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KEY
DEFINITIONS
YOU
MUST
KNOW!
Nucleus
Organelle within the cell that contains the genetic information.
DNA
Deoxyribonucleic Acid – a nucleic acid (chemical) that carries the genetic code.
DNA is the chemical from which chromosomes are composed.
Triplet Code
Sequence of 3 bases on DNA or ‘triplet’ that codes for a particular amino acid.
Gene (s) Genes
A section of DNA that contains coded information as a sequence of bases.
Genes code for polypeptides (small proteins).
It is these polypeptides that determine the nature and development of organisms.
Allele
An alternative form of the same gene. Gene e.g. Height – alleles – tall, small.
Chromosome
A single DNA strand that has been supercoiled/condensed/contracted.
Can only be seen when the cell begins to divide. One is paternal (from father) one is maternal (from mother).
Gene locus
The fixed position on DNA that a particular gene occupies.
Polypeptide
A small protein (consisting of 10-100 amino acids)
Genome
The term used to describe all the different genes that make up an individual.
Intron OR Interruption sequence
A section of DNA that codes not code for a polypeptide (non-coding information). Found within genes and there are multiple repeats.
Exon
A section of DNA that does for a polypeptide (coding information).
Eukaryotic Cells
Animal Cell
Plant Cell
PLANT and ANIMAL Cells are EUKARYOTIC
They have a distinct MEMBRANE bound NUCLEUS (Double membrane).
What's in the Nucleus of a Cell?
GENETIC INFORMATION
DISPERSED IN THE FORM OF CHROMATIN (DNA + PROTEIN)
What happens when a cell divides?
Chromatin
Chromosomes
Chromatin CONDENSES – gets SHORTER and FATTER
and forms CHROMOSOMES.
Chromosomes always come in pairs: Paternal and Maternal
Human Male
PM
Human Female
PM
A chromosome is a single DNA molecule that is highly condensed
Recognising Chromosomes
This chromosome:
This chromosome:
1. Has NOT Replicated
2. Single stranded
3. Consists of 1 DNA
strand/helix
1. Has Replicated
2. Double stranded
3. Consists of 2 DNA strands/helices
4. 2 DNA strands are called
Chromatids
Chromsomes from a Eukaryotic Cell
These chromosomes are condensed (or coiled up)
‘Spikey’ edges show part of the DNA that is not tightly coiled.
Chromosomes, Genes and DNA
Gene =section
of DNA that
coded information
as a sequence of
bases
Chromosomes are a single DNA
molecule that has been supercoiled.
A chromosome
= single DNA
molecule
Alleles – different
forms of a gene
Different Gene
Paternal
Chromosome
Maternal
Chromosome
Chromosomes, Genes and DNA
Chromosomes are a
single DNA molecule
that has been
supercoiled.
DNA, Chromatin and Chromosomes - notes
• EUKARYOTIC CELLS are those with distinct MEMBRANE BOUND NUCLEUS.
• Animal, Plant, Fungi, all have eukaryotic cells.
• The nucleus of the cell contains the GENETIC INFORMATION in the form of DNA.
• DNA is a HELICAL molecule that is LINEAR (forms a straight line).
• When a cell is not dividing the DNA exists as CHROMATIN.
• When the cell divides the CHROMATIN condenses (gets shorter and fatter).
• This condensed CHROMATIN is forms CHROMOSOMES.
• A CHROMOSOME is simply a HIGHLY CONDENSED SINGLE DNA molecule.
• Chromosomes always come in PAIRS.
• One is PATERNAL (from the father) one is MATERNAL (from the mother).
• GENES are SECTIONS of DNA from which the chromosomes are composed of.
• Genes contain coded information that code for single POLYPEPTIDE chains or
PROTEINS.
• It is these polypeptides/proteins that determine our CHARACTERSITICS.
• ALLELES are ALTERNATIVE forms of a gene.
• E.g. Gene for Height can have alleles – Tall and Small.
Summary Questions
1. What is visible in a cell when it is not dividing?
2. What becomes visible during cell division?
3. What does condense mean?
4. What is a chromosome?
5. What is a gene?
6. What do genes code for?
7. How can you tell if a chromosome has replicated?
8. Assume human chromosome 22 has 450 genes.
(i) How many DNA molecules does chromosome 22 carry
(ii) How many copies of chromosome 22 are found in:
Sperm cell
Egg cell
Embryo with 16 cells
(iii) How many polypeptides does chromosome 22 code?
Summary Answers
1. Chromatin.
2. Chromosomes.
3. Means that the genetic material gets shorter and fatter.
4. A single DNA molecule that is highly condensed.
5. A section of DNA / A section of a chromosome – as chromosomes are DNA.
6. Polypeptides or proteins – these make up our characteristics.
7. It becomes double stranded.
8.
(i) Chromosome 22 consists of 1 DNA molecule. As 1 chromosome = 1 DNA
(ii) 1 copy of chromosome 22 would be found in a sex cell.
(iii) 2 copies of chromosome 22 (one paternal/one maternal).
(iv) 450 genes – each gene codes for 1 polypeptide so 4 polypeptides.
How is DNA Organised in Eukaryotic Cells?
How does DNA fit into the cell?
DNA is 1000mm long (100cm)
This is 1000,000µm (x1000 as 1mm = 1000µm)
An average cell is 100µm
So DNA is 1000,000/100 = 10,000x bigger
So how does it fit into the cell?
Answer: coiling and supercoiling
Linear DNA
DNA wrapped around
histone protein to form a
Nucleosome
10-11nm in size
Histone
Protein
Beads on a string
arrangement
Beads on a string
supercoiled to form
chromatin
In Non-Dividing cells
chromatin is the main
form in which DNA is
packaged.
In Dividing cells
chromatin supercoils
further to form
chromosomes
Organisation of Eukaryotic DNA
Chromatin
DNA is coiled around histone proteins to form nucleosomes – ‘beads on a string’.
Histone
DNA
Nucleosomes are further coiled (super coiled) to form chromatin.
Nucleosomes are further coiled to form chromosomes - during cell division.
Eukaryotic DNA Organisation into Chromosomes
Supercoiling of Chromatin to
give Chromosomes
Chromatin
Histone
Protein
Nucleosomes 10-11 nm
Linear DNA Molecule
• Nucleosome = DNA wrapped around histone proteins.
• Chromatin = Supercoiling of nucleosomes. DNA is dispersed throughout the nucleus as chromatin.
• Chromosome = Further coiling of chromatin to form chromosomes.
Eukaryotic DNA Organisation into Chromosomes
• Nucleosome = DNA wrapped around histone proteins.
• Chromatin = Supercoiling of nucleosomes. DNA is dispersed throughout the nucleus as chromatin.
• Chromosome = Further coiling of chromatin to form chromosomes.
Organisation of DNA into Chromosomes - notes
What are chromosomes?
• A chromsome is a single DNA molecule that has been supercoiled.
• A supercoil is simply a coil within a coil.
• Chromosomes look thread-like under the microscope.
How is DNA packaged to form Chromosomes?
•
In order to form a chromosome the DNA is packaged by:
1. Wrapping it around proteins called Histones
2. This forms nucleosomes - about 10-11nm in size.
3. Often referred to as ‘beads on a string’.
4. The nucleosomes are then supercoiled to form chromatin.
5. Chromatin is the collective complex of ‘nucleosomes’.
6. During cell division chromatin coils further to form short fat bundles called
chromosomes.
Chromosome anatomy
• When fully condensed each chromosome consists of:
1. Two chromatids (or DNA molecules)
2. Centromere (region when the chromosomes join).
•
Where chromosomes consists of two chromatids they have replicated.
Summary Questions: Chromosome Structure
1.
How is the considerable length of DNA compacted into a chromosome?
2.
What is the function of the proteins found in a chromosome?
3.
What are the main biochemical components of chromosomes.
4.
What is chromatin?
5.
How is the genetic information different prior to cell division and at the start of cell
division?
6.
What do the two chromatids of a chromosome represent?
7.
Why is DNA wrapped around histone proteins?
8.
If there are 46 chromosomes in a human cell, each with an average of 5cm
uncoiled length, estimate the length of DNA contained within a single cell in
meters.
9.
Suppose the total length of DNA in a single human muscle cell was 2.3m. If all the
DNA were distributed equally between the chromosomes what would be the
length of each one.
10. What do you think is the length of DNA in a human brain cell?
Summary Answers: Chromosome Structure
1.
DNA is wrapped around proteins to form nucleosomes – which are in turn supercoiled
to form chromosomes.
2.
The protein fixes the DNA into position (DNA is wrapped around it).
3.
DNA and protein
4.
Supercoiled nucleosomes – the way in which the DNA is dispersed throughout the
cell/packaged before cell division.
5.
Prior to cell division DNA exists are chromatin (DNA and histone proteins). During cell
division DNA exists as chromosomes. This is chromatin that is further coiled.
6.
Two arms of the chromosome represent – separate DNA helices/molecules.
7.
Wrapping the DNA around histones reduces the size of the DNA i.e. It condenses DNA
into a tiny length. This allows DNA to fit into the nucleus of a cell.
8.
1 cell has 46 chromosomes, therefore 46 x5 = 230 cm in one cell. Given that 100cm = 1
meter, then 230/100 = 2.3m in total.
9.
Each chromosome= single DNA molecule. So if we have 2.3m of DNA and 46
chromosomes – each chromosome would have 2.3/46 = 0.05m.
10.
2.3 m as all cells have the same quantity of DNA.