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Genes, Genome and DNA. The base materials for inheritance. Human genome. Genome refers to the total genetic material within a cell. In humans there are 30000 genes. Most is in the nucleus.

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genes genome and dna

Genes, Genome and DNA

The base materials for inheritance.

human genome
Human genome
  • Genome refers to the total genetic material within a cell. In humans there are 30000 genes.
  • Most is in the nucleus.
  • Some is in mitochondria. (Believed to be engulfed organisms in the distant evolutionary past of eukaryotic cells)
  • Is the main component of chromosomes.
  • It bears the genetic material of organisms.
  • DNA’s structure is similar to a twisted ladder with 4 nucleic bases Adenosine, Thymine, Cytosine and Guanine.
  • The study of DNA and its manipulation is known as Genetics.

The four nucleic acids always join together in a particular way, in complementary base pairs.

  • Adenosine with Thymine (A=T)
  • Cytosine with Guanine (C=G)
  • Each pair is called the base pair.
  • The 2 strands are called complementary strands.
  • There are 2.9 billion combinations of A-T, G-C
structure of nucleotides

Phosphate: Links neighboring sugars

Sugar: One of two types possible: ribose in RNA and deoxyribose in DNA

Base: Four types are possible in DNA: adenine, guanine, cytosine and thymine. RNA has the same except uracil replaces thymine.

Structure of Nucleotides

Purine – double ringed structure. (A & G)

Single ringed structure called pyrimidine.

( T & C)

  • The chemical structure of nucleotides:

Symbolic form


The complementary strands of DNA are referred to as antiparallel because one runs 5’ to 3’, while the other runs 3’ to 5’ ( ‘ read as prime).

  • 5’ end has an unlinked phosphate group
  • 3’ end has an unlinked ribose sugar
  • In eukaryotes the DNA is coiled around small proteins called histones.
  • The DNA wrapped around the histone proteins is called a nucleosome. In fact there is about twice as much protein as DNA in chromosomes.
  • This packaging protects it from enzymatic degradation and is an efficient form of packaging.
  • When cells prepare to divide the nucleosomes fold in a regular manner to produce supercoils that become visible under the light microscope.
  • A gene is a unit of heredity made up of a unique sequence of DNA. Together with proteins, it is organised into chromosomes.
  • The whole set of genes in an organism is a genome.
  • Are the basic units of heredity. They are made up of a unique sequence of DNA.
  • The complete set of genes make up the genome.
  • Genes are located on chromosomes.
  • A gene is a section of DNA with a specific sequence of nitrogenous bases. This sequence is the code that directs the manufacture of a protein.
  • Some genes code for tRNA or rRNA.
  • The position of the gene on a chromosome is called the locus (pl. loci)
  • Genes are named for the function they control or there observable effect in an organism. Eg. RH gene controls Rhesus blood type.
  • Genes are linked if they are carried on the same chromosome. Eg. Sex-linked diseases.
genes how do they control things
Genes – how do they control things?


  • Genes are inherited from parents through egg and sperm cell
  • Genes are made of DNA
  • Genes differ from each other in their length and in the order of their DNA base pair.
gene s job
Gene’s Job
  • Genes job is to control the production of proteins which serve critical roles in cellular structure and function and in development.
  • One gene-one protein hypothesis has now been modified to one gene- one polypeptide
genes in action
Genes in action
  • Genes act by specifying the production of polypeptides which form proteins.
  • Proteins perform a myriad of essential functions in the cell.
  • Eg:
      • Enzymes, transport, carrying oxygen, antibodies
  • All living cells & viruses contain genetic information in chromosomes.
  • Therefore they are made of DNA & found in the nucleus.
  • They are supercoils of DNA.
  • They are the most compact form of data storage known.
  • Each gene has its own position on the chromosome. (locus)
numbers of chromosomes
Numbers of Chromosomes
  • Chromosome numbers vary considerably among organisms.
  • The numbers may differ markedly even between closely related species:
how many chromosomes
How many chromosomes?
  • All nucleated cells of an organism contain a fixed number of chromosomes.
  • Somatic cells (body cells, except gametes) contain a fixed number of chromosomes per species.
  • humans 46, koala 16, fern 1260,
  • The ploidy level of a cell refers to the number of chromosome sets it carries.
  • Diploid (2n) means 2 sets of chromosomes (somatic cells – body cells)
  • Haploid (n) means 1 set of chromosomes (gametes – sex cells)
genes in eukaryote cells

Nucleus contains inherited information: The total collection of genes located on chromosomes in the nucleus has the complete instructions for constructing a total organism.

Nuclear pores are involved in the active transport of substances into and out of the nucleus

Nuclear membrane

encloses the nucleus in eukaryotic cells

Cytoplasm: The nucleuscontrols cell metabolism; the many chemical reactions that keep the cell alive and performing its designated role.


Structure of the nucleus

Chromosomes are made up of DNA and protein and store the information for controlling the cell

Nucleolus is involved in the construction of ribosomes

Genes in Eukaryote Cells
  • Eukaryotes have genetic information stored in chromosomes in the nucleus of each cell:
genes outside the nucleus in eukaryote cells




Chloroplast DNA



Genes Outside the Nucleusin Eukaryote Cells
  • Eukaryotes have two types of organelles with their own DNA:
    • mitochondria
    • chloroplasts
  • The DNA of these organelles is replicated when the organelles are reproduced (independently of the DNA in the nucleus).
genes in prokaryote cells

Cytoplasm(no nucleus)

Single, circular



Cell membrane

Genes in Prokaryote Cells


  • Bacteria have no membrane-bound organelles.
  • Cellular reactions occur on the inner surface of the cell membrane or in the cytoplasm.
  • Bacterial DNA is found in:
    • One, large circular chromosome.
    • Several small chromosomal structures called plasmids.


Cell wall

plasmid dna

Sex pilus conducts the plasmid to the recipient bacterium

A plasmid about to pass one strand of the DNA into the sex pilus

Plasmid of the

non-conjugative type

Plasmid of the conjugative type

Plasmid DNA


  • Bacteria have small accessory chromosomes called plasmids.
  • Plasmids replicate independently of the main chromosome.
  • Some conjugativeplasmids can beexchanged with otherbacteria in a processcalled conjugation.
  • Via conjugation, plasmidscan transfer antibiotic resistance to other bacteria.


the big questions
The BIG Questions…
  • How are genes turned on & off in eukaryotes?
  • How do cells with the same genes differentiate to perform completely different, specialized functions?
why turn genes on off
Why turn genes on & off?
  • Specialization
    • each cell of a multicellular eukaryote expresses only a small fraction of its genes
  • Development
    • different genes needed at different points in life cycle of an organism
      • afterwards need to be turned off permanently
  • Responding to organism’s needs
    • homeostasis
    • cells of multicellular organisms must continually turn certain genes on & off in response to signals from their external & internal environment
molecular biology
Molecular Biology
  • the central DOGMA of Molecular Biology can be summarised in the form:
how do we make proteins
How do we make proteins?
  • In order to make proteins we first must have a set of instructions.
  • That is the DNA or more particularly the genes and their nucleotide bases.
  • It can be considered our blue print.
  • DNA never leaves the nucleus, but proteins are synthesized in the cytoplasm, so a copy of each gene is made to carry the “message” from the nucleus to the cytoplasm. This copy is mRNA, and the process of copying is called transcription.
  • We don’t want to lose our good copy of instructions so the first step is to make a copy of DNA which we can use outside the nucleus.
  • This process is called transcription and the copy is called messenger RNA
  • As the RNA polymerase moves along the RNA is formed.
  • RNA has slight differences to DNA it is single stranded and instead of thymine it has uracil. The nucleotides pair exactly the same.

Here is how it works:

  • DNA is separated by RNA polymerase which is an enzyme.
  • This makes a copy of DNA and then attaches nucleotides from within the cell according to base pair rules.

RNA polymerase enzyme

Free nucleotides

used to construct

the mRNA strand

Template strand of DNA contains the information for the construction of a functional mRNA product (e.g. a protein)


chromosome as found in non-dividing cell

Direction of synthesis

Formation of a single strand of mRNA that is complementary to the template strand (therefore the same “message” as the coding strand)



  • A mRNA strand is formed using the DNAmolecule as the template.
  • Free nucleotides with bases complementary to the DNA are joined together by the enzyme RNApolymerase.

Coding strand

The two strands of DNA coil up into a double helix

5’ is the 5 prime end which has a free phosphate group.
  • 3’ is the 3 prime end which has a free hydroxyl group
  • This is why it is known as an anti parallel double stand.
  • Replication starts at the 5 prime end.
the next step
The next step
  • Once we have a copy of DNA in the form of mRNA we can then send the instructions outside the nucleus where we can begin to make protein.
  • This is called Translation
  • The mRNA copy is made with the help of RNA polymerase. This enzyme joins up the mRNA nucleotides to make a mRNA strand.
  • This mRNA strand is a complementary copy of the DNA (gene)
  • The mRNA molecule leaves the nucleus via a nuclear pore into the cytoplasm
movement of mrna



Movement of mRNA
  • In eukaryotic cells, the two main steps in protein synthesis occur in separate compartments: transcription in the nucleus and translation in the cytoplasm.
    • mRNA moves out ofthe nucleus, to thecytoplasm, through pores inthe nuclear membrane.
  • In prokaryotic cells, there is no nucleus, and the chromosome is in direct contact with the cytoplasm, and protein synthesis can begin even while the DNA is being transcribed.

Nuclear pore through

which the mRNA passes

into the cytoplasm



  • In the process of translation the sequence of bases in the mRNA is read and the amino acids are assembled in a corresponding order to form a polypeptide.
  • As the ribosome moves along the mRNA then another amino acid is joined with a peptide bond.
  • As the amino acid is joined the tRNA moves off.
  • mRNA is read by the ribosome which attaches amino acids via transfer RNA
  • The sequence is the genetic code.
genetic code
Genetic code
  • The gene code works in triplets.
  • Every three letters represent a amino acids. For example UAA means STOP.
  • The Three letter code is called a CODON
  • Gene code
  • First the mRNA attaches itself to a ribosome (to the small subunit).
  • Six bases of the mRNA are exposed.
  • A complementary tRNA molecule with its attached amino acid (methionine) base pairs via its anticodon UAC with the AUG on the mRNA in the first position P.
  • Another tRNA base pairs with the other three mRNA bases in the ribosome at position A.
  • An enzyme initiates the formation of a peptide bond between the amino acids.
  • The first tRNA (without its amino acid) leaves the ribosome.
translation 2
Translation 2

The ribosome moves along the mRNA to the next codon (three bases).

The second tRNA molecule moves into position P.

Another tRNA molecule pairs with the mRNA in position A bringing its amino acid.

A growing polypeptide is formed in this way until a stop codon is reached.

end of translation
End of Translation

A stop codon on the mRNA is reached and this signals the ribosome to leave the mRNA. A newly synthesised protein is now complete!

gene regulation
  • Virtually every cell in your body contains a complete set of genes
  • But they are not all turned on in every tissue
  • Each cell in your body expresses only a small subset of genes at any time
  • During development different cells express different sets of genes in a precisely regulated fashion
  • While the expression of some genes is continuous, the expression of others is regulated in cells.
  • Although all somatic cells in an organism carry identical gene sets, only a proportion of those genes are switched on in a given cell type or stage of development.
gene regulation1
  • In eukaryotic organisms like ourselves there are several methods of regulating protein production
  • Most regulatory sequences are found upstream from the promoter
  • Genes are controlled by regulatory elements in the promoter region that act like on/off switches or dimmer switches
gene regulation2
  • Specific transcription factors bind to these regulatory elements and regulate transcription
  • Regulatory elements may be tissue specific and will activate their gene only in one kind of tissue
  • Sometimes the expression of a gene requires the function of two or more different regulatory elements







Both exons and introns are transcribed to produce a long primary RNA transcript

Double stranded molecule of genomic DNA



Exons are spliced together

Messenger RNA is an edited copy of the DNA molecule (now excluding introns) that codes for a single functional RNA product, e.g. protein.







messenger RNA

The primary RNA transcript is edited

Primary RNA transcript


Introns are removed


Introns and Exons

Most eukaryotic genes contain segments of protein-coding sequences (exons) interrupted by non-protein-coding sequences (introns).

  • Introns in the DNA are long sequences of codons that have no protein-coding function.
  • Introns may be remnants of now unused ancient genes.
  • Introns might also facilitate recombination between exons of different genes; a process that may accelerate evolution.
  • Most eukaryotic genes contain segments of protein-coding sequences (exons) interrupted by non-protein-coding sequences (introns).
    • Introns in the DNA are long sequences of codons that have no protein-coding function.
    • Introns may be remnants of now unused ancient genes.
    • Introns might also facilitate recombination between exons of different genes; a process that may accelerate evolution.
introns and exons
  • After the initial transcript is produced the introns are spliced out to form the completed message ready for translation
  • Introns can be very large and numerous, so some genes are much bigger than the final processed mRNA