DNA structure

The structural unit of chromosome is nucleosome.

- A nucleosome is made up of DNA and histone proteins. DNA is coiled around histone

proteins.

 DNA (doc.d p39)

- DNA (Deoxyribo Nucleic Acid) is located inside the nucleus of every nucleated cell.

N.B. Mitochondria also contains DNA

 DNA structure:

- The structural unit (repeated unit) of DNA is called nucleotide.

- The nucleotide is made up of Deoxyribose sugar (5 carbon sugar) + phosphate +

nitrogenous base

Histone protein

DNA

DNA

Histone protein

Nucleosome

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- There are 4 nitrogenous bases: Adenine (A), Guanine (G), Cytosine (C), and Thymine

(T)

- Structure of nucleotide

The DNA structure is double helix.

- DNA has a twisted ladder shape (double helix).

- DNA molecule is made up of two strands attached by hydrogen bonds.

- DNA has a ladder shape. Deoxyribose sugar and phosphate form the backbones

(sides) of the DNA strand, while the nitrogenous bases form the inner sides that

connect the two strands.

- Base pairing complementarity rule: (A) pairs with (T) forming a double bond while

(C) pairs with (G) forming a triple bond. The bond between nitrogenous bases is

weak.

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 How is DNA formed?

1. A bond is formed between phosphate of one nucleotide and the sugar of another

nucleotide to form the backbones (sides) of DNA.

2. Nitrogenous bases of the nucleotides pair together (by hydrogen bonds) to form

double strand.

3. DNA is twisted to form a double helix.

DNA REPLCATION Identical reproduction and cell cycle(DNA replication)

 Cell cycle :

 The phases of Interphase are:

G1 , S, and G2

- G0 or G1: Growth or Gap 1: The organelles duplicate and develop. The cell prepares

for synthesis phase by synthesizing the proteins (enzymes) needed for S-phase.

Chromosomes do not duplicate during G1 .

The non-dividing cells (not undergoing mitosis) and differentiated cells stay in this

phase.

- S: synthesis: The chromosomes duplicate by a process called DNA replication, each

chromosome which was made up of 1 chromatid becomes made up of 2 chromatids.

- G2: Gap 2 or growth: organelles are doubled and increase in number. The cell

Synthesizes proteins (enzymes) needed for mitosis. Proofreading of the synthesized

DNA strands takes place.

 DNA replication:

- It occurs during S-phase of Interphase.

- It takes place inside the nucleus.

- One DNA molecule duplicates into two DNA molecules.

 Steps of DNA replication:

1. Unwind the double helix.

2. Unzip the double stranded DNA by breaking the weak hydrogen bonds under the action

of Helicase enzyme.

3. DNA polymerase starts adding new complementary nucleotides to the nucleotides in the

original DNA strands. (according to base pairing complementarity)

4. DNA strands rewind forming two new DNA molecules.

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 DNA replication is semi conservative

The newly formed DNA molecules are made up of one old strand and one new strand.

Therefore DNA replication is semi conservative.

Aspect of chromosomes and DNA during different phases of interphase

Proteins, an association of amino acid

 Proteins:

 Proteins in food are found in meat, egg white, chicken, soy beans, lentils.... Proteins are

needed to build up new matter.

 Proteins in the human body are enzymes, hormones, antibodies, receptors, channels in the

cell membrane.

 Human proteins are synthesized in the body by a process called protein synthesis.

 Proteins can be structural or functional; structural enter in the building up of the cell

example hemoglobin. Functional have a certain function to do example hormones and

enzymes.

Structure of proteins:

 Proteins are made up of long chain of amino acids connected by peptide bonds.

 Proteins have primary, secondary, tertiary, and quaternary structure.

- Primary structure: amino acids form long chains called polypeptides.

- Secondary structure: the polypeptide chain is twisted.

- Tertiary structure: Polypeptides are folded to form an active protein and a molecule

is added example iron.

- Quaternary structure: more than one polypeptide chains are attached together.

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Structure of amino acids:

 There are only 20 kinds of amino acids in nature.

 All amino acids have: carboxyl group (COOH), amino group (NH2 ), H and R group (side

chain). The side chain differs from one amino acid to another.

 The difference between proteins:

 Proteins differ in:

- number of amino acids

- kind of amino acids

- Sequence of amino acids

 The factors that affect the activity of enzymes.

- Number of amino acids

- sequence of amino acids

- spatial configuration (3D structure)

The gene, structure and information unit

 A gene is a fragment of DNA (part of the chromosome) that has specific sequence of

nucleotides which codes for a specific characteristic.

 Homologous chromosomes have the same genes on the same locus ( location).

 Notion of a gene:

Gene (sequence of DNA nucleotides) codes Protein (sequence of amino acid) phenotype (trait)

 Define transgenesis.

It is the transfer of a gene from one individual to another belonging to the same or

different species.

 Aim of transgenesis

The aim of transgenesis is to produce organisms having better characteristics.

 Define transgenic organism. (Genetically modified organisms- GMO)

Is the organism that receives a foreign gene from another organism and this leads to the

formation of a new protein that results in the appearance of a new phenotype (trait).

 A codon : is a triplet of nucleotides (3 nucleotides that code for one amino acid)

 What is an allele?

The different sequences of DNA of the gene are called alleles.