Summary Life Sciences Paper 2 – Grade 12 Exam Revision Notes on Genetics, Evolution and Human Evolution

LIFE SCIENCES - QUICK FIX - PAPER TWO
MEIOSIS

STRUCTURE AND ARRANGEMENT OF CHROMOSOMES IN A NORMAL HUMAN KARYOTYPE.
- Each chromosome comprises two chromatids
- held together by a centromere
- There are 23 pairs/46 chromosomes in
- human somatic cells/body cells
- which are arranged into homologous pairs
- that are similar in length
- carry genes for the same characteristics
- have alleles of a particular gene at the same lociand
- have the same centromere position
- Each somatic cell has 22 pairs/44 autosomesand
- a pair/2 gonosomes/sex chromosomes/X and Y chromosomes
- Autosomes are arranged in pairs from largest to smallest in a karyotype
- Males have XY chromosomes
- Females have XX chromosomes
- The X chromosome is larger than the Y chromosome

BEHAVIOUR OF THE CHROMOSOMES DURING THE DIFFERENT PHASES OF MEIOSIS I.
- During prophase I
- chromosomes pairup/homologous pairs /bivalents form
- Crossing overexchange of genetic material occurs
- between chromatids/adjacent chromosome pairs
- During metaphase I of meiosis
- homologous chromosomes/chromosome pairs are arranged
- at the equatorof the cell
- in a random way
- with the chromosome attached to the spindle fibre
- During anaphase  I
- chromosome pairs separate/chromosomes move to opposite poles
- During telophase  I
- the chromosomes reach the poles of the cell




1

,EVENTS THAT LEAD TO DOWN SYNDROME.
- Non-disjunction occurred/A homologous pair of chromosomes
- at position 21
- failed to separate
- during Anaphase
- resulting in one gamete with 24 chromosomes / an extra Chromosome /
2 chromosomes at position 21
- The fertilisation of this gamete with a normal gamete/gamete
with 23 chromosomes /1 chromosome at position 21
- results in a zygote with 47 chromosomes
- There are 3 chromosomes/an extra chromosome at position 21/ this is Trisomy 21

HOW MEIOSIS CONTRIBUTES TO GENETIC VARIATION
- Crossing over
- occurs during prophase I
- Homologous chromosomes / chromatids overlap
- at points called chiasma/ chiasmata
- Genetic material is exchanged
- resulting in new combinations of genetic material from both parents
- Random arrangement of chromosomes
- occurs during metaphase I / II
each pair of homologous chromosomes  /each chromosome
− may line up in different ways on the equator of the spindle
− allowing the gametes to have different combinations of maternal and paternal
chromosomes 
- so that they separate in a random/ independent manner
- resulting in new combinations of genetic material

HOW MUTATIONS CONTRIBUTE TO GENETIC VARIATION
Two types of mutations, viz a gene mutation and a chromosome mutation

- A gene mutation occurs
- as a result of a change in sequence of nitrogen bases in the DNA molecule
- A chromosomemutation occurs as a
- result of a change in the structure of a chromosome/
number of chromosomes during meiosis
- Mutations that occur in sex cells
- are passed on to the new generations
- creating new characteristics




2

, DNA & PROTEIN SYNTHESIS
THE LOCATION, STRUCTURE AND FUNCTIONS OF A DNA MOLECULE
- The DNA is located in the nucleus/chromosome/genes/mitochondria
- DNA is a nucleic acid
- It has a doublestranded - helix
- consisting of building blocks called nucleotides
The three components of a nucleotide are as follows:
- Nitrogenous bases - Phosphate portion
- Deoxyribose sugar portion (in DNA)
- 4 nitrogenous bases are A,T,C,G of DNA:
- adenine (A) bonds with thymine (T)  - cytosine (C) bonds with guanine (G) 
- by weak hydrogen bonds
- Sections of DNA carry hereditaryinformation
- DNA contains coded information for protein synthesis

STRUCTURAL DIFFERENCES BETWEEN DNA AND RNA.
DNA RNA

Double stranded molecule Single strandedmolecule
Has a helixshape Is a straight molecule
One of the nitrogen bases is thymine The nitrogen base uracilin place of thymine
Contains deoxyribose sugars Contains ribosesugars
A longermolecule A shortermolecule
Paired bases Unpaired bases

DNA REPLICATION.
The DNA- double helix molecule unwinds within the nucleus
- Weak Hydrogen bonds between the two strands break/ the molecule
unzips
- Each strand serves as a template to form a new strand.
- Free DNA nucleotides attach to the individual strands - with complementary
nitrogen bases pairing. (A to T and C to G)
- Two genetically identical DNA molecules are formed
- Process is controlled by enzymes




3