Chapt 06

GENERAL BIOLOGY

SCHOOL OF MLTFACULTY OF HEALTH SCIENCE

PREPARED BY: MANEGA

HDL 121DNA REPLICATION & PROTEIN

SYNTHESIS

DNA REPLICATION & PROTEIN SYNTHESIS

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Learning Outcomes

After completing this lecture, students will be able to:

(a) Describe basic chemical substances in cell

(b) Explain physical & chemical characteristic of

the substances

(c) Understand & describe the physiological

functions

Topics© 2010 Cosmopoint


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Topic Outlines

1.1. Basic Chemical Substances In Cell1.1.1 Characteristics1.1.2 Physical & chemical1.1.3 Physiological functions

© 2010 Cosmopoint


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Nucleic acids

A macromolecule composed of chains of monomeric nucleotide (linked together)

Polynucleotides are formed via polymerisation of nucleotides Form polynucleotides by condensation with the formation of phosphodiester

bonds Composed of

(a) A base (purine or pirimidine)

Purines: Adenine, guanine which are double ring structures

Pirimidines: Cytosine, thymine & uracil are single ring structures

(b) A sugar (ribose & deoxyribose)

(c) A phosphate group

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1.1. Basic Chemical Substances In Cell


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In biochemistry, these molecules carry genetic information or form structures within cells

The most common nucleic acids are

(a) DNA

(b) RNA (including mRNA, tRNA & rRNA)

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RNA Ribonucleic acid A close cousin of deoxyribonucleic acid (DNA) Polymer of ribonucleoside-phosphates. Its backbone is comprised of alternating ribose & phosphate

groups Contain ribose while DNA contains deoxyribose

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Three major types:

(a) messenger RNA (mRNA) carries information about a protein sequence to the ribosomes, the protein synthesis factories in the cell

(b) transfer RNA (tRNA) is a small RNA chain that transfers a specific amino acid to a growing polypeptide chain at the ribosomal site of protein synthesis during translation

(c) ribosomal RNA (rRNA) & protein combine to form a nucleoprotein called a ribosome. The ribosome binds mRNA & carries out protein synthesis

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Functions

Plays several important roles in the processes of translating genetic information from deoxyribonucleic acid (DNA) into proteins

RNA acts as a messenger between DNA & the protein synthesis complexes known as ribosomes

RNA forms vital portions of ribosomes Serves as an essential carrier molecule for amino acids to be used

in protein synthesis

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Differences between DNA & RNA

DNA RNA

Double strands of polynucleotide Single strand of polynucleotide

Bigger molecule of more than a million bases

Smaller molecule of less than 1000 bases

Entirely double helix strand Single &/or double helix of intra-strand

Pentose within is deoxyribose Pentose within is ribose

Bases consist of A, C, G and T Bases consist of A, C, G and U

The ratio of A + G : C + T = 1 : 1 The ratio of A + G : C + U = 1 : 1

Almost all DNA is found in nucleus Found in both nucleus & cytoplasm

Its amount is constant in all cells, gametes have half the amount

Its amount varies among cells & it can be altered change

Chemically stable Not so stable

Not being broken down, it exists forever

Can be broken down by enzymes

Only one type Three types: rRNA, tRNA & mRNA

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Introduction - DNA

Within the nucleus of every cell are long strings of DNA The code that holds all the information needed to make & control

every cell within a living organism. Nucleic acid that contains the genetic instructions used in the

development & functioning of all known living organisms. Functionally divided into genes. Within the molecule, it has specific sequence of bases. This sequence of bases codes for a specific sequence of amino

acids in a protein.

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1.1. DNA Replication


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It consists of just a few kinds of atoms: carbon, hydrogen, oxygen, nitrogen & phosphorus

Combinations of these atoms form the sugar-phosphate backbone of the DNA @ the sides of the ladder

Other combinations of these atoms form the four bases: thymine (T), adenine (A), cytosine (C) & guanine (G).

These bases are the rungs of the DNA ladder. It takes two bases to form a rung – one for each side of the ladder

04/10/2023DML 202 General Biology & Human

Genetics (Chapter 11: DNA & Protein Synthesis)

13

1.1. DNA Replication


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DNA Replication Process in which a molecule of DNA forms 2 molecules

of identical ones that are also identical to the original one

Takes place in the nucleus during S

interphase i.e. occurs just before cell division

in both mitosis & meiosis Aim: Prepare another identical set of DNA molecules to

make identical sister

chromatids so that chromosome are formed

before cell division can occur

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1.1.1.Process Involved


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Occurs in semi-conservative way as proved by the experiment of Meselson & Stahl

An uncoiling enzyme helicase initially catalyses the uncoiling of small parts of DNA & then proceeds to the whole DNA

Can proceed simultaneously in

several spots of a DNA molecule

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http://www.vcbio.science.ru.nl/images/cellcycle/mcell-replication_eng_zoom.gif


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Protein

Large organic compounds made of amino acids arranged in a linear chain & joined together by peptide bonds between the carboxyl & amino groups of adjacent amino acid residues.

The sequence of amino acids in a protein is defined by a gene & encoded in the genetic code.

Genetic code: a system of representation in which a code of 3 bases in the DNA or RNA codes for an amino acid in the protein.

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1.2 Protein Definition


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A primer RNA strand is added to the side of one DNA strand that acts as a template by the addition of nucleotide triphosphates forming the new DNA strand called leading strand

It is catalysed by the enzyme DNA polymerase that adds nucleotides to the 3’ end of DNA, so replication proceeds in 5’ to 3’ fashion in the growing new DNA strand

The bases on these nucleotides are very particular about what they connect to.

Cytosine (C) will pair to guanine (G); adenine (A) will pair to thymine (T)

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However, the process is different in the other complementary strand called lagging strand of DNA that is bent by the enzyme polymerase & short strands called Okazaki fragments are formed also in 5’ to 3’ way

DNA ligase: enzyme to join the fragments formed to become continuous long complementary strand

Finally when the primer is removed by DNA polymerase, 2 identical strands of DNA are formed

Each contains one side of the original DNA & one side made of ‘new’ nucleotide

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Protein

Large organic compounds made of amino acids arranged in a linear chain & joined together by peptide bonds between the carboxyl & amino groups of adjacent amino acid residues.

The sequence of amino acids in a protein is defined by a gene & encoded in the genetic code.

Genetic code: a system of representation in which a code of 3 bases in the DNA or RNA codes for an amino acid in the protein.

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Protein SynthesisCan be divided into

(a) transcription(b) activation of amino acids

(c) translation

(d) formation of functional

protein

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1.2.1 Protein Synthesis (transcription and translation)

http://www.vcbio.science.ru.nl/images/cellcycle/mcell-transcription-translation_eng_zoom.gif


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Translation

The sequence of bases in mRNA is translated to become a sequence of amino acids in a polypeptide

Initially, mRNA forms a complex with the subunits of ribosome Two sites, P and A are created on the surface of the ribosome with

the first & second codes of the mRNA in them respectively The ribosome binds to the mRNA at the start codon (AUG) Two suitable amino-acyl tRNAs are then attached to the P site & the

A site respectively each with its anti-codon complementary to that of the mRNA.

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A peptide bond is formed between the two amino acids, which are brought close together.

The process is catalysed by a ligaseA relative movement occurs in which the 3rd code of

the mRNA is shifted to the A site & its 1st code is shifted out liberating the 1st tRNA & the 2nd code sits on the P site

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Protein Synthesis

Purpose: create a polypeptide – a protein made out of a chain of amino acids

In a hair follicle cell, a protein called keratin is made. Many ribosomes can be working on a single strand of mRNA at once.

Is not a slow processA protein chain with 400 amino acids long can be

assembled in 20 seconds.

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A peptide bond is formed between the 2nd & 3rd amino acid lengthening the polypeptide

Another relative movement occurs & the process is repeated with further lengthening of the polypeptide until the last code gets into A site

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The last codon is a ‘nonsence’ codon (UAG, UAA & UGA) indicating a ‘full stop’. No amino acyl-tRNA can get into the A site. The final relative movement would liberate the completed polypeptide, mRNA, tRNA & the subunits of the ribosomes

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DML 202 General Biology & Human Genetics (Chapter 11: DNA &

Protein Synthesis)35



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THANK YOU

Chapt 06

Technology

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