Understanding Genes, Chromosomes, Alleles, and Mutations in DNA

 
Genes, Chromosomes, Alleles
and Mutations
 
 
Review of DNA
 
7.1.1
 
Nucleosomes
 
In eukaryotes, DNA associates with proteins called
histones
DNA is wound around an octamer of histones and is
secured to a H1 histone, forming a nucleosome
Nucleosomes serve two main functions:
They protect DNA from damage
They allow long lengths of DNA
    to be packaged (supercoiled)
    for mitosis / meiosis
 
7.1.2 / 7.1.3
 
Eukaryotic Chromosomes
 
Eukaryotic chromosomes consist of DNA wrapped
around histone proteins
This forms the basic structure of the nucleosome,
which is packed together to form chromatin (in a
'beads on a string' arrangement)
Chromatin will supercoil and condense during
prophase to form chromosomes
 
4.1.1
 
Key terms
 
Gene: 
 A heritable factor that controls a specific
characteristic, consisting of a length of DNA
occupying a particular position on a chromosome
(locus)
 
Allele: 
 One specific form of a gene, differing
from other alleles by one or a few bases only and
occupying the same locus as other alleles of the
gene
 
Genome: 
 The whole of the genetic information
of an organism
 
4.1.2
 
Composition of the Genome
 
DNA is either made up of unique sequences (genes) or non-coding
sequences (which includes repetitive sequences)
 
7.1.4
 
Mutations!
 
Gene mutation:
  A change in the nucleotide
sequence of a section of DNA coding for a
particular feature
 
An example of a gene mutation that causes
disease by altering the amino acid sequence
of a polypeptide is sickle cell anaemia
 
 
4.1.3
 
Cause of Sickle Cell Anaemia
 
A base substitution mutation is the change of a single base in
a sequence of DNA, resulting in a change to a single mRNA
codon during transcription
 
In sickle cell anaemia, the 6th codon for the beta chain of
haemoglobin is changed from GAG to GTG (on the non-coding
strand)
 
This causes a change in the mRNA codon (GAG to GUG),
resulting in a single amino acid change of glutamic acid to
valine (Glu to Val)
 
The amino acid change alters the structure of haemoglobin,
causing it to form fibrous, insoluble strands
 
4.1.4
 
Red Blood Cells
 
Consequences of Sickle Cell Anaemia
 
The insoluble haemoglobin cannot effectively carry
oxygen, causing individual to feel constantly tired
The sickle cells may accumulate in the capillaries and
form clots, blocking blood supply to vital organs and
causing a myriad of health problems
Also causes anaemia (low RBC count), as the sickle cells
are destroyed more rapidly than normal red blood cells
Sickle cell anaemia is an autosomal recessive condition
and is thus caused by having two recessive alleles
Heterozygous individuals have increased resistance to
malaria due to the presence of a single recessive allele
(heterozygous advantage)
 
Anaemia
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Explore the intricate world of genetics through genes, chromosomes, alleles, and mutations. Delve into the fundamental structures of DNA, such as nucleosomes and eukaryotic chromosomes. Gain insights into key genetic terms like genes, alleles, and genome composition. Learn about mutations, including gene mutations like in sickle cell anemia, and understand the genetic basis of diseases. Unravel the complex mechanisms by which DNA dictates inheritable characteristics and influences genetic diversity.


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  1. Genes, Chromosomes, Alleles and Mutations

  2. Review of DNA 7.1.1 C G A T G C

  3. Nucleosomes 7.1.2 / 7.1.3 In eukaryotes, DNA associates with proteins called histones DNA is wound around an octamer of histones and is secured to a H1 histone, forming a nucleosome Nucleosomes serve two main functions: They protect DNA from damage They allow long lengths of DNA to be packaged (supercoiled) for mitosis / meiosis

  4. Eukaryotic Chromosomes 4.1.1 Eukaryotic chromosomes consist of DNA wrapped around histone proteins This forms the basic structure of the nucleosome, which is packed together to form chromatin (in a 'beads on a string' arrangement) Chromatin will supercoil and condense during prophase to form chromosomes

  5. Key terms 4.1.2 Gene: A heritable factor that controls a specific characteristic, consisting of a length of DNA occupying a particular position on a chromosome (locus) Allele: One specific form of a gene, differing from other alleles by one or a few bases only and occupying the same locus as other alleles of the gene Genome: The whole of the genetic information of an organism

  6. Composition of the Genome 7.1.4 DNA is either made up of unique sequences (genes) or non-coding sequences (which includes repetitive sequences)

  7. Mutations! 4.1.3 Gene mutation: A change in the nucleotide sequence of a section of DNA coding for a particular feature An example of a gene mutation that causes disease by altering the amino acid sequence of a polypeptide is sickle cell anaemia

  8. Cause of Sickle Cell Anaemia 4.1.4 A base substitution mutation is the change of a single base in a sequence of DNA, resulting in a change to a single mRNA codon during transcription In sickle cell anaemia, the 6th codon for the beta chain of haemoglobin is changed from GAG to GTG (on the non-coding strand) This causes a change in the mRNA codon (GAG to GUG), resulting in a single amino acid change of glutamic acid to valine (Glu to Val) The amino acid change alters the structure of haemoglobin, causing it to form fibrous, insoluble strands

  9. Red Blood Cells

  10. Consequences of Sickle Cell Anaemia The insoluble haemoglobin cannot effectively carry oxygen, causing individual to feel constantly tired The sickle cells may accumulate in the capillaries and form clots, blocking blood supply to vital organs and causing a myriad of health problems Also causes anaemia (low RBC count), as the sickle cells are destroyed more rapidly than normal red blood cells Sickle cell anaemia is an autosomal recessive condition and is thus caused by having two recessive alleles Heterozygous individuals have increased resistance to malaria due to the presence of a single recessive allele (heterozygous advantage)

  11. Anaemia

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