Basic concepts: chromosomes, cell division, gene structure, transcription and translation,

Question 1

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Answer 1

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Question 2

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Question 4

What is DNA?

Answer 4

• DNA (deoxyribonucleic acid) stores genetic information
• Made of nucleotides
• Each nucleotide contains:
  • Sugar
  • Phosphate
  • Base
• 4 bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G)
• Base pairing rule:
  • A pairs with T
  • C pairs with G
• Forms a double helix

Question 5

What is a chromosome?

Answer 5

• Long strand of DNA
• Wrapped around histone proteins
• Tightly packed inside the nucleus
• Packaging allows ~2 metres of DNA to fit inside a microscopic nucleus
• Humans have 46 chromosomes
• Arranged as 23 pairs
  • 22 pairs of autosomes
  • 1 pair of sex chromosomes (XX or XY)
• Somatic (body) cells: 46 chromosomes
• Gametes (sperm and egg): 23 chromosomes

Question 6

What are the structural parts of a chromosome?

Answer 6

• Centromere: joins two sister chromatids
• Telomeres: protective ends of the chromosome
• p arm: short arm
• q arm: long arm
• After DNA replication:
  • Each chromosome consists of two sister chromatids
  • Joined at the centromere

Question 7

What is cell division?

Answer 7

• Process by which cells reproduce
• Two types:
  • Mitosis
  • Meiosis

Question 8

What is mitosis and what is its purpose?

Answer 8

• Cell division in somatic (body) cells
• Purpose:
  • Growth
  • Repair
  • Replacement of cells
• Stages:
  • Prophase: chromosomes condense
  • Metaphase: chromosomes line up in the middle
  • Anaphase: sister chromatids separate
  • Telophase: two nuclei form
• Produces 2 genetically identical daughter cells
• Each has 46 chromosomes
• Maintains chromosome number

Question 9

What is meiosis and where does it occur?

Answer 9

• Produces gametes (sperm and eggs)
• Occurs in:
  • Testes
  • Ovaries
• Consists of:
  • Meiosis I
  • Meiosis II

Question 10

What happens in Meiosis I?

Answer 10

• Homologous chromosomes pair
• Crossing over occurs
• Chromosome number is halved

Question 11

What happens in Meiosis II?

Answer 11

• Similar to mitosis
• Sister chromatids separate

Question 12

What is the result of meiosis?

Answer 12

• Produces 4 genetically different cells
• Each has 23 chromosomes
• Reduces chromosome number from 46 to 23
• Crossing over occurs in Meiosis I
• Creates genetic variation

Question 13

What is a gene?

Answer 13

• A segment of DNA
• Codes for a protein
• Proteins carry out most functions in the body

Question 14

What are the main components of a gene?

Answer 14

• Promoter: region before the gene; controls when gene is switched on
• Exons: coding regions; remain in final mRNA; code for protein
• Introns: non-coding regions; removed during RNA splicing
• Start codon (AUG): signals start of protein synthesis
• Stop codon: signals end of protein synthesis
• Exons code; introns are removed
• Genes are transcribed into RNA before becoming protein

Question 15

What is transcription?

Answer 15

• DNA → mRNA
• Occurs in the nucleus
• Enzyme: RNA polymerase

Question 16

What are the steps of transcription?

Answer 16

• RNA polymerase binds to the promoter
• DNA unwinds
• Complementary RNA strand is made
• mRNA leaves the nucleus
• After transcription:
  • Introns removed
  • Exons joined
  • Process called RNA splicing
• Produces messenger RNA (mRNA)

Question 17

What is translation?

Answer 17

• mRNA → protein
• Occurs in the cytoplasm
• Takes place at the ribosome

Question 18

What are the steps of translation?

Answer 18

• mRNA attaches to ribosome
• Read in groups of 3 bases
• Each 3 bases = codon
• Each codon codes for 1 amino acid
• tRNA brings amino acids
• Amino acids are linked to form protein
• 3 bases = 1 codon
• 1 codon = 1 amino acid
• Start codon = AUG

Question 19

What are the absolute core exam points?

Answer 19

• Humans have 46 chromosomes
• Gametes have 23 chromosomes
• Mitosis → identical cells
• Meiosis → genetic variation
• Crossing over occurs in Meiosis I
• Gene contains promoter, exons, introns
• Exons code
• DNA → mRNA = Transcription
• mRNA → protein = Translation
• 3 bases = 1 codon