replicating the ends of DNA Molecules

• The replication process provides no way to complete the 5’ ends, thus, after many rounds of replication it produces shorter DNA molecules with uneven ends

  • Not an issue for prokaryotes

• Eukaryotes have a special nucleotide sequence at their ends called telomeres

  • Helps prevent the shortening of DNA molecules

  • Shortening of telomeres has possible connection to aging

DNA Packaging’

• In bacteria DNA is supercoiled

• In eukaryotes, DNA is combined with proteins to form chromatin

  • Unfloded, chromatin resembles beads on a string

  • Each “bead“ is a nucleosome

Histones are made of a set of 8, histone tails are like hooks which DNA can attach onto

• Folding upon folding

Every time there is cell division, we have to unfold and refold the histones for the purposes of transcription and translation of DNA

6.2: DNA Transcription

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Flow of genetic information

• Central dogma: DNA → RNA → Protein

  • Transcription: DNA → RNA

  • Translation: RNA → Protein

    • Ribosome = site of translation

RNA

    ↳ Ribonucleic acid

* RNA in comparison to DNA found in Unit 1 *

RNA is a messenger between DNA and the ribosomes

• RNA decodes DNA by making an RNA copy

• Like keeping a copy of important documents that way the originals don’t get damaged or lost

  • RNA assembles amino acids into proteins = protein synthesis

Types of RNA

the Genetic code

One DNA strand 3’ → 5’, is the template strand

• Called noncoding, minus, or antisense strand

• The complementary mRNA strand is read in the 5’ → 3’ direction

ALL LIFE FORMS USE THE SAME UNIVERSAL CODE

Transcription

    ↳ the process of making mRNA from a DNA template

transcription unit: a stretch of DNA that codes for a polypeptide or RNA

RNA polymerase and Transcription

RNA polymerase attaches to a promoter (start of gene) and stop at the terminator (end of gene).

• Uracil replaces thymine when pairing with adenine

Steps to transcription

1. Initiation (Prokaryotes)

• RNA Polymerase binds directly to Promoter in DNA

1. Initiation (Eukaryotes)

• TATA BOX =

  • DNA sequence (TATAAAA) in promoter region upstream from transcription start site

• Transcription factors must recognize TATA box before RNA polymerase can bind to DNA Promoter

• Transcription factors + RNA Polymerase = Transcription initiation complex

2. Elongation

• RNA polymerase adds RNA nucleotides to the 3’ end of the growing chain (A - U, G - C)

• As RNA polymerase moves, it untwists DNA, then retwists after mRNA is made

3. Termination

• RNA Polymerase transcribes a terminator sequence (prok.) or polyadenylation signal sequence (euk).

• Now pre-mRNA for eukaryotes

• Prokaryotes = mRNA ready for use

Flow of genetic information in Prokaryotes vs Eukaryotes

Addititions to Pre-mRNA

6.3: DNA Translation

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6 pages

6.4: Mutations

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4 pages

6.5: Regulation of Gene Expression

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5 pages

6.6: Biotechnology

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4 pages

Kahoot/Additional notes

Who used X-ray crystallography to study the structure of DNA?

• Franklin and Wilkins

What are bacteriophages?

• Viruses that infect bacteria

Which enzyme is found in retroviruses that produce DNA from an RNA template?

• Reverse transcription

In what process is external DNA taken up by a bacterial cell?

• Transformation

Telomerase is a telomers enzyme and makes them

Exons with a mutation would be very damaging for cell

Describe the structure of plasmid DNA

• It is double stranded and circular

Introns must be removed in…

• Eukaryotic cells

Why are introns removed?

• To produce a protein with only active genes

A growing polypeptide chain bonds together the Amino acids with ______ bonds

• Covalent

What do protein kinases do?

• They in/activate proteins

What does the operon model attempt to explain?

• Gene expression in bacteria

Which of the following describes a post transcriptional modification?

• Poly A-Tail is added

An enhancer site is…

• located at a distance from the gene it affects

What is the impact of alternative RNA splicing?

• It produces different polypeptides from the mRNA

The trp operon is cosidered a repressible operon because…

• tryptophan stops the operon from working so trp production stops

DNA packing affects gene expression by..

• controlling access to DNA
  

Essential Questions

• Explain the central dogma.

• How do transcription and translation work to produce proteins from DNA codes?

• How are these processes different in prokaryotes and eukaryotes?

• How do changes in DNA and RNA sequences impact an organisms' phenotype?

• What are some of the impacts of point and frameshift gene mutations?

• How do prokaryotes control gene expression?

• How do eukaryotes control gene expression?

• Gene expression is regulated at multiple levels in eukaryotes.... describe some of them.

• How are cells in multicellular organisms differentiated?

• What are the current biotechnologies that deal with gene editing? How do they work?

• What are some of the ethical concerns associated with gene editing technology?

• How is it possible for one living organisms to express genes found in other organisms?