Biological Concepts: Central Dogma and Cell Biology

Reminders for Upcoming Assessments

  • Chapter 5 Quiz

    • Open for taking and retaking any number of times until the end of the weekend.
    • Useful study tool, as it pulls from a question bank to provide variety of questions each attempt.
    • Highest score will be recorded (e.g., if a 9 is scored first and a 1 second, a score of 9 will be kept).

  • Next Exam

    • Exam 2 has a due date set.
    • Serves as another helpful study tool.

  • New Assignment

    • Posted and due next Friday.
    • Focuses on active cells, including structures and the secretory system discussed in class.

Overview of Central Dogma

Concept Introduction

  • The Central Dogma describes the flow of genetic information.
    • DNA: The original storage of genetic information.
    • RNA: Transcribes the information temporarily to provide instructions for protein synthesis.
    • Proteins: Final products that execute various functions in the cell.

Flow of Information

  • The arrows representing the flow between molecules do not denote physical movement, but the flow of information from DNA to RNA and from RNA to proteins.
  • Every living organism uses DNA as the genetic code, which must be replicated and passed to daughter cells or new organisms.

Processes Involved

  • DNA Replication
  • Transcription
  • Translation
  • By the end of the week, students should be able to deduce the RNA copy and resulting protein sequence given a nucleotide sequence.

DNA Structure Recap

  • Double-Stranded DNA
    • Comprised of nucleotides, where each nucleotide consists of a phosphate group, a sugar, and a nitrogen base.
    • Backbone structure: Sugar-phosphate repeating units.
    • Base pairing rules:
    • A pairs with T (adenine with thymine)
    • C pairs with G (cytosine with guanine)

Terminology

  • Chromosome: A single piece of DNA.
    • Bacterial chromosomes are circular; human chromosomes are linear.
    • Humans have 46 chromosomes total.
    • Chromosome size varies; average length is approximately 100 million base pairs, resulting in about 3.2 billion base pairs for humans.

Spatial Organization

  • DNA is compactly packaged within the nucleus. If stretched out, DNA from a cell could measure about 2 meters (or approximately 6 feet).
  • The nucleus's dimensions allow packing of DNA in a regulated manner using proteins called histones.
    • Histones form complexes called nucleosomes around which DNA wraps, allowing dynamic compaction.
  • Chromatin is the term for this packed state of proteins and DNA in the cell.

Chromosome Visualization and Numbering

  • At cell division, DNA condenses so chromosomes can be seen under a light microscope.
  • Chromosomes are counted in pairs: 23 pairs or 46 total; one from each parent.

Karyotypes

  • A karyotype is a categorization of chromosomes used for diagnosing certain genetic conditions (e.g., Down syndrome diagnosed by an extra chromosome 21).

Classification of Chromosomes

  • Chromosomes 1 through 22 are autosomes (non-sex chromosomes).
  • The 23rd pair consists of sex chromosomes (XX for females, XY for males).

Cell Cycle and DNA Replication

Phases of the Cell Cycle

  • G1 Phase: Cell preparing to divide, synthesizing proteins and organelles.
  • S Phase: DNA synthesis occurs; every chromosome duplicates.
  • G2 Phase: Preparing for cell division.
  • M Phase (Mitotic Phase): Chromosomes segregate, and the cell divides into two daughter cells.

DNA Replication Details

  • Semiconservative Replication: Each new DNA double helix consists of one old and one new strand.
    • Directionality is crucial: strands have 5' to 3' ends.
Role of Enzymes
  • Helicase: Unzips the DNA by breaking hydrogen bonds.
  • DNA Polymerase: Synthesizes the new complementary DNA strand.

Transcription and Translation Processes

Overview of Transcription

  • In transcription, RNA polymerase synthesizes a complementary RNA strand from a DNA template.
    • It only transcribes the template strand, as opposed to using both strands like in DNA replication.
Key Components of Transcription
  • Template Strand: The strand used for RNA synthesis.
  • Coding Strand: The non-template strand.
  • Promoter: Begin transcription signal.
  • Terminator: End transcription signal.
Steps in Transcription
  1. Initiation: RNA polymerase binds to the promoter region and unwinds the DNA strands.
  2. Elongation: RNA strand elongates as RNA polymerase moves along the DNA.
  3. Termination: RNA polymerase detaches from the DNA upon reaching the terminator region.

Post-Transcriptional Modifications (Eukaryotes)

  • RNA Processing: Includes splicing out of introns and joining exons, adding a 5' cap and a poly A tail to stabilize the mRNA.
    • Introns are non-coding segments; exons are coding segments.

Translation Process

  • Translation occurs at ribosomes in the cytoplasm.
    • Messenger RNA (mRNA) contains the sequence that is translated into amino acids using transfer RNA (tRNA).
Genetic Code
  • The genetic code is read in codons (three-letter sequences of nucleotides) that correspond to specific amino acids.
  • There are 64 codons, covering all 20 amino acids.
    • Codons include start (AUG, for methionine) and stop signals.
Using the Genetic Code Table
  • Students must be able to transcribe given DNA sequences to RNA, then translate that RNA back into an amino acid sequence using the genetic code table.
Importance of Sequence Accuracy
  • The primary structure of proteins hinges on the order of amino acids dictated by the original DNA sequence.
  • Variations in the sequence lead to changes in protein structure and function.

Resources for Further Study

  • Introduction to transcription and translation through interactive modules that visually explain the processes.