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DNA, RNA & Protein Synthesis

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DNA, RNA & Protein Synthesis. A DNA nucleotide has three parts: a sugar molecule called DEOXYRIBOSE, a phosphate group (which consists of P and O atoms), and a NITROGEN BASE.

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Protein Synthesis

  • A DNA nucleotide has three parts: a sugar molecule called DEOXYRIBOSE, a phosphate group (which consists of P and O atoms), and a NITROGEN BASE.
The four nitrogen bases found in DNA nucleotides are ADENINE (A), GUANINE (G), CYTOSINE (C), and THYMINE (T). A and G are called PURINES (two rings of carbon). C and T are called PYRIMIDINES (one ring of carbon).
In 1953, James Watson and Francis Crick suggested a model for the structure of DNA that looked like a spiral staircase: DOUBLE HELIX.
Nitrogen bases: COMPLEMENTARY BASE PAIRS. Cytosine always pairs with guanine; adenine always pairs with thymine.


a cell

*Two nucleotide chains separate

by unwinding; each chain serves as a

template for a new nucleotide chain.

Steps of DNA replication:
    • 1. Two chains separate at the REPLICATION FORK by enzymes (HELICASES). As the helicase enzymes move along the DNA molecule, they break hydrogen bonds between the bases, and the chains separate.
2. Enzymes (DNA POLYMERASES) bind to the separated chains of DNA; one nucleotide a time, it constructs a new complementary chain of nucleotides.
  • * Product: two identical copies of the original DNA.

Suppose that the sequence of nucleotides in one chain of the original DNA molecule is A-T-T-C-C-G. DNA polymerases would produce a new nucleotide chain with the sequence of ________________.

The process of DNA replication occurs with one error in every 10,000 paired nucleotides. A change in the nucleotide sequence at even one location, (MUTATION), may have serious effects on new cells.
      • Besides a mistake in replication, what else can cause mutations?

Sickle Cell Anemia: caused by a point mutation that substitutes adenine for thymine; results in defective hemoglobin/sickle shaped RBCs.

RIBONUCLEIC ACID (RNA)- responsible for the movement of genetic information from the DNA in the nucleus to the site of protein synthesis in the cytoplasm.
  • Like DNA, RNA is made up of
  • repeating nucleotides.
  • **3 Differences between DNA & RNA
  • 1. The sugar molecule of every RNA nucleotide is RIBOSE (instead of deoxyribose.)

2. URACIL replaces thymine. So, uracil (not thymine) pairs with adenine in RNA.

  • 3. RNA is single-stranded, not double-stranded.
There are three types of RNA:
    • 1. Messenger RNA (mRNA)- carries genetic information

from the DNA in the nucleus to the cytoplasm of a eukaryotic cell.

    • 2. Transfer RNA (tRNA)- transfers each amino acid to the ribosome as it is specified by a coded messages in the mRNA.
    • 3. Ribosomal RNA (rRNA)- makes up the ribosomes where proteins are made.
TRANSCRIPTION- process by which genetic information is copied from DNA to RNA.
  • - RNA POLYMERASE binds to DNA and separates the DNA strands; then uses one strand of DNA as a template from which nucleotides are assembled into a strand of RNA.
    • 1. RNA polymerase intiates RNA transcription by binding to specific regions of DNA called PROMOTERS (marks the beginning of the DNA chain that will be transcribed.)
    • 2. When RNA polymerase binds to a promoter, the DNA molecule separates. One of the chains, called the TEMPLATE, is used for transcription.
    • 3. RNA polymerase adds complementary RNA nucleotides to the newly forming RNA molecule. (A-U, G-C)
    • 4. Transcription continues until the RNA polymerase reaches the TERMINATION SIGNAL, where it releases both the DNA molecule and the newly formed RNA molecule.
The products of transcription (transcripts): three types of RNA molecules (mRNA, tRNA, rRNA). Following transcription, mRNA moves through the nuclear membrane into the cytoplasm of the cell, where it directs protein synthesis.
-PROTEIN SYNTHESIS: production of proteins. Proteins carry out the genetic instructions in an organism’s DNA.
  • -During protein synthesis, mRNA is translated into a sequence of amino acids. A GENETIC CODE is used to translate mRNA transcripts into proteins. How many amino acids are there?
  • -CODON- three mRNA nucleotides; codes for a specific amino acid. There are 64 codons. The START CODON engages a ribosome to start translating an mRNA molecule. STOP CODONS cause the ribosome to stop translating an mRNA.
TRANSLATION: cell uses information from mRNA to produce proteins. tRNA anticodons pair with corresponding mRNA codons, and amino acids are joined together to form a polypeptide.
steps of translation
Steps of translation
  • 1. mRNA is transcribed from DNA and released into the cytoplasm.
  • 2. mRNA attaches to a ribosome. As each codon moves through the ribosome, a tRNA matches up a codon to an amino acid. The three bases on the tRNA molecule, (ANTICODON), are complimentary to one of the mRNA codons.
  • 3. The ribosome connects the amino acids to each other, releases the tRNA, and moves down to the next tRNA molecule.
  • 4. The polypeptide chain continues to grow until the ribosome reaches a STOP CODON on the mRNA molecule.

That Spells DNA

Genes for proteins regulate growth throughout an organism, controlling its size and shape. PROTEINS ARE THE KEYS TO ALMOST EVERYTHING THAT LIVING CELLS DO!

Extra credit opportunity!