HUMB1000 Lecture Notes - Lecture 11: Noncoding Dna, Homologous Chromosome, Somatic Cell
12 Jun 2018
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Introduction to DNA
❖ DNA: deoxyribonucleic acid
❖ Genetic information contained in nucleus
❖ Contains genetic information for protein formation
❖ Approximately 23 000 genes in human genome
❖ Genes code for proteins
❖ Only 1.5% of DNA is due to genes
❖ 98.5% of DNA is non-coding – e.g regulatory sequences, introns, and noncoding DNA
– e.g repeat elements
Structure of DNA
• Double-stranded (double helix)
• Sugar-phosphate backbone
• Complementary nitrogen bases
o adenine – thymine
o guanine – cytosine
Organisation of DNA
❖ Double strands of DNA → helix
❖ DNA wrapped arounds proteins called histones
❖ Histones & DNA bundled together → chromatin
❖ Chromatin twists around to make chromosomes
How much DNA is in a cell?
• Each somatic human cell has two copies of each
chromosome – one you inherited from Mum and one from
Dad.
• The maternal and paternal chromosomes of a pair are called
homologous chromosomes (make a “homologous pair)
• Humans have 22 pairs of autosomal chromosomes and 1 pair of sex chromosomes
– Women have 2 X chromosomes and men have an X and Y
• Somatic cells with 46 chromosomes (23 pairs) are said to be diploid (have the full
amount of DNA)
• Gametes (sperm and egg) only have 1 chromosome of each homologous pair (have
23 chromosomes) and are called haploid (have half the normal amount of DNA)
• When cells are dividing, the chromosomes become easier to see and we can arrange
them next to their pair – this kind of map is a karyotype
Karyotype
The number and visual appearance of the chromosomes
in the cell nuclei of an organism or species.
Other Words
• Genetics - study of heredity
• Gene – piece of DNA that codes for a protein
• Allele – alternative form of a gene
• Genotype – the actual gene (allele) a
• Phenotype - person’s appearance
• Dominant and recessive alleles
• Sex-linked traits: traits affected by genes on sex
chromosomes
Protein Synthesis
The “Proteome”
• Cells are protein factories that constantly synthesize many different proteins
• These proteins are used for cell functions or can be exported
• The cell’s DNA contains all the instructions the cell needs for making proteins
• Not all cells make all proteins - some proteins are needed only by specific cells.
o The “proteome” of a cell is all the proteins that a cell makes, and
“proteomics” is the study of the proteins in a cell.
o The proteome of one cell can be compared to another to see how they are
different.
▪ a muscle cell vs a skin cell
▪ a melanoma vs a normal melanocyte
Transcription
❖ DNA has two strands, but only one strand of the DNA is used as a template to make
RNA
❖ Genetic information (a gene) is copied from a strand of DNA to make a strand of
ribonucleic acid (RNA) called mRNA (messenger RNA)
❖ RNA is like DNA except –
o Sugar: ribose instead of deoxyribose
o it is single-stranded
o contains uracil instead of thymine
❖ RNA acts as an intermediary between DNA and protein
❖ Initiated by transcription factors that recruit RNA polymerase enzyme
❖ The mRNA produced is called an RNA transcript
❖ There are special sequences/signals in DNA that indicate when a gene starts and
stops
❖ Within a gene there are exons (coding) and introns (non-coding)
❖ The initial mRNA transcript (pre-mRNA) contains both the exons and introns.
❖ The RNA introns are then cut out and the exons are all joined together. This
transcript is called processed RNA
❖ Three kinds of RNA are transcribed from DNA
o Messenger RNA (mRNA): is translated in the cytoplasm to make proteins
o Ribosomal RNA (rRNA): together with ribosomal proteins rRNA makes up the
ribosomes
o Transfer RNA (tRNA): each tRNA can bind specifically to one of the 20
different amino acids used to build proteins, important in translating mRNA
into amino acid peptide
Document Summary
Approximately 23 000 genes in human genome. Only 1. 5% of dna is due to genes. 98. 5% of dna is non-coding e. g regulatory sequences, introns, and noncoding dna. Structure of dna: double-stranded (double helix, sugar-phosphate backbone, complementary nitrogen bases, adenine thymine, guanine cytosine. Histones & dna bundled together chromatin. How much dna is in a cell: each somatic human cell has two copies of each chromosome one you inherited from mum and one from. Dad: the maternal and paternal chromosomes of a pair are called homologous chromosomes (make a homologous pair, humans have 22 pairs of autosomal chromosomes and 1 pair of sex chromosomes. The number and visual appearance of the chromosomes in the cell nuclei of an organism or species. Other words: genetics - study of heredity, gene piece of dna that codes for a protein, genotype the actual gene (allele) a chromosomes. Sex-linked traits: traits affected by genes on sex.
