DNA and Genes Chapter 13

What is DNA? Our genes are located on chromosomes. Chromosomes are made of DNA - deoxyribonucleic acid. DNA contains the instructions for making proteins.

Structure of DNA DNA is a long polymer of repeating subunits called nucleotides. Nucleotides – deoxyribose (sugar), phosphate group, and a nitrogen base. Four possible nitrogen bases: adenine (A), guanine (G), cytosine (C), thymine (T).

The Four Bases The four bases can be broken down into two groups: Pyrimidines Thymine Cytosine Purines Adenine Guanine

Structure of DNA cont. Nucleotides join together to form long chains, with the phosphate group of one bonding to the sugar of the next. Backbone – sugar and phosphate groups Middle (rungs) – base pairs held together by hydrogen bonds

Structure of DNA cont. Adenine always pairs with thymine. Cytosine always pairs with guanine. Two chains of DNA join together to form a double helix (twisted ladder).

Discovering DNA’s Structure Chargaff determined base-pairing in 1949. Franklin & Wilkins discovered helical structure in 1952. Watson & Crick built first 3-D model in 1953.

Base Pairing Ex: ATTGCTATCGGCA TAACGATAGCCGT Try writing a match for the following strand: G T A C G C T A G C T A G C T _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ The answer: C A T G C G A T C G A T C G A

Nucleotide Sequence All organisms contain DNA made of nucleotides with the same four bases. Differences between organisms come from the sequence of the four bases along the DNA strand. Nucleotide sequence is the unique genetic information of an organism. The closer the relationship between two organisms, the greater the similarity in their DNA.

Replication of DNA DNA Replication – process that copies the DNA in the chromosomes. Occurs during interphase, before cell division. Each single strand of DNA serves as a template to reconstruct the other strand.

How DNA Replicates Hydrogen bonds between bases are broken and the DNA molecule is unwound and “unzipped” by DNA helicase. Free nucleotides bond to the exposed single strands by base pairing and are linked together by DNA polymerase. This continues until the whole molecule has been unzipped and replicated.

How DNA Replicates cont. 1958 – Meselson and Stahl found that DNA Replication was Semi-Conservative compared to being Conservative They used heavy Nitrogen (15N) to determine this step.

How DNA Replicates cont. Each new DNA molecule formed contains one parent strand and one newly synthesized strand.

RNA RNA – ribonucleic acid Three major differences between RNA and DNA: - RNA is a single strand - RNA’s sugar is ribose - RNA contains the base uracil (U) instead of thymine (T)

RNA vs DNA

RNA cont. RNA molecules are responsible for protein synthesis. There are three types of RNA: - Messenger RNA (mRNA) - Ribosomal RNA (rRNA) - Transfer RNA (tRNA)

Central Dogma In order to make protein, DNA must be transcribed into mRNA. mRNA leaves the nucleus and is translated into protein in the cytoplasm.

Transcription First the DNA “unzips”, just like during replication. Free RNA nucleotides match up with the exposed DNA nucleotides by base pairing. (A – U, G – C) The completed mRNA molecule breaks away and the DNA molecule “zips” back together. mRNA can leave the nucleus, carrying instructions to the cytoplasm. Transcription Animation

RNA Genetic Code mRNA carries the instructions for building a protein. Instructions are written in three-letter words called codons. Each codon stands for one of twenty amino acids. Proteins are chains of amino acids in the order coded on the mRNA

RNA Genetic Code

Translation Translation – process of converting the information in a sequence of nitrogen bases in mRNA into a sequence of amino acids in a protein. Takes place at the ribosomes (rRNA) in the cytoplasm. As the ribosome moves down the mRNA strand, transfer RNA (tRNA) brings in the appropriate amino acid.

Translation

Translation cont. Each tRNA has an anticodon of three nitrogen bases. The tRNA anticodon matches with a mRNA codon. tRNA also carries an amino acid to add to the growing protein.