Molecular genetics 1 win q

Chapter 20 Molecular Genetics

Deoxyribonucleic acid (DNA)

• Deoxyribonucleic acid (DNA) is a molecule that carries genetic information.

• These genetic information is important for all cellular functions, such as cell division and cell differentiation.

• Almost all cells in our body contain DNA inside their nuclei.

DNAEach DNA molecule consists of two parallel strands twisted around each other to form a double helix.

A molecule of DNA is wrapped around proteins to form a single chromatin thread.

During cell division, the chromatin threads coil more tightly to form chromosomes inside the cell nucleus.

proteins

nuclear pore

nucleus

nuclear envelope

What is DNA made of?

Double helix ‘untwisted’

One strand of nucleotides

Components of a single nucleotide

DNA molecule: a long double helix

basedeoxyribosesugar

phosphate group

Sugar phosphate ‘backbone’

Basic units of DNA

adenine cytosine guanine thymine deoxyribosesugar

phosphategroup

Bases

Nucleotides

base joins with the phosphate group and deoxyribose sugar group

Basic units of DNA• The basic unit of DNA is a

nucleotide.

• Each nucleotide is made of - a sugar called deoxyribose; - a phosphate group; and- a nitrogen-containing base,

all joined together

• The four bases of nitrogen-containing bases are

- adenine (A) - thymine (T) - cytosine (C) - guanine (G)

adenine cytosine guanine thymine deoxyribosesugar

phosphategroup

Bases

Nucleotides

adenine nucleotide thymine nucleotide

guanine nucleotide cytosine nucleotide

base joins with the phosphate and deoxyribose sugar group

The building blocks of DNA• Nucleotides are joined together to form long chains called

polynucleotides.

• Each gene is made up of a sequence of nucleotides. This sequence can vary.

bases

sugar-phosphate backbone

polynucleotide

Rule of base pairing• The bases of one strand form bonds with bases of the

other strand according to the rule of base pairing.

• Adenine (A) bonds with thymine (T), while cytosine (C) always bonds with guanine (G).

• Bases that bond with each other are known as complementary base pairs.

base pair

Guide to be a God!

1) Colour each of the individual structures on the worksheet with a different colour:

Example: adenine = red thymine = greenguanine = blue cytosine = yellowphosphate = brown deoxyribose = purple

Guide to be a God!

2) Cut out each structure.

3) Using the small symbols (squares, circles and stars) on the structures as guides, line up the bases, phosphates and sugars.

4) Glue the appropriate pairs together to form nucleotides.

Example:

Guide to be a God!

5) Construct the right side of your DNA molecule by putting together in sequence a cytosine, thymine, guanine and adenine nucleotide.

6) Complete the left side of the DNA ladder by adding complementary nucleotides or nucleotides that fit. Your finished model should resemble a ladder.

Guide to be a God!

7) To show replication of your model, separate the left side from the right side on your desk, leaving a space of about 15 to 20 cm.

8) Using the remaining nucleotides, add to the left side of the model to build a new DNA molecule. Do the same with the separated right side.

Questions?!?!

1) When constructing the DNA molecule, what did you notice about the orientation of the two strands?

2) What DNA strand would bond opposite?3) What is a similarity and a difference in

DNA between Homo sapien and Blattaria?

The DNA double helix The DNA molecule has a spiral structure known as the double

helix. Both strands of DNA that run in opposite directions are twisted to form this double helix.

a base pair

sugar-phosphate backbone

coiling of DNA

double helix structure of DNA

Genes

• A DNA molecule contains many genes along its length.

• A gene is a small segment of DNA which controls the formation of a protein, such as an enzyme.

gene

DNA molecule

Genes• Each gene stores a message that determines how a

protein should be made in a cell.• The message stored by a gene is known as the

genetic code.• Proteins are responsible for the development of

certain characteristics in the body.

gene

DNA molecule

part of a DNA molecule unzipped to show a gene

M E S S A G E

a gene is a segment of DNAprotein coded by the gene

Structure of a gene• Each gene consists of two polynucleotide chains. One of the chains

determines the type of protein made. This chain is called the template.

• The template is made up of a sequence of nucleotide bases.

• Three sequential bases code for one amino acid. This is known as the triplet code or codon.

DNA template

Process of decoding and protein synthesis

polypeptide made of five amino acids

triplet code/ codon

How are proteins made?• Proteins in the cell are made through a two-step process — transcription and translation.

• Transcription occurs when the message in the template has to be copied into an RNA molecule called messenger RNA (mRNA).

• Transcription occurs in the nucleus.

• Three bases in the mRNA made up a codon.

DNA template

Transcription mRNA - RNA contains U

(uracil) instead of T (thymine)

codon

How are proteins made?• The mRNA moves out of the nucleus and carries the

message to the cytoplasm.

• A ribosome helps to translate the sequence of codons on the mRNA into a protein molecule.

mRNA

polypeptide

Translation

Comparing DNA and RNA

DNA (double helix) RNASugar unit is deoxyribose. Sugar unit is ribose.

Nitrogen-containing bases are adenine (A), thymine (T), cytosine (C) and guanine (G).

Nitrogen-containing bases are adenine (A), uracil (U), cytosine (C) and guanine (G).

Permanent molecule in the nucleus

Temporary molecule that is made when needed

Found only in nucleus Found in nucleus and cytoplasm

1 part of a gene

Transcription and Translation

First, the gene unzips.

1 part of a gene


template

mRNA molecule is made

One of the strands in the gene is used as the template to make mRNA. This is transcription. The mRNA molecule copies the genetic code in the DNA template, following the rule of base pairing.

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Note that mRNA does not contain T (thymine). A (adenine) in DNA pairs with U (uracil) in mRNA.


mRNA molecule is made

ribosome

mRNA

nuclear envelope

The mRNA leaves the nucleus and attaches to a ribosome in the cytoplasm.

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nuclear pore


tRNA • In the cytoplasm are amino acids and transfer RNA (tRNA). Transfer RNA or tRNA is another RNA molecule also needed for protein synthesis.

• tRNA molecules have amino acids attached to one end of their structure.Each tRNA is very specific and attaches only to its own amino acid For example, a tRNA with the anticodon UAC always attaches to the amino acid M.

• Each tRNA also has three bases at one end. This is an anticodon that can bind to complementary codons on mRNA.

cytoplasmamino acids

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anticodon


codonThe anticodons on tRNA bind with their respective codons on mRNA.

tRNA

amino acid attached to tRNA

peptide bond

ribosome

4 • Translation starts with mRNA attaching to a ribosome.

• The first two tRNAs together with their amino acids also fit into the ribosome. They attach to the codons on the mRNA according to the rule of base pairing.

• A peptide bond is formed between the two amino acids.


5peptide bond between amino acids

first tRNA is released

a new tRNA fits into the ribosome

• Once the peptide bond is formed between the first two amino acids, the ribosome moves along one codon to the right of the mRNA.

• As the ribosome moves to this position, the first tRNA is released.

• At the same time, the third tRNA and its amino acid slots into the ribosome.

codon ribosome moves along the mRNA strand


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another amino acid is attached to the chain

direction of movement of ribosome

• Another amino acid is attached to the chain.


polypeptide formed• The process continues as the ribosome moves along the mRNA.

• At the end of the mRNA is a stop codon such as UGA, UAA or UAG. A stop codon does not have any tRNA with complementary codons. This means that anticodons ACU, AUU or AUC do not exist.

• Eventually, the whole chain of polypeptide is produced. The ribosome leaves the mRNA.

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polypeptide formed• The ribosome may attach to the same mRNA for another round of translation.

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insulin gene

• Obtain the human chromosome containing the insulin gene.

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How the human insulin gene is inserted into bacterial DNA

Genetic Engineering

insulin gene


• Cut the gene using a restriction enzyme. This enzyme cuts the two ends of the gene to produce ‘sticky ends’.

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cut by restriction enzyme


fragment of DNA containing the insulin gene

sticky end

Genetic Engineering

insulin gene


• Cut the gene using a restriction enzyme. This enzyme cuts the two ends of the gene to produce ‘sticky ends’.

• Each ‘sticky end’ is a single strand sequence of DNA bases. These bases can pair with complementary bases to form a double strand.

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sticky end


Genetic Engineering

insulin gene

• Obtain a plasmid from a bacterium.

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sticky end

plasmid


Genetic Engineering

insulin gene

• Obtain a plasmid from a bacterium.

• Cut the plasmid with the same restriction enzyme. This produces complementary sticky ends.

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sticky end

plasmid

cut by same restriction enzyme

sticky ends


Genetic Engineering

insulin gene

• Mix the plasmid with the DNA fragment containing the insulin gene.

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sticky end

plasmid


sticky ends


Genetic Engineering

insulin gene

• Mix the plasmid with the DNA fragment containing the insulin gene.

• Add the enzyme DNA ligase to join the insulin gene to the plasmid.

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sticky end

plasmid


sticky ends

insulin gene inserted into plasmid


DNA ligase

Genetic Engineering

insulin gene

• Mix the plasmid with E. coli bacteria.

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sticky end

plasmid


sticky ends


E. coli

bacterial DNA


DNA ligase

Genetic Engineering

Genetic Engineeringinsulin gene

• Mix the plasmid with E. coli bacteria.

• Apply temporary heat or electric shock. This opens up pores in the cell surface membrane of each bacterium for the plasmid to enter.

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sticky end

plasmid


sticky ends


plasmid

bacterial DNA

plasmid enters the bacterium

trangenic bacterium

E. coli

bacterial DNA


DNA ligase

Molecular genetics 1 win q

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