it is explaining the structure of DNA according to 0620 syllabus

1. How is DNA organized?
§ DNA is stored in the nucleus of cells,
and it’s the cells genetic information.
§ A gene is a section of DNA that codes
for one protein.
§ A chromosome is a long length of
DNA which carries genetic information
in the form of genes.

2. Diploid vs Haploid
§ A diploid nucleus is a nucleus that
contains two sets of chromosomes.
This means 23 pairs (so 46 in total)
§ All of your normal somatic cells are
diploid
§ A haploid nucleus is a nucleus that
contains one set of chromosomes.
This means 23 chromosomes only.
§ Gametes (sperm/egg cells) are haploid

3. Mitosis
§ Mitosis is a nuclear division giving rise
to genetically identical cells
§ Mitosis is needed for growth, repair of
damaged tissues and cell
replacement. It is also used for
asexual reproduction in some
organisms.

4. Mitosis: The steps
§ Step 1: Duplicate the chromosomes
§ Step 2: Line up the chromosomes
along the centre
§ Step 3: Separate the chromosomes
§ Step 4: Nucleus divides
§ Step 5: Cytoplasm divides
§ This creates diploid cells

5. Stem cells
§ Stem cells are undifferentiated cells
that divide asymmetrically by mitosis to
produce specialized cells
§ Almost all cells in the body have the
same DNA, but they become
specialized because some genes are
switched on and some are switched
off.

6. Meiosis
§ Step 1: Duplicate the chromosomes
§ Step 2: Line up the chromosomes
along the centre
§ Step 3: Separate the chromosomes
§ Step 4: Nucleus divides
§ Step 5: Cytoplasm divides
§ Step 6: Nuclei divide again
§ Step 7: Cytoplasm divides again
§ This creates haploid gametes

7. How does this build on what you already know?
How does link to the rest of biology?
§ A gene is a section of DNA that
codes for a protein
§ These proteins could be anything
including enzymes, hormones,
neurotransmitters and cell membrane
carrier proteins

8. Let’s look closer - Transcription
§ Each three bases of DNA is called a
triplet.
§ An enzyme will come along, unzip
the DNA strands so the bases are
exposed, and transcribe a
complimentary strand of mRNA.
§ mRNA is a lot like DNA, but it uses
the letter U instead of T.
§ This process is called transcription.
§ This is so DNA stays in the
nucleus.

9. Let’s look closer - Translation
§ Each three bases of mRNA is called a
codon.
§ The mRNA will move out of the nucleus
into the cytoplasm, and attach to a
ribosome.
§ The ribosome will read the code and
choose the correct amino acid for each
codon.
§ Each codon codes for a different amino
acid, and there are 20 amino acids in
total.
§ The ribosomes assembles the amino
acids into proteins.

10. Overview

11. What are alleles?
§ Every person has two versions of
every gene.
§ These versions of genes are called
alleles
§ Inheritance is the passing on of
genes to the next generation

12. There are two types of alleles
§ An allele is either dominant or
recessive.
§ If a person has a dominant allele, it will
always be expressed
§ If a person has a recessive allele, it will
only be expressed if there is no
dominant

13. What are the combinations?
§ This means for each gene, you can
either have 2 dominant alleles, 2
recessive alleles, or one of each.
§ Homozygous dominant means you
have 2 dominant alleles
§ Homozygous recessive means you
have 2 recessive alleles
§ Heterozygous means you have 1
dominant and 1 recessive allele

14. Genotype vs phenotype
§ The genotype of an individual is
whether they are homozygous
dominant, homozygous recessive or
heterozygous for a particular gene
§ The phenotype of an individual is the
characteristic you can see in the
organism

15. Punnett squares
§ We can use Punnett squares to
predict the genotypes and phenotypes
of the offspring when two individuals
mate

16. Sex inheritance
§ The 23rd pair of chromosomes are the
sex chromosomes
§ If you are female you have 2 X
chromosomes
§ If you are male you have 1 X and 1 Y
chromosome
§ This means males determine the sex of
the offspring

17. Genetic diagrams

18. Pedigree diagrams

19. What is codominace?
§ Sometimes, the alleles for a gene
can be codominant which means in
a heterozygote, both alleles are
expressed.

20. What are blood groups?
§ Blood groups can show
codominance.
§ The alleles are IA (for group A), IB (for
group B) and I (for group O).

21. Sex linkage
§ Sex linkage is where a gene is
located on a sex chromosome.
§ This means a trait or disease may be
more likely to be passed on to a
specific sex
§ A disease can be X-linked or Y-linked

22. Sex linkage
§ Red-green
colourblindness is an X-
linked recessive disorder
§ This means males are
more likely to have it has
they only have one X
chromosome.