Reproduction and DNA Notes

AQA GCSE Biology: Revision notes

Key facts

  • Sexual reproduction gives genetic variation; asexual reproduction gives clones.
  • DNA is a double helix. Nucleotides have a sugar, a phosphate and a base, and bases pair A with T and C with G.
  • A gene is a sequence of DNA bases coding for the amino acid sequence of a protein.
  • The code is a triplet code: three bases (a codon) code for one amino acid.
  • Proteins are made by transcription then translation (Higher Tier).
  • The Human Genome Project (1990 to 2003) sequenced the whole human genome.

Sexual

  • Two parents
  • Variation

Asexual

  • One parent
  • Clones

Sexual and asexual reproduction

Sexual reproduction is slower but varied and adaptable; asexual is fast and cheap but makes clones.

Sexual reproduction needs two parents and the fusion of gametes. Asexual reproduction needs one parent and no gametes.

  • Sexual: variation lets a population adapt and evolve, but it is slower and costs more energy.
  • Asexual: fast, low energy and one parent, but no variation, so a change in the environment can harm all the clones.

Sexual

Parents:
Two parents
Offspring:
Genetically different offspring
Speed and cost:
Slower, high energy cost
Adaptation:
Can adapt to change

Asexual

Parents:
One parent
Offspring:
Clones
Speed and cost:
Faster, low energy cost
Adaptation:
Cannot adapt if conditions change

Why are asexual offspring genetically identical to the parent?

Where variation comes from

Meiosis shuffles chromosomes, crossing over swaps segments, and fertilisation joins two different gametes.

Three things create variation:

  1. Meiosis shuffles chromosomes, so gametes have different combinations of alleles.
  2. Crossing over swaps segments of DNA between homologous chromosomes.
  3. Fertilisation combines DNA from two parents.

Asexual reproduction uses only mitosis, so offspring are clones, apart from rare mutations.

  1. 1

    Meiosis

    gametes with different allele combinations

  2. 2

    Crossing over

    segments exchanged between homologous chromosomes

  3. 3

    Fertilisation

    gametes from two parents fuse

  4. 4

    Offspring

    a unique genetic mix

Sources of variation

Which process swaps segments of DNA between chromosomes?

DNA structure

DNA is a double helix of two strands of nucleotides, joined by pairs of bases: A with T and C with G.

DNA is a double helix. Each nucleotide has a deoxyribose sugar, a phosphate group and one base: adenine (A), thymine (T), cytosine (C) or guanine (G).

The sugar of one nucleotide bonds to the phosphate of the next, forming a backbone. Bases on opposite strands pair by hydrogen bonds. The strands run in opposite directions.

  • A pairs withT (two hydrogen bonds)
  • C pairs withG (three hydrogen bonds)
  1. 1

    Backbone

    sugar to phosphate in each strand

  2. 2

    Base pairing

    A with T, C with G

  3. 3

    Double helix

    strands twist, held by hydrogen bonds

Building the double helix

Which base pairs with cytosine?

The genetic code

Every three bases (a codon) code for one amino acid, and the order of codons sets the order of the amino acids in the protein.

A gene is a sequence of bases that codes for a sequence of amino acids, which form a protein.

The code is a triplet code. Four bases in pairs give only 16 combinations, but in threes give 64, enough for 20 amino acids and stop signals.

  • Several codons can code for the same amino acid (degenerate).
  • Almost all organisms use the same code (universal).
  • AUG is the start codon. UAA, UAG and UGA are stop codons.
  • 3 bases1 codon
  • 1 codon1 amino acid

Worked example

An mRNA sequence reads AUG-GCU-UAC-CAA-UGA. How many amino acids does it code for?

A gene section has 12 bases. How many codons is that?

Making a protein

DNA is transcribed to mRNA in the nucleus, then mRNA is translated at a ribosome (Higher Tier).

Transcription (nucleus): RNA polymerase unwinds the DNA and pairs free RNA nucleotides with the template strand. Uracil (U) replaces thymine, so A on the template pairs with U. The mRNA leaves for a ribosome.

Translation (ribosome): tRNA anticodons match mRNA codons, bringing amino acids, which are joined by peptide bonds. A stop codon releases the protein.

  1. 1

    DNA in nucleus

    gene to be copied

  2. 2

    Transcription

    RNA polymerase makes mRNA

  3. 3

    mRNA to ribosome

    leaves the nucleus

  4. 4

    Translation

    tRNA anticodons match mRNA codons

  5. 5

    Protein

    amino acids joined by peptide bonds

Gene to protein

Where does translation happen?

The Human Genome Project

An international project that sequenced all human DNA, about 3 billion base pairs, and mapped the genes.

The Human Genome Project identified roughly 20 000 to 25 000 human genes and gave a complete reference sequence.

Uses:

  • diagnosis of genetic diseases such as cystic fibrosis
  • targeted treatments and gene therapy
  • personalised medicine
  • drug design
  • comparing human DNA with other species to study evolution.

  1. 1990

    Project starts

    International effort to map all human genes.

  2. 2003

    Project completed

    The whole genome, about 3 billion base pairs, sequenced.

What is one use of the Human Genome Project?

Try an exam question

Describe how sexual reproduction produces genetic variation.

[3 marks]

That's the notes covered.

Carry on to the next subtopic.