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BIOL1000 Block 2 Part 2 Summary: Chromosomes, Mitosis, Meiosis + Mendelian Genetics R100,00
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BIOL1000 Block 2 Part 2 Summary: Chromosomes, Mitosis, Meiosis + Mendelian Genetics
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A summary of part 2 of block 2 of ILS. This document covers chromosomes, mitosis, meiosis and Mendelian Genetics.

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  • June 7, 2022
  • June 22, 2022
  • 37
  • 2021/2022
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CHROMOSOMES
Cellular organisation of genetic material

● All DNA in a cell is the cells genome

○ The genome can consist of a single DNA molecule of many DNA

molecules

■ Prokaryotic cells generally only have a single, circular molecule

making up the genome

■ Eukaryotic organisms have a number of DNA molecules

● DNA are associated with proteins (histones in eukaryotes) that package the DNA

● Chromosome: a single piece of DNA associated with packing proteins

○ DNA interacts with certain dyes

○ When positively charged dyes are added to DNA (which is negatively

charged), strong interactions occur

● 2m of DNA is contained within one cell

○ This is made possible by the very tight packing of DNA in chromosomes

■ As well as the fact that DNA is very thin



Condensed chromosomes

● During cell division chromosomes condense to become more clearly visible

○ (This is after DNA replication)



● The sister chromatids have exactly the

same DNA sequence

● The end regions are called telomeres

○ They are made up of repetitive

sequences of non coding DNA

, ○ Recall DNA replication: the primer is added before DNA pol III can add

nucleotides however after these primers are removed by DNA pol I, there

is just empty space at the end of the DNA strand

■ The longer strand therefore shortens so it can be the same length

as the replicated strand

○ Telomerase act as a buffer to delay gene shortening - telomerase is an

enzyme that extends the telomeres

■ Telomerase uses repeated sequences to do this

○ Human germ cells contain telomerase to ensure that DNA is not lost from

one generation to another



The types of chromosomes

Based on the position of the centromere




● Note that telocentric chromosomes are not found in human cells




Chromosomes in human cells

● Somatic cells (not sex cells) have 23 pairs of chromosomes

, ○ 22 pairs of chromosomes and 1 pair of sex chromosomes

● Karyotype: an ordered display of the pairs of chromosomes from a cell

○ Chromosomes are ordered according to size and staining pattern, position

of chromosome

■ Note that the staining pattern comes about from the way that the

chromosomes interact with certain dyes - different genes interact

differently with different dyes

● Why are chromosomes paired? Every person has a set of chromosomes from their

mother and another set from their father



● Human chromosomes are grouped into 7 classes A-G according to length and

position of centromere




You don’t need to know this table

, ● Two chromosomes in each pair are homologous chromosomes. One chromosome

is a homolog of another

○ They carry genes that control the same characters (e.g both have genes

that code for eye colour, or both have genes that code for the enzyme

helicase)

○ The genes occur at the same position on each homologous pair.

● The sex chromosomes are X and Y

○ Note that the sex chromosomes in males (X and Y) are not homologous



Preparation of a karyotype

● Somatic cells are harvested and grown in a culture

● They are stimulated to divide - this is important at because the condensed form of

chromosomes only occurs in dividing cells

● The division process is stopped during metaphase - they are in their most highly

condensed form at this phase

● The chromosomes are stained and a photograph is taken

○ The most common staining technique is the Giemsa stain which produces

the banding patterns on chromosomes

■ The bands are called G bands

■ Dark stained areas are rich in A and T, called heterochromatic

regions. These are the inactive chromosomal regions (tightly

packed)

■ Light stained regions are euchromatic - they are associated with

active genes (loosely packed)

,LARGE SCALE CHROMOSOMAL CHANGES

● Some genetic disorders occur as a result of alterations of the chromosome

number or structure

● The karyotype can be used to detect abnormal changes

○ Having missing or extra chromosomes is called aneuploidy

● Almost all changes in the chromosome number or structure are so disastrous to

the developing organism that it will immediately abort

○ This is because the lack of / addition of certain genes can upset the genetic

balance

● Some organisms are able to survive with this genetic imbalance but they will

have a syndrome that is characteristic of that aneuploidy

Down syndrome

● Trisomy 21

● It is an aneuploid condition that results in three copies of chromosome 21 (instead

of 2)

● The extra copy affects the organism but not so much that it dies

● it is different from the trisomy of other autosomes in that children can survive into

adulthood

○ Trisomy 18 is Edwards Syndrome

○ Trisomy 13 is Patau syndrome

■ Humans with these syndromes do not survive long after birth

Aneuploidy of sex chromosomes

● These do not affect genetic balance as much as changes in autosomes

● XXX chromosomes - have no unusual physical features

● Monosomy X - produces sterile females

, ● XXY individuals have Klinefelter syndrome - usually does not become noticeable

until adulthood

○ They have a low sperm count




How genetic imbalance occurs

● Duplicating a single gene usually does not have severe consequences

● But duplicating (or removing) ALL the genes on a chromosome does - there are

too many genes in the organism

○ That is why offspring with aneuploidy are not viable



X inactivation in Female mammals

● All humans operate with at least one X chromosome

● One of the X chromosomes in human females is randomly inactivated during

embryonic development

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