1. IB Topic 4.1- Chromosomes and Karyotyping
Focus Question:
What are genes?
IB Topic 4.1

2. Chromosome Genes Initial terms to know: (which are made of) (contains)
DNA (nucleotide unit)

3. DNA
Deoxyribonucleic acid, a molecule made up of repeating (nucleotides) that form a double helix.
In eukaryotes DNA is always associated with a protein
In prokaryotes:
chromosomes are circular
Extra DNA in small loops called plasmids
No proteins associations

4. Chromosome
a large strand of DNA (usually million base pairs long in humans)
contains many genes. (although only 5% or so of DNA are genes that code for protein.)
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5. Genes are represented by letters
Ex: Hair Color
B = brown
b = blond
Start with the general rule that you have two genes for each trait. (One comes from mom and one from dad)
               
               

6. Allele = ONE specific form of a gene- differs from other alleles by one or a few bases only and occupies the same gene locus
Ex:
B = brown
b = blond
r = red
l = black

7. A gene pool may have multiple alleles, but only two are found in each individual.
Ex: BB Bb bb B r L b

8. Genotype-The Type O’ Gene
Genotype- The two genes you have for a trait or the letters. BB Bb bb
Homozygous = BB, bb
Heterozygous = Bb

9. Dominance B b B dominates over b
Ex: Bb contains a blond gene in the genotype, but hair will appear brown.

10. Phenotype
What those two genes make you look like
“Brown”
“Blond”

11. Summary: Genotype vs. PhenotypeBB
“Brown” Bb bb
“Blond”

12. Clarification of Terms
Gene- a heritable factor that controls a specific characteristic.
Allele- one specific form of a gene, differing from other alleles by one or a few bases only and occupying the same gene locus as other alleles of the gene.
Genome- the whole of the genetic information of an organism (sum total of all possible alleles available in a particular species)

13. Genes aren’t the only thing on chromosomes…
Most ladder steps on a chromosome are made of repetitive, non-coding sequences called __?
In particular, mammals have accumulated considerable repetitive elements and noncoding regions, which account for the majority of their DNA sequences (52% non-coding and 44% repetitive DNA). Only 1.2% of the mammalian genome thus encodes for protein function.

14. Chromosome Structure
(1) Chromatid (2) Centromere (3) Short (p) arm (for petite) (4) Long (q) arm (the next letter in the alphabet)

15. Allele Nomenclature
The chromosomal locus of a gene might be written "6p21.3".
Component
Explanation 6
The chromosome number. p
The position is on the chromosome's short arm (p for petit in French); q indicates the long arm (chosen as next letter in alphabet after p).
21.3
The numbers that follow the letter represent the position on the arm: region 2, band 1, sub-band 3. The bands are visible under a microscope when the chromosome is suitably stained. Each of the bands is numbered, beginning with 1 for the band nearest the centromere. Sub-bands and sub-sub-bands are visible at higher resolution.

16. Karyotypes
The karyotype of an organism is usually displayed in photomicrographs wherein chromosomes are arranged in homologous pairs, and in descending order of size and relative position of the centromere.
Karyotype is used to study chromosomal aberrations, cellular function, or taxonomic relationships, or to gather information about past evolutionary events

17. Karyotyping
Karyotype- Identification of individual chromosomes using a photograph of the DNA in a dividing cell. This can be used to identify certain genetic disorders.
The chromosomes are arranged in pairs according to their structure.

18. Normal Genes
When a genes is acting like it should, that part of your body is normal.

19. Extra-Missing Chromosome
Chromosome Mutation- occurs when there is an extra or missing chromosome. Ex: Down Syndrome, Turner, Kleinfelter

20. Gene Mutation- occurs when there is a change in the base sequence of a gene.
Ex. Sickle cell Anemia.
GAG has mutated to GTG causing glutamic acid to be replaced by valine, hence sickle cell anemia.
Ss = resistant to malaria, ss = sickle cell, SS = no sickle cell.

21. Mutations
Mutations are changes in the genetic material of a cell (or virus).
These include large-scale mutations in which long segments of DNA are affected (for example, translocations, duplications, and inversions).
A chemical change in just one base pair of a gene causes a point mutation.
If these occur in gametes or cells producing gametes, they may be transmitted to future generations.

22. For example, sickle-cell disease is caused by a mutation of a single base pair in the gene that codes for one of the polypeptides of hemoglobin.
A change in a single nucleotide from T to A in the DNA template leads to an abnormal protein.
Fig

23. A point mutation that results in replacement of a pair of complementary nucleotides with another nucleotide pair is called a base-pair substitution.
Some base-pair substitutions have little or no impact on protein function.
In silent mutations, alterations of nucleotides still indicate the same amino acids because of redundancy in the genetic code.
Other changes lead to switches from one amino acid to another with similar properties.
Still other mutations may occur in a region where the exact amino acid sequence is not essential for function.

24. Other base-pair substitutions cause a readily detectable change in a protein.
These are usually detrimental but can occasionally lead to an improved protein or one with novel capabilities.
Changes in amino acids at crucial sites, especially active sites, are likely to impact function.
Missense mutations are those that still code for an amino acid but change the indicated amino acid.
Nonsense mutations change an amino acid codon into a stop codon, nearly always leading to a nonfunctional protein.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

25. Fig

26. Insertions and deletions are additions or losses of nucleotide pairs in a gene.
These have a disastrous effect on the resulting protein more often than substitutions do.
Unless these mutations occur in multiples of three, they cause a frameshift mutation.
All the nucleotides downstream of the deletion or insertion will be improperly grouped into codons.
The result will be extensive missense, ending sooner or later in nonsense - premature termination.

27. Mutations can occur in a number of ways.
Errors can occur during DNA replication, DNA repair, or DNA recombination.
These can lead to base-pair substitutions, insertions, or deletions, as well as mutations affecting longer stretches of DNA.
These are called spontaneous mutations.

28. Chemical mutagens may operate in several ways.
Mutagens are chemical or physical agents that interact with DNA to cause mutations.
Physical agents include high-energy radiation like X-rays and ultraviolet light.
Chemical mutagens may operate in several ways.
Some chemicals are base analogues that may be substituted into DNA, but that pair incorrectly during DNA replication.
Other mutagens interfere with DNA replication by inserting into DNA and distorting the double helix.
Still others cause chemical changes in bases that change their pairing properties.