Structure of Dna & Rna

Only available on StudyMode
  • Topic: DNA, RNA, Gene
  • Pages : 16 (2356 words )
  • Download(s) : 63
  • Published : January 19, 2013
Open Document
Text Preview
MOLECULAR STRUCTURE OF DNA AND RNA
•Adapted from Chapter 9 in Genetics: Analysis and Principles (Robert J. Brooker)

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

IDENTIFICATION OF DNA AS THE GENETIC MATERIAL
• To fulfill its role, the genetic material must meet several criteria – 1. Information: It must contain the information necessary to make an entire organism – 2. Transmission: It must be passed from parent to offspring – 3. Replication: It must be copied • In order to be passed from parent to offspring

– 4. Variation: It must be capable of changes
• To account for the known phenotypic variation in each species

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

A little history on the identification of genetic material

In early 1900’s, it was generally agreed that the genetic material must be ‘something’ from the chromosome

A chromosome consists of DNA & proteins!

Is the genetic material DNA or Protein?
•DNA consists of only four different subunits (A, T, C, G) •Chromosomes contain less DNA than protein by weight •Chromosomes contain more protein than DNA by •weight •Protein has 20 different subunits (amino acids) so it has a greater potential for variety of combinations •Diverse proteins were identified

Experiments demonstrating that DNA is the genetic material
•Frederick Griffith experiments with Streptococcus pneumoniae •The experiments of Avery, MacLeod and McCarty •Hershey and Chase experiments with Bacteriophage T2

Frederick Griffith Experiments with Streptococcus pneumoniae • Griffith studied a bacterium (Diplococcus pneumoniae) now known as Streptococcus pneumoniae • S. pneumoniae comes in two strains –S Smooth • Secrete a polysaccharide capsule

– Protects bacterium from the immune system of animals

• Produce smooth colonies on solid media

–R

Rough

• Unable to secrete a capsule • Produce colonies with a rough appearance Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

9-5

• In addition, the capsules of two smooth strains can differ significantly in their chemical composition

Figure 9.1

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

In 1928, Griffith conducted experiments using two strains of S. pneumoniae: type IIIS and type IIR 1. Inject mouse with live type IIIS bacteria
Mouse died

2. Inject mouse with live type IIR bacteria
Mouse survived

3. Inject mouse with heat-killed type IIIS bacteria
Mouse survived

4. Inject mouse with live type IIR + heat-killed type IIIS cells Mouse died

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Figure 9.2

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

9-8

• Griffith concluded that something from the dead type IIIS was transforming type IIR into type IIIS – He called this process transformation

• The substance that allowed this to happen was termed the transformation principle – Griffith did not know what it was

• Research began for the identity of the “transformation principle”

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

9-9

The Experiments of Avery, MacLeod and McCarty
• To determine the nature of the transformation principle • Avery, MacLeod and McCarty realized that Griffith’s observations could be used to identify the genetic material • They carried out their experiments in the 1940s – At that time, it was known that DNA, RNA, proteins and carbohydrates are major constituents of living cells • They prepared cell extracts from type IIIS cells containing each of these macromolecules

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

9-10

Understanding Avery et al’s experiments:
•S. pneumoniae bacteria can take up...
tracking img