Do Now 3.21Objectives:

1.Define transcription.

2.Explain the differences in structure and function between DNA and RNA

Task:

A DNA strand has the following sequence:

TAC-CCC-AAA-GCT-ATA

What is the sequence of the complementary strand?

The strands of DNA are Antiparallel

• Antiparallel means the two strands are parallel, but have opposite orientations.

5’ & 3’: The two different ends of a nucleotide strand

• The numbers 5’ and 3’ refer to different carbon atoms in the sugar of a nucleotide.

• The 5’ end of a strand has a phosphate attached to it. The 3’ end does not

DNA Replication: Key Ideas• ENZYMES:

– Helicase “unzips” dsDNA– DNA polymerase builds a complementary strand of DNA.– DNA ligase joins pieces of DNA together

• DIRECTION:– Polymerases add nucleotides to the 3’ end of a growing

polynucleotide. In other words, new strands are built in the 5’-3’ direction.

– The “lagging strand” of DNA is oriented in the 3’-5’ direction and must be built “backwards” in small pieces called Okazaki fragments that must be put together by DNA Ligase.

• SEMI-CONSERVATIVE:– Each new dsDNA molecule produced by replication has one

old strand and one new strand.

“Replisome” (lots of detail!)

Transcription

• Transcription makes an RNA copy of a DNA sequence.

• All of a cell’s RNA is made by transcription. (mRNA, tRNA, rRNA, etc.)

• RNA molecules process information (DNA is storage)

RNA vs. DNA

1. Single strand

2. Ribose sugar

3. Thymine replaced by Uracil (A=U)

The Big Idea (aka “central dogma”)

DNA mRNA Protein

Transcrip

tion

TranslationReplication

DNA & RNA Polymerase

• Polymerases are enzymes that make nucleotide polymers. In other words, they make new strands of DNA or RNA.

• DNA polymerase makes new DNA strands.

• RNA polymerase makes new RNA strands.

Polymerases Follow a Template Strand

• Notice that RNA polymerase is copying the “template strand” of DNA. The “coding strand” has the same sequence as the new RNA molecule.

• Only correctly base-paired matches are added

• Polymerases add nucleotides to the 3’ end of a molecule

RNA Polymerase

at Work

mRNA

• mRNA (messenger RNA) is a single stranded nucleic acid molecule that carries the information in a gene from the nucleus to the cytoplasm. There, it is used by ribosomes as instructions for building a protein molecule.

tRNA and rRNA

• tRNA and rRNA are used to build proteins during the process of translation which we will learn about soon.

• Translation is the process of making a protein using the instructions in an mRNA molecule.

RNA’s Shape• Because RNA is single stranded, it can fold up

in many different shapes by hydrogen bonding with itself. This is why RNA can have so many different structures and functions.

tRNA rRNA

Recap of Key Ideas• Transcription makes an RNA copy of a DNA sequence.

• RNA has a different structure than DNA.– Single stranded

– Ribose sugar

– Uracil replaces Thymine

• RNA polymerase performs transcription.

• It is the mRNA molecule which exits the nucleus and contains the insturctions used to make a protein.

• tRNA and rRNA are other forms of RNA used by a cell to make protein