Chapter 12 Notes:

STRUCTURE OF RNA AND TRANSCRIPTION

RNA: Ribonucleic Acid

·    A nucleic acid molecule that is similar to DNA but has different functions.

·    Structure: Composed of RNA nucleotides and is a helix.

RNA differs from DNA in four ways:

1. RNA is single stranded not doubled.

 

2. Has uracil instead of thymine (still a pyrimidine).

 

 

3. Contains the 5-Carbon sugar ribose instead of deoxyribose.

 

4. There are 3 types of RNA and only one type of DNA

Types of RNA:

1. mRNA (messenger): copies instructions in DNA and carries these to the ribosome. The message.

 

2. tRNA (transfer): carries amino acids to the ribosome.  Contains the anti-codon (group of three complementary nitrogen bases to the codon on mRNA). Acts as a taxi-cab.

3. rRNA (ribosomal): makes up the ribosome. It “reads” the mRNA by grouping mRNA together into codons (groups of 3 nitrogen bases.)

 

 

 

Protein Synthesis: the process where information in DNA is used to build proteins. 

 

·    Consists of two processes transcription and translation.

 

Transcription: A strand of DNA is copied into a complementary strand of mRNA.

 

Why? DNA cannot leave the nucleus because it is too big so it needs a “messenger” to carry the information for making proteins to the ribosomes in the cytoplasm.  mRNA is much smaller because it is single stranded and only a portion of the DNA is copied so it can leave the  nucleus.

 

 

Location: the nucleus of cells

 

Process:

1. DNA is untwisted in only the parts that contain the information needing to be copied.

2. DNA is unzipped by enzymes breaking the hydrogen bonds.

3. Free floating mRNA nucleotides pair with one of the unzipped strands of the DNA following the base pairing rule. 

 

4. Important Note: Uracil replaces thymine. (G with C, C with G, T with A and A with U).

5. Only one strand of DNA acts as a template.  The other just “hangs out” until the process is over.

6. Enzymes separate the mRNA from the DNA template and the mRNA enters the cytoplasm and goes to a ribosome.

 

7. The two strands of DNA zip back up, reforming the hydrogen bonds between the nitrogen bases.

 

RESULT: A single strand of mRNA is made with information copied from DNA for the building a specific protein.