Lecture 11- 3:04:2017 BIO PDF

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BIO1011 Lecture 11 Molecular Genetics – How DNA works

Saw-Hoon Lim 23 Rainforest walk, G03 [email protected]

References: Campbell 10th ed. Concepts 16.1, 17.1-17.3, 17.5, 21.3.

Applications of DNA technology Examples: Identification of individuals using DNA profiling or fingerprinting – forensics, paternity. Food and pharmaceutical production - transgenic organisms. Medicine - gene therapy, stem cell technology.

1. DNA is the genetic material Chromosomes contain both DNA and proteins, which was the genetic material? Hershey-Chase: Used bacteriophage - virus that infects bacteria.

bacterial chromosome

Was it DNA or protein that entered?

35S

32P

Fig 16.4

2. DNA structure DNA = deoxyribonucleic acid. Polymer of nucleotides, each with 3 components: a nitrogenous base, a deoxyribose sugar, and a phosphate group.

Single strand of DNA

Campbell Fig 16.5

How was the DNA structure solved? 1953 – James Watson and Francis Crick. - Used Xray crystallography data from Wilkins and Franklin. - Plus data of Chargaff: Chargaff’s rule: A=T, G=C

Campbell Fig 16.6

Brooker Fig 9.13

Proposed DNA was double stranded and helical in shape – double helix. - bases face inwards, like rungs in ladder. - form pairs - 10 base pairs per turn of helix

Strands are anti-parallel

Campbell Fig 16.7

A pairs with T:

2 H bonds

C pairs with G:

3 H bonds (stronger)

Structure suggested how it is copied DNA replication is semi-conservative. Shown experimentally by Meselson and Stahl - 1958.

3. The flow of genetic information How is the information in DNA accessed? Gene expression (strictly controlled)

DNA transcription

mRNA translation

Polypeptide

Functional RNA

Transcription DNA is copied into RNA = ribonucleic acid. In RNA uracil (U) is used instead of thymine (T).

Campbell Fig 17.4

RNA = ribonucleic acid Single stranded

Why is RNA intermediate needed?

Campbell Fig 17.3

2. By using intermediate can have expendable information carriers.

RNA processing

promoter

Translation How is information to make a polypeptide encoded in the DNA/mRNA? - Triplet code (gives 64 alternatives), non overlapping. - Each set of 3 bases called a codon.

Campbell Fig 17.4

The Genetic Code

Fig 17.5

The genetic code is universal. This means genes from one organism can often work in another.

4. Genomes Genome – entire DNA sequence of an organism.

Initially small genomes were sequenced, such as bacteria (~4Mb), first complete one in 1995. The human genome was sequenced in 2001 (3,200Mb), annotation completed in 2006. www.ncbi.nlm.nih.gov/genome/guide/human/index Many other organisms genomes sequenced: www.ncbi.nlm.nih.gov/sites/entrez?db=genomeprj

What’s in a genome? In human genome only 1.5% is coding sequence.

Rest is non-coding DNA: ~24% is gene-related, introns and regulatory regions. ~75% is other noncoding DNA.

5. DNA mutation DNA sequence differences arise due to mutation. *All genes mutate spontaneously and randomly. Different levels of mutation: - point mutations - chromosomal aberrations - changes in chromosome number

Can have dramatic effects on gene product, or none.

Silent mutation

Missense mutation

Nonsense mutation

Lecture 11 Summary • The Hershey-Chase experiment established that DNA was the genetic material. • Watson and Crick determined that DNA is a double-stranded helix using data from Xray crystallography and biochemical analyses. • DNA replication is semi-conservative. • Accessing the information in DNA is called gene expression, and involves transcription to produce a mRNA intermediate, and translation into a polypeptide. • Not all DNA encodes for proteins or RNAs. • DNA mutation causes changes in DNA sequence between individuals.

Past exam question When compared with DNA, RNA (A) (B) (C) (D)

is single stranded is found only in nuclei has ribose in place of dextrose has thymine in place of uracil

• Hershey and Chase carried out an experiment (the blender experiment) in which they incorporated radioactive sulphur or phosphorus into bacteriophages. What is a bacteriophage (what does it infect and how)? What molecules in the bacteriophage became radioactive upon incorporation of sulphur? And phosphorus? What was the blender for? When the samples were centrifuged, the radioactive sulphur stayed with the liquid, and the radioactive phosphorus stayed with the intact bacteria. What did this prove? • What sort of bonds hold together the phosphate, sugar and nucleotide? • Which two of these three parts makes up the backbone of DNA? • What is the structure of DNA that was proposed by Watson and Crick? • Which bases pair with which bases in DNA? What does it mean that the strands are antiparallel? • What does it mean that DNA replication is semi-conservative? • What is transcription? What is translation? • All genes are expressed all the time. True or false? • What are the three differences between DNA structure and RNA structure? • What is a genome? • DNA uses a Triplet code. What does this mean? • All genes encode proteins. True or false? • What is a silent mutation? Missense mutation? Nonsense mutation?...