Nature, structure and organisation of the genetic material

Chapter 10

How Mendel did it• Prevented self-fertilisation and artificially crossed them (look at examples on

p.341 of text)

• He started by studying one trait at a time (monohybrid crosses) then he crossed plants differing in two traits and termed that dihybrid crosses.

• Others tried to study variation in many traits and got confused!

• Mendel was methodical - known history of parents (pure breeding)- recorded parentage- counting offspring

• Mendel demonstrated that inherited factors retain their identity across generations & disproved the 'blending' inheritance model.

Read Mendel's Model of Inheritance (p.343 & 344)

Mendel's Principle of Segregation

• Individuals carry pairs of alleles of each gene, which segregate into gametes during meiosis so that each gamete carries one allele of each gene. p.347 Fig. 10.13

• Also known as:Mendel‘s first law

Principle of Independent Assortment

• The alleles of a gene controlling one trait assort independently of alleles of another gene controlling a different trait p.347 Fig. 10.14

• However, some genes do not behave according to Mendel's Laws.

• Can you think of situations where genes are not assorted independently?

sex -linked & linkage (tendency to be inherited together)

Quick check p.345 Q1-4

What are genes made of?• early 1900s: Bateson's & Punnett's experiments found Mendel’s factors did not

always behave independently

• 1902 (same time): Sutton (US) studied Mendel's results & recognised that chromosomes are the mechanism for inheritance

• 1910: Mendel's factors were found to be located on chromosomes. Morgan demonstrated that specific genes are located on specific chromosomes. He won a Nobel Prize for his work in genetics in 1933.

• 1928: Griffith's studied rough and smooth bacteria causing pneumonia "Transforming Factor“ he knew that genetic material was a chemical substance

• 1943: Avery identified the chemical nature of the transforming factor as being DNA Enzymes that destroyed Carbs, Proteins & Lipids did not affect the ability of dead bacteria

'transform' Enzymes that destroy DNA, meant that extract could no longer 'transform' DNA

Griffith‘s experiment

Living smooth cells found How had this happend??????

Quick check p.351 Q5-8

Nature of genes

• Adenine (A)• Thymine (T)• Cytosine (C)• Guanine (G)

• Chargaff’s rule:Proportions of A & T are about equal

and proportions of C & G are about equal

SUGAR

PHOSPHATE

Mad Minute1) What does DNA stand for?

2) What are the basic building blocks of DNA?

3) What is the name given to the sugar part of DNA?

4) Which "group" forms the head of the 5' end of the 'chain'?

5) The proportions of A to T and G to C are about equal. This idea is know as ???

6) The 3-D DNA molecule is known as a ... ?

7) What types of weak bonds are formed between the base pairs?

8) List the complementary base sequence for the following G G T A C G T A

9) What is the name given to the separation of a double strand of DNA? (not denaturation)

10) What is the name given to complementary DNA chain with parts of chains from different sources?

Deoxyribonucleic Acid

Nucleotides

Deoxyribose

Phosphate

Chargaff‘s ruleDouble helix

Hydrogen bonds

C C A T G C A T

Dissociation

Hybridisation

DNA forms a double helix

• ... used Rosalind Franklin's and Maurice Wilkins X-ray crystallography work to develop the 3D structure of DNA

• the total amount of DNA in a haploid human cell is about 3000 million bp

• In an average chromosome, the total length of DNA is about 4cm consists of about 120 milllion base

pairs constitutes several thousand genes

WATSON & CRICK

Genes range in size from 8000 base pairs to more than 2 million base pairs (an average gene consists ofseveral thousands of base pairs)

• the genome is the total set of genes carried by an individual or cell Feb 2001 the 1st draft of the human

genome was first published

MITOCHONDRIAL DNA double stranded & circular 13 genes code for proteins 2 genes for ribosomal RNA 22 genes for tRNAs D-loop does not carry any genes (see Fig 10.32 p.358)

Gene structure• Two DNA strands: Template & Complementary

strand• Information in a particular gene is located on

template strand• Scanning Tunnelling Microscopy (STM)• A single strand can be shown inmany ways:Nucleotides OR P-sugar-base ORbases

Introns/Exons

• During Transcription, mRNA makes a complementary strand of both the introns and exons (this produces precursor mRNA/ pre-mRNA). The introns are then spliced out and the exons join together to produce mature mRNA.

Introns are the non-coding pieces of DNA Exons are coding regions of DNA

Gene sequencing

• When the order of the nucleotides in a gene are identified, the gene is said to be sequenced

• Gene sequencing: involves the process of identifying the order of nucleotides along a gene

• All living things have the same DNA (the same four bases).

• Retroviruses are the only known exception (but they're not living). They only have RNA.

Quick Check p.362 Q15-17

Coding codons• A code can be permanently encoded so that is can be decoded one or

many times.

• eg: music as an audio file can be burnt onto a CD (it is encoded as microscopic pits on the surface) then, the CD player will decode (translate) the information

• SO... 4 bases can convey very complex information

• The Genetic Code typically contains information for joining amino acids to form proteinseg: the coded info = nucleotide sequence

the decoded info = order of amino acids in proteins

Quick Check p.364 Q18-23

Use of „genetic code“• What DNA bases make the starting codon

• What would be the matching mRNA codon

• What DNA bases code for STOP

• What would be the matching mRNA codons

TAC

AUG

ATT, ACT or ATC

UAA, UGA or UAG

Read p.365-369 and do Quick Check Q24- 27 p.370

Mutations

• Mutations: change the instructions that are encoded in genes• Spontaneous mutations: no causative agent• Induced mutations: agent that cause mutations (mutagenic)

e.g. radiation (X-rays & UV & nuclear), some chemicals• Substitution: may cause no

change / change in aa produced• Addition: single addition

produces major change, due to frameshift mutations

• Deletion: single deletion produces major change, due to frameshift mutations

• Trinucleotide repeat expansion (TRE): involves additional repeats of such sequences and is the cause of several inherited conditions (e.g. Huntington’s disease)

• These are different to "Errors in Karyotypes" (p.295 Ch 9)• Change in the total number of chromosomes e.g.: Trisomal 21 = Down's Syndrome often caused by non-disjunction

• Change to part of a chromosome eg: Duplication or Deletion

• Rearrangement of part of a chromosome eg: Translocation READ p.374 &375

DNA songDNA is in my genes, Tells me how to make proteins

Got my genes from Mum & Dad, Mixed them up and made me gladDNA is in my genes, Tells me how to make proteins

DNA bases read time 3, Always starting with TACmRNA codon would be AUG, DNA triplets tells the story of me,

Always starting with TAC, DNA bases read times 3

ATT ACT ATC, STOP making proteins for memRNA codons for this would be, UAA UGA UAG

STOP making proteins for me, ATT ACT ATC